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PROFESSOR: So I got much
more than one request

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00:00:27,420 --> 00:00:29,460
to do some stuff on
nuclear materials,

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00:00:29,460 --> 00:00:31,840
and I think it's just
about the right time.

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That you guys know
enough about radiation

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00:00:34,105 --> 00:00:35,730
interacting with
matter and everything,

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00:00:35,730 --> 00:00:37,435
and stopping power,
and processing,

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to actually make sense of
nuclear materials and radiation

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00:00:40,300 --> 00:00:40,800
damage.

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00:00:40,800 --> 00:00:43,432
And this is my whole
theme, so happy

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00:00:43,432 --> 00:00:44,890
to come talk to
you guys about this

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00:00:44,890 --> 00:00:46,640
and show you why I
think it's interesting.

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00:00:46,640 --> 00:00:48,530
Because it all goes--

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00:00:48,530 --> 00:00:51,160
this slide kind of gets onto it.

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00:00:51,160 --> 00:00:54,180
It starts off with the
single-atom atomic defects

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00:00:54,180 --> 00:00:57,000
that make up the basic
building blocks of damage

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00:00:57,000 --> 00:01:00,090
and ends up with things that
break in nuclear reactors

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00:01:00,090 --> 00:01:01,188
under radiation.

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00:01:01,188 --> 00:01:02,730
And so to understand
the whole thing,

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00:01:02,730 --> 00:01:05,459
you've got to know everything
from the single atoms

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00:01:05,459 --> 00:01:07,405
on the sort of
femtosecond scale,

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00:01:07,405 --> 00:01:09,030
all the way up to
the engineering scale

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00:01:09,030 --> 00:01:12,930
where things evolve over
years or even decades.

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00:01:12,930 --> 00:01:13,840
So we'll be talking--

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first, probably
today, we're going

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00:01:15,030 --> 00:01:16,690
to go over a material
science primer.

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00:01:16,690 --> 00:01:20,500
So who here has had any
courses in material science?

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00:01:20,500 --> 00:01:21,000
No one.

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00:01:21,000 --> 00:01:23,950
That's good because I'm
assuming that there is a--

36
00:01:23,950 --> 00:01:25,355
see, no one knows anything here.

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00:01:25,355 --> 00:01:27,730
I know there's a couple material
scientists in the class,

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00:01:27,730 --> 00:01:30,430
and I'll apologize ahead of
time if it's a bit of a review.

39
00:01:30,430 --> 00:01:33,250
But we'll be going mostly
through what are materials

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00:01:33,250 --> 00:01:35,560
and what are the defects
that change their material

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00:01:35,560 --> 00:01:37,850
properties, and
how do they behave.

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00:01:37,850 --> 00:01:39,710
That'll take us
through about today.

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00:01:39,710 --> 00:01:43,150
So then tomorrow, we can see how
radiation causes those defects

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00:01:43,150 --> 00:01:47,418
and actually changes
material properties.

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00:01:47,418 --> 00:01:49,460
So there's a whole laundry
list of different ways

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00:01:49,460 --> 00:01:51,620
that materials
fail, and most folks

47
00:01:51,620 --> 00:01:54,510
are concerned with
all of these--

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00:01:54,510 --> 00:01:57,380
everything from simple
overload, which means you

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00:01:57,380 --> 00:01:59,900
stress something too
much and it just breaks,

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00:01:59,900 --> 00:02:02,000
to all the different
forms of corrosion.

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00:02:02,000 --> 00:02:03,788
That's a whole field in itself.

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00:02:03,788 --> 00:02:05,330
And then there's
the things that just

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00:02:05,330 --> 00:02:08,150
we have to worry about
because they're only activated

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with radiation damage.

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00:02:10,100 --> 00:02:13,170
And in this case, this isn't
quite ionization by radiation,

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but it's actual radiation
slamming into nuclei

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00:02:15,920 --> 00:02:18,810
and moving atoms
out of their place.

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00:02:18,810 --> 00:02:20,850
And we've got one figure
that we had recently

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00:02:20,850 --> 00:02:23,850
in a paper that sums up the
entire multi-scale picture

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00:02:23,850 --> 00:02:27,810
of radiation damage, from the
femtosecond to, let's say,

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00:02:27,810 --> 00:02:28,918
the megasecond scale.

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00:02:28,918 --> 00:02:30,210
Or I think it's more than that.

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00:02:30,210 --> 00:02:32,670
Maybe gigasecond would be
the right word for that.

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00:02:32,670 --> 00:02:35,970
And all the way down from the
angstrom to the meter scale.

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And I want to walk you
through sort of a lens scale

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00:02:39,750 --> 00:02:42,840
by lens scale depiction
of radiation damage.

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It all starts with knocking
atoms out of place.

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We've mentioned
this a little bit

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when we talked about
nuclear stopping power,

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and this is where it
actually comes into play.

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Sometimes an incoming
neutron or photon or ion

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can displace an atom
from its original site,

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and we call that a physical--
it's a displacement.

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And then that atom comes
off with quite a bit

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of kinetic energy and can
knock into a whole bunch

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00:03:07,590 --> 00:03:09,090
of other atoms.

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Now this loss of the solid
crystalline structure,

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you can't really tell what the
original structure looked like,

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00:03:15,510 --> 00:03:16,710
right?

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00:03:16,710 --> 00:03:19,590
It actually comprises a
very small, localized zone

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00:03:19,590 --> 00:03:22,050
of melting called
a thermal spike.

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If you think about,
all these atoms

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are vibrating at
fractions of an eV--

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00:03:26,370 --> 00:03:28,320
at thermal energies,
like the thermal neutrons

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00:03:28,320 --> 00:03:29,970
we talked about in the reactor.

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Then you hit them
with an MeV neutron.

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They might transfer
100 keV of energy.

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00:03:35,160 --> 00:03:36,660
And a bunch of these
atoms will then

89
00:03:36,660 --> 00:03:40,440
be moving about at, let's
say, a few hundred eV.

90
00:03:40,440 --> 00:03:43,350
That's way beyond
liquid temperature.

91
00:03:43,350 --> 00:03:45,660
So actually, it's been
theorized that there's

92
00:03:45,660 --> 00:03:49,020
a little pocket of atoms around
three to five nanometers wide

93
00:03:49,020 --> 00:03:52,200
that reaches, like, 10,000
Kelvin for a very, very

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00:03:52,200 --> 00:03:54,270
short amount of time--

95
00:03:54,270 --> 00:03:55,830
less than a picosecond.

96
00:03:55,830 --> 00:03:57,690
Because almost
instantly, those atoms

97
00:03:57,690 --> 00:04:00,090
knock into the ones
around them, and this

98
00:04:00,090 --> 00:04:02,940
is how the process of
heat transfer occurs.

99
00:04:02,940 --> 00:04:04,440
And so, very quickly,
you get what's

100
00:04:04,440 --> 00:04:07,050
called the quench, where
most of those atoms

101
00:04:07,050 --> 00:04:08,730
very quickly knock
into other ones,

102
00:04:08,730 --> 00:04:12,180
slowing down, finding their
equilibrium positions again,

103
00:04:12,180 --> 00:04:14,250
but not every one.

104
00:04:14,250 --> 00:04:17,310
You can see there's a few
places where the atoms are still

105
00:04:17,310 --> 00:04:19,079
out of their original location.

106
00:04:19,079 --> 00:04:21,450
And it's those residual
defects that actually

107
00:04:21,450 --> 00:04:24,090
comprise radiation damage.

108
00:04:24,090 --> 00:04:27,860
And as those defects build
up, they start to move.

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00:04:27,860 --> 00:04:29,240
They can diffuse.

110
00:04:29,240 --> 00:04:31,040
They can be transported
ballistically

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00:04:31,040 --> 00:04:32,977
by more radiation damage.

112
00:04:32,977 --> 00:04:35,060
They can move by all sorts
of different mechanisms

113
00:04:35,060 --> 00:04:39,008
and eventually find each other,
forming what's called clusters.

114
00:04:39,008 --> 00:04:41,300
So a bunch of those missing
atoms could find each other

115
00:04:41,300 --> 00:04:43,490
and make a hole,
which we call a void.

116
00:04:43,490 --> 00:04:45,320
A bunch of the extra
atoms shoved in

117
00:04:45,320 --> 00:04:47,870
between the other
ones can form things

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00:04:47,870 --> 00:04:49,970
called interstitial clusters.

119
00:04:49,970 --> 00:04:52,160
We say interstitial because
it's like in the space

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00:04:52,160 --> 00:04:56,090
in between where you'd
normally find some atoms.

121
00:04:56,090 --> 00:04:59,270
So let's say you had a whole
bunch of those missing atoms

122
00:04:59,270 --> 00:05:01,520
come together, forming a void.

123
00:05:01,520 --> 00:05:04,060
This is an actual Transmission
Electron Microscope,

124
00:05:04,060 --> 00:05:06,380
or TEM, image of a void--

125
00:05:06,380 --> 00:05:09,380
pockets of vacuum in materials.

126
00:05:09,380 --> 00:05:12,263
Notice anything interesting
about its shape?

127
00:05:12,263 --> 00:05:14,110
AUDIENCE: It's, like, rounded.

128
00:05:14,110 --> 00:05:17,350
PROFESSOR: It's rounded, but
what's most striking to me

129
00:05:17,350 --> 00:05:19,850
is it isn't actually round.

130
00:05:19,850 --> 00:05:23,160
So you would expect a void or a
bubble to be kind of spherical,

131
00:05:23,160 --> 00:05:23,660
right?

132
00:05:23,660 --> 00:05:27,740
That's the minimum energy
configuration of most things.

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00:05:27,740 --> 00:05:30,600
Not so when you have a
little pocket of vacuum.

134
00:05:30,600 --> 00:05:33,370
It's where crystallinity
comes into play.

135
00:05:33,370 --> 00:05:36,350
And these voids can end up
forming superstructures.

136
00:05:36,350 --> 00:05:38,570
What curious thing
do you notice here?

137
00:05:41,380 --> 00:05:42,775
For this whole
ensemble of voids.

138
00:05:45,760 --> 00:05:46,260
Yeah?

139
00:05:46,260 --> 00:05:48,220
AUDIENCE: It seems like
they're all in line.

140
00:05:48,220 --> 00:05:50,827
PROFESSOR: They are all
in the same direction.

141
00:05:50,827 --> 00:05:51,410
Kind of funny.

142
00:05:51,410 --> 00:05:53,182
That's definitely not
an accident, right?

143
00:05:53,182 --> 00:05:54,890
That's not like they're
randomly aligned.

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00:05:54,890 --> 00:05:56,432
There's a reason
for this, that we'll

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00:05:56,432 --> 00:05:57,630
go into in a couple slides.

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00:05:57,630 --> 00:05:58,130
Yeah?

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00:05:58,130 --> 00:05:59,980
AUDIENCE: What's
the size scale here?

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00:05:59,980 --> 00:06:01,780
PROFESSOR: The size scale?

149
00:06:01,780 --> 00:06:05,218
I think these are on the
order of 20 nanometers or so.

150
00:06:05,218 --> 00:06:07,510
Yeah, I cropped these images
just to get points across.

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Let's see if it says
in the older one.

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Not quite.

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00:06:10,450 --> 00:06:14,110
Yeah, but these voids can get
upwards of tens of nanometers.

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As small as single atoms.

155
00:06:15,300 --> 00:06:15,800
Yeah?

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00:06:15,800 --> 00:06:18,250
AUDIENCE: Sorry, what is this?

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00:06:18,250 --> 00:06:20,080
PROFESSOR: This is
the accumulation

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00:06:20,080 --> 00:06:23,960
of radiation defects
into what's called voids.

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00:06:23,960 --> 00:06:24,460
Yeah.

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00:06:24,460 --> 00:06:28,390
Don't worry, we'll go over
it in more detail again.

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00:06:28,390 --> 00:06:31,140
And if you get little pockets
of vacuum in your material,

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00:06:31,140 --> 00:06:32,890
you're not creating
or destroying mass.

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00:06:32,890 --> 00:06:34,330
You're just moving it.

164
00:06:34,330 --> 00:06:37,990
So those voids, where that mass
was has to go somewhere else,

165
00:06:37,990 --> 00:06:39,730
and you actually get
things that swell

166
00:06:39,730 --> 00:06:41,480
in the reactor on their own.

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00:06:41,480 --> 00:06:43,780
They don't change mass
but they change volume.

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00:06:43,780 --> 00:06:46,810
They just kind of puff up
like Swiss cheese, sometimes

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00:06:46,810 --> 00:06:50,470
upwards of 20% or 30% changes
in diameter and length

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00:06:50,470 --> 00:06:51,640
for some tubing.

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Now if you're depending
on these fuel rods

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00:06:53,950 --> 00:06:56,740
being a certain space
apart in a reactor

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00:06:56,740 --> 00:06:59,920
and they start to swell,
squeezing out the coolant,

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00:06:59,920 --> 00:07:01,995
you lose the ability
to cool the reactor.

175
00:07:01,995 --> 00:07:04,120
Because then how can you
get water around something

176
00:07:04,120 --> 00:07:06,248
where the tubes have
then swelled together?

177
00:07:06,248 --> 00:07:08,290
There's lots of other bad
things that can happen,

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00:07:08,290 --> 00:07:10,290
which we'll get into.

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00:07:10,290 --> 00:07:12,390
And so then that's the
origin of void swelling.

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00:07:12,390 --> 00:07:15,045
From single missing
atoms called vacancies,

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00:07:15,045 --> 00:07:17,880
they can cluster
into voids which

182
00:07:17,880 --> 00:07:21,120
then cause physical dimensional
changes of materials

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00:07:21,120 --> 00:07:23,650
on the scale of
centimeters to meters.

184
00:07:23,650 --> 00:07:26,610
And that's why we say it's
this full multi-scale picture

185
00:07:26,610 --> 00:07:28,550
of radiation damage.

186
00:07:28,550 --> 00:07:30,230
But to understand,
what is damage,

187
00:07:30,230 --> 00:07:32,420
you have to know what is
an undamaged structure

188
00:07:32,420 --> 00:07:33,508
to begin with.

189
00:07:33,508 --> 00:07:36,050
So it doesn't make sense to say,
how does a structure change,

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00:07:36,050 --> 00:07:38,580
if you don't know
how it behaves.

191
00:07:38,580 --> 00:07:42,350
So I want to give a very quick
primer to material science.

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00:07:42,350 --> 00:07:44,660
And apologies to any material
scientists in the room

193
00:07:44,660 --> 00:07:46,910
because this is going
to seem really basic,

194
00:07:46,910 --> 00:07:50,060
but this is a very quick
intro to this whole field.

195
00:07:50,060 --> 00:07:53,220
I want to go over quickly,
what is a crystalline solid?

196
00:07:53,220 --> 00:07:55,520
A perfectly undamaged
material would

197
00:07:55,520 --> 00:07:59,510
be a set of atoms lined up
in a very regular lattice

198
00:07:59,510 --> 00:08:02,750
and of regular array, where you
move over a certain distance

199
00:08:02,750 --> 00:08:04,280
and you find another atom.

200
00:08:04,280 --> 00:08:06,770
And this extends forever
and ever and ever,

201
00:08:06,770 --> 00:08:09,417
all the way out to when
you reach the free surface.

202
00:08:09,417 --> 00:08:11,750
And so this is what we would
call an undamaged material.

203
00:08:11,750 --> 00:08:15,320
A pristine, perfect,
single crystal.

204
00:08:15,320 --> 00:08:17,510
By crystal, I mean
an arrangement

205
00:08:17,510 --> 00:08:19,830
of atoms in a certain direction.

206
00:08:19,830 --> 00:08:23,090
So notice here, all of the atoms
are lined up in, let's say,

207
00:08:23,090 --> 00:08:25,730
some cubic xyz way.

208
00:08:25,730 --> 00:08:28,970
That's what we would call
one crystal or one grain.

209
00:08:28,970 --> 00:08:31,550
You'll hear both of those.

210
00:08:31,550 --> 00:08:34,640
And you'll notice also that
the arrangement of the atoms

211
00:08:34,640 --> 00:08:38,600
tends to determine what the
physical objects look like.

212
00:08:38,600 --> 00:08:42,169
Or we like to say
that form follows

213
00:08:42,169 --> 00:08:44,780
structure in material science.

214
00:08:44,780 --> 00:08:47,540
So for materials
like pyrite, which

215
00:08:47,540 --> 00:08:50,510
follows a simple cubic
structure, that's the crystals

216
00:08:50,510 --> 00:08:51,980
you pull out of the ground.

217
00:08:51,980 --> 00:08:53,960
They mimic their
atomic configurations

218
00:08:53,960 --> 00:08:57,030
in physical
centimeter-sized space.

219
00:08:57,030 --> 00:09:00,570
For gold atoms, they adopt a
slightly different structure.

220
00:09:00,570 --> 00:09:02,730
It's still cubic
but there is atoms

221
00:09:02,730 --> 00:09:04,710
shoved into the cube faces.

222
00:09:04,710 --> 00:09:07,650
It's what we call
Face-Centered Cubic, or FCC.

223
00:09:07,650 --> 00:09:10,170
And you start to see
cube-looking structures

224
00:09:10,170 --> 00:09:13,400
all over single
crystals of gold.

225
00:09:13,400 --> 00:09:14,780
Another one, gypsum.

226
00:09:14,780 --> 00:09:17,660
It's got a very different type
of structure called monoclinic,

227
00:09:17,660 --> 00:09:21,170
where none of the sides of
this parallelogram are the same

228
00:09:21,170 --> 00:09:22,602
and there are some funny angles.

229
00:09:22,602 --> 00:09:24,560
But if you look at the
arrangement of the atoms

230
00:09:24,560 --> 00:09:27,760
and the actual crystals
of gypsum that grow,

231
00:09:27,760 --> 00:09:31,772
you see a striking similarity,
which I find pretty neat.

232
00:09:31,772 --> 00:09:35,240
I also want to mention, what
is the absence of structure

233
00:09:35,240 --> 00:09:36,110
in material science?

234
00:09:36,110 --> 00:09:38,360
We call that something
that's amorphous.

235
00:09:38,360 --> 00:09:40,430
Amorphous means without form.

236
00:09:40,430 --> 00:09:44,057
So for example, crystalline
indium phosphide

237
00:09:44,057 --> 00:09:45,890
would have this regular
structure like this.

238
00:09:45,890 --> 00:09:47,270
You move over a
certain distance,

239
00:09:47,270 --> 00:09:50,790
you see another green atom,
and so on and so on and so on.

240
00:09:50,790 --> 00:09:53,150
In an amorphous material,
it can still be a solid,

241
00:09:53,150 --> 00:09:56,840
but there is no fixed distance
between any certain types

242
00:09:56,840 --> 00:09:58,010
of atoms.

243
00:09:58,010 --> 00:10:01,220
And radiation can cause a
lot of this amorphization

244
00:10:01,220 --> 00:10:03,320
by knocking the atoms about
and having them freeze

245
00:10:03,320 --> 00:10:05,300
in random configurations.

246
00:10:05,300 --> 00:10:08,170
This is one of the ways
that radiation damage can

247
00:10:08,170 --> 00:10:10,660
embrittle materials because--

248
00:10:10,660 --> 00:10:12,720
well, we'll get into that.

249
00:10:12,720 --> 00:10:15,050
So now let's talk
about the defects

250
00:10:15,050 --> 00:10:17,742
that can be created
in a perfect crystal.

251
00:10:17,742 --> 00:10:19,450
The simplest ones, we
call point defects.

252
00:10:19,450 --> 00:10:21,770
They're zero-dimensional
because they're just

253
00:10:21,770 --> 00:10:23,990
single atoms out of place.

254
00:10:23,990 --> 00:10:26,490
You can have what's called
a vacancy, where if you had,

255
00:10:26,490 --> 00:10:29,780
let's say, a face-centered
cubic lattice of atoms,

256
00:10:29,780 --> 00:10:32,942
where you have atoms on every
cube corner and every face,

257
00:10:32,942 --> 00:10:34,400
if you just pull
one out somewhere,

258
00:10:34,400 --> 00:10:36,940
we refer to that as a vacancy.

259
00:10:36,940 --> 00:10:38,212
A missing atom.

260
00:10:38,212 --> 00:10:39,920
It had to go somewhere,
though, and we'll

261
00:10:39,920 --> 00:10:42,280
get to where it is
in just a second.

262
00:10:42,280 --> 00:10:44,680
So it might be kind of hard
to conceptualize, how do we

263
00:10:44,680 --> 00:10:48,520
know that there are missing
atoms in all these little cubes

264
00:10:48,520 --> 00:10:49,960
or lattices?

265
00:10:49,960 --> 00:10:52,980
We do have direct evidence.

266
00:10:52,980 --> 00:10:54,650
They're what's called
quenching studies,

267
00:10:54,650 --> 00:10:58,010
where you can measure the
resistance or resistivity

268
00:10:58,010 --> 00:11:00,020
of a piece of material
after heating it

269
00:11:00,020 --> 00:11:01,590
to a certain temperature.

270
00:11:01,590 --> 00:11:04,040
Because it turns out that the
hotter you make something,

271
00:11:04,040 --> 00:11:07,310
the more of those vacancies
just naturally occur.

272
00:11:07,310 --> 00:11:10,610
You won't actually ever
find an absolutely perfect

273
00:11:10,610 --> 00:11:13,460
single crystal
anywhere in nature,

274
00:11:13,460 --> 00:11:15,890
unless you go to zero
Kelvin for infinite time,

275
00:11:15,890 --> 00:11:18,320
then the atoms arrange
themselves thusly.

276
00:11:18,320 --> 00:11:21,770
There's always some amount
of atomic vibration going on.

277
00:11:21,770 --> 00:11:26,120
And there's actually some
thermodynamic energy gain

278
00:11:26,120 --> 00:11:28,910
to having a few defects
in your structure.

279
00:11:28,910 --> 00:11:32,120
And that number of
defects increases

280
00:11:32,120 --> 00:11:33,730
with increasing temperature.

281
00:11:33,730 --> 00:11:35,730
Once you get to the melting
point of a material,

282
00:11:35,730 --> 00:11:38,750
or like right before something
melts, you can have up to 1

283
00:11:38,750 --> 00:11:41,000
in 10,000 atoms just missing.

284
00:11:41,000 --> 00:11:42,590
Moved somewhere else.

285
00:11:42,590 --> 00:11:44,360
We call that the
thermal equilibrium

286
00:11:44,360 --> 00:11:45,940
vacancy concentration.

287
00:11:45,940 --> 00:11:49,420
And we can measure that using
these resistivity measurements,

288
00:11:49,420 --> 00:11:52,640
where you heat materials up to
higher and higher temperatures,

289
00:11:52,640 --> 00:11:54,920
cool them down suddenly
in, like, liquid nitrogen

290
00:11:54,920 --> 00:11:58,820
or liquid helium, and measure
the change in resistivity.

291
00:11:58,820 --> 00:12:01,500
The more defects there are,
the harder it is for electrons

292
00:12:01,500 --> 00:12:02,275
to flow through.

293
00:12:02,275 --> 00:12:04,650
And the only thing that could
really be responsible there

294
00:12:04,650 --> 00:12:09,030
in a single element
would be vacancies.

295
00:12:09,030 --> 00:12:12,170
So we do know that
these really exist.

296
00:12:12,170 --> 00:12:14,030
They can also cluster up.

297
00:12:14,030 --> 00:12:15,710
It turns out that
every time you have

298
00:12:15,710 --> 00:12:17,810
a vacancy in a material,
the other atoms

299
00:12:17,810 --> 00:12:21,170
move in a little bit towards
it, relaxing the pressure they

300
00:12:21,170 --> 00:12:23,270
feel from the atoms nearby.

301
00:12:23,270 --> 00:12:25,340
And one way for a whole
bunch of vacancies

302
00:12:25,340 --> 00:12:28,700
to lower the stress of the
whole atomic configuration

303
00:12:28,700 --> 00:12:30,720
is to cluster together.

304
00:12:30,720 --> 00:12:32,870
So if you have a whole
bunch of vacancies,

305
00:12:32,870 --> 00:12:36,380
they may not allow as
much stress accommodation

306
00:12:36,380 --> 00:12:40,370
as if they were separate,
when they're together.

307
00:12:40,370 --> 00:12:44,257
Now you might ask, what
happened to the original atoms?

308
00:12:44,257 --> 00:12:46,340
You can't just take atoms
away and then go nowhere

309
00:12:46,340 --> 00:12:48,458
because you can't just
destroy matter, right?

310
00:12:48,458 --> 00:12:50,000
Unless you turn it
into energy, which

311
00:12:50,000 --> 00:12:52,070
is what we do in
nuclear engineering.

312
00:12:52,070 --> 00:12:55,010
So in the material
science world,

313
00:12:55,010 --> 00:12:57,410
they end up as what's
called interstitials,

314
00:12:57,410 --> 00:13:00,350
where you kind of have a
vacancy created from somewhere

315
00:13:00,350 --> 00:13:02,210
that knocks that
atom out, and it

316
00:13:02,210 --> 00:13:07,040
gets stuck in the next biggest
space between some other atoms.

317
00:13:07,040 --> 00:13:08,750
And we refer to those
as interstitials.

318
00:13:08,750 --> 00:13:13,610
And those can cluster up, too,
to reduce their total stress

319
00:13:13,610 --> 00:13:15,250
in the lattice.

320
00:13:15,250 --> 00:13:17,740
They can cluster up into
what's called split dumbbell

321
00:13:17,740 --> 00:13:18,940
interstitials.

322
00:13:18,940 --> 00:13:21,820
Instead of having one
extra atom shoved in here,

323
00:13:21,820 --> 00:13:23,470
you might rearrange
a couple so there's

324
00:13:23,470 --> 00:13:26,260
two atoms in the center
of a cube instead of one.

325
00:13:26,260 --> 00:13:29,320
And that tends to be a lower
energy or a more stable

326
00:13:29,320 --> 00:13:32,322
configuration.

327
00:13:32,322 --> 00:13:34,780
So let's look a little bit at
the energetics of these point

328
00:13:34,780 --> 00:13:36,880
defects because
understanding how they move

329
00:13:36,880 --> 00:13:40,803
and why will tell us a lot about
how radiation damage happens.

330
00:13:40,803 --> 00:13:42,220
So it turns out
that interstitials

331
00:13:42,220 --> 00:13:43,990
are very hard to make.

332
00:13:43,990 --> 00:13:45,670
It's really hard
to shove an atom

333
00:13:45,670 --> 00:13:47,300
where it doesn't want to be.

334
00:13:47,300 --> 00:13:52,350
But once you get it there,
it moves very easily.

335
00:13:52,350 --> 00:13:55,480
Let's draw a quick,
simple cubic lattice

336
00:13:55,480 --> 00:13:57,460
to do a little
thought experiment

337
00:13:57,460 --> 00:13:58,930
and explore why that might be.

338
00:14:01,630 --> 00:14:05,590
Let's say I want to shove
an interstitial atom in here

339
00:14:05,590 --> 00:14:07,330
between these other atoms.

340
00:14:07,330 --> 00:14:10,120
Well their electron
clouds are going to repel,

341
00:14:10,120 --> 00:14:12,340
and it's going to push
all the nearby atoms away

342
00:14:12,340 --> 00:14:13,690
by just a little bit.

343
00:14:13,690 --> 00:14:16,600
And these ones might
push the other atoms away

344
00:14:16,600 --> 00:14:21,660
by just a little bit,
stretching out the lattice,

345
00:14:21,660 --> 00:14:23,280
or adding some
compressive stress

346
00:14:23,280 --> 00:14:25,830
wherever that interstitial is.

347
00:14:25,830 --> 00:14:28,255
But then how would it move?

348
00:14:28,255 --> 00:14:29,630
What's the biggest
barrier it has

349
00:14:29,630 --> 00:14:32,660
to overcome to get to the
next adjacent location?

350
00:14:38,753 --> 00:14:40,170
Well, which direction
would it go?

351
00:14:40,170 --> 00:14:42,458
Would it go this way?

352
00:14:42,458 --> 00:14:43,000
Probably not.

353
00:14:43,000 --> 00:14:44,710
There's an atom in the way.

354
00:14:44,710 --> 00:14:48,150
So it's going to find the
path of least resistance

355
00:14:48,150 --> 00:14:51,690
to try to get over here,
because like we've talked about

356
00:14:51,690 --> 00:14:54,120
before, all atoms
are always in motion.

357
00:14:54,120 --> 00:14:55,158
Vibrating.

358
00:14:55,158 --> 00:14:56,700
Some of them will
be energetic enough

359
00:14:56,700 --> 00:14:59,140
to squeeze through
these two atoms

360
00:14:59,140 --> 00:15:00,390
and get over to the next site.

361
00:15:00,390 --> 00:15:03,390
And that turns out to be
a pretty easy process.

362
00:15:03,390 --> 00:15:06,840
We can look at the energy
required for an interstitial

363
00:15:06,840 --> 00:15:07,800
to move.

364
00:15:07,800 --> 00:15:11,520
We notice it's really small
fractions of an electron volt,

365
00:15:11,520 --> 00:15:14,310
whereas creating them takes
two or three electron volts.

366
00:15:14,310 --> 00:15:18,510
In atomic land, that's a
very high energy penalty.

367
00:15:18,510 --> 00:15:19,730
Now let's look at vacancies.

368
00:15:19,730 --> 00:15:21,640
They're quite the opposite.

369
00:15:21,640 --> 00:15:23,850
They're rather easy
to make but they're

370
00:15:23,850 --> 00:15:27,360
very hard to move,
compared to interstitials.

371
00:15:27,360 --> 00:15:28,890
Notice that the
energy of movement

372
00:15:28,890 --> 00:15:32,070
is about the same as the energy
of formation for vacancies.

373
00:15:32,070 --> 00:15:33,930
To take an atom out
or to pluck it out,

374
00:15:33,930 --> 00:15:37,860
you have to break every
bond between nearby atoms.

375
00:15:37,860 --> 00:15:43,310
So you actually have to put
energy in to break those bonds

376
00:15:43,310 --> 00:15:46,550
and then remove the
atoms somewhere else.

377
00:15:46,550 --> 00:15:49,320
Now these things are
usually made in pairs,

378
00:15:49,320 --> 00:15:51,740
so if you think about
how much energy would it

379
00:15:51,740 --> 00:15:55,310
take to cause a single radiation
damage event where you have

380
00:15:55,310 --> 00:15:58,870
one vacancy, which let's say
would have been right here,

381
00:15:58,870 --> 00:16:03,110
and one interstitial, it takes
the sum of these two energies--

382
00:16:03,110 --> 00:16:05,480
usually about four
electron volts.

383
00:16:05,480 --> 00:16:07,880
That's not something
that tends to happen

384
00:16:07,880 --> 00:16:11,360
chemically or from stress
or from something like that.

385
00:16:11,360 --> 00:16:14,330
But radiation coming
in with hundreds of keV

386
00:16:14,330 --> 00:16:17,750
or even MeV neutrons,
anything's on the table

387
00:16:17,750 --> 00:16:20,870
because it's high enough energy.

388
00:16:20,870 --> 00:16:21,490
Yeah?

389
00:16:21,490 --> 00:16:23,660
AUDIENCE: What would take
about three or four eV?

390
00:16:23,660 --> 00:16:25,618
PROFESSOR: So it would
take about three or four

391
00:16:25,618 --> 00:16:28,720
eV to make a pair of a
vacancy and an interstitial.

392
00:16:28,720 --> 00:16:30,220
If you just add these two up.

393
00:16:30,220 --> 00:16:34,110
It comes usually to about three
or four eV, or electron volts.

394
00:16:34,110 --> 00:16:35,590
And that's a very
difficult thing

395
00:16:35,590 --> 00:16:40,000
to do in sort of chemical world,
where reactions might proceed

396
00:16:40,000 --> 00:16:42,070
with fractions of
an electron volt.

397
00:16:42,070 --> 00:16:45,010
But when you have MeV
neutrons coming in,

398
00:16:45,010 --> 00:16:47,110
they do whatever they want.

399
00:16:47,110 --> 00:16:48,955
They'll do whatever they will.

400
00:16:48,955 --> 00:16:50,580
So someone actually
asked me yesterday,

401
00:16:50,580 --> 00:16:54,180
what sort of materials can
you put in the way of neutrons

402
00:16:54,180 --> 00:16:56,370
to stop them from doing damage?

403
00:16:56,370 --> 00:16:59,470
And the answer is,
pretty much nothing.

404
00:16:59,470 --> 00:17:03,210
Fast neutrons tend to travel
about 10 centimeters, even

405
00:17:03,210 --> 00:17:05,910
in things like steel
or water, and they're

406
00:17:05,910 --> 00:17:07,630
going to hit what
they're going to hit.

407
00:17:07,630 --> 00:17:10,980
There's not much you can do
but put more things in the way.

408
00:17:10,980 --> 00:17:13,619
And we can only get to a certain
density with regular matter.

409
00:17:13,619 --> 00:17:17,190
And I think osmium has
upwards of, like, 22 grams

410
00:17:17,190 --> 00:17:18,960
per cubic centimeter density.

411
00:17:18,960 --> 00:17:21,060
That's not enough
to stop neutrons,

412
00:17:21,060 --> 00:17:23,190
even over a
considerable distance.

413
00:17:23,190 --> 00:17:25,470
Unless you had, like,
liquid neutron star,

414
00:17:25,470 --> 00:17:28,807
that you could pack nuclei in
at a way higher number density,

415
00:17:28,807 --> 00:17:29,640
not much you can do.

416
00:17:32,300 --> 00:17:34,850
So moving up in the
dimensions, there's

417
00:17:34,850 --> 00:17:38,030
another type of defect
called a dislocation, where

418
00:17:38,030 --> 00:17:40,940
it's actually energetically
favorable to slide

419
00:17:40,940 --> 00:17:45,920
an extra half-plane of atoms
in between two sets in here

420
00:17:45,920 --> 00:17:49,220
in the crystal lattice, creating
a sort of bulged-out structure

421
00:17:49,220 --> 00:17:51,010
like you see right here.

422
00:17:51,010 --> 00:17:53,930
And dislocations are one of
the most important defects

423
00:17:53,930 --> 00:17:56,210
in material science
and radiation damage.

424
00:17:56,210 --> 00:17:59,120
They're what I like to call
the agents of plasticity.

425
00:17:59,120 --> 00:18:02,150
If you deform a material
enough that it doesn't just

426
00:18:02,150 --> 00:18:07,610
spring back, then most likely,
you were creating and moving

427
00:18:07,610 --> 00:18:09,430
dislocations in the material.

428
00:18:09,430 --> 00:18:11,720
If you think about a
couple of different ways

429
00:18:11,720 --> 00:18:13,850
to cause deformation--

430
00:18:13,850 --> 00:18:15,410
let's bring our
perfect lattice back

431
00:18:15,410 --> 00:18:20,280
without all these
extra notations.

432
00:18:20,280 --> 00:18:25,350
If you want to slide or shear
two planes of atoms across,

433
00:18:25,350 --> 00:18:33,203
and they're all
bonded to each other,

434
00:18:33,203 --> 00:18:34,620
what do you
physically have to do?

435
00:18:40,028 --> 00:18:42,320
How can you get these atoms
to slide across each other?

436
00:18:42,320 --> 00:18:44,870
What sort of energy do
you have to put into it?

437
00:18:47,940 --> 00:18:48,517
Yeah?

438
00:18:48,517 --> 00:18:49,740
AUDIENCE: [INAUDIBLE] energy.

439
00:18:49,740 --> 00:18:50,670
PROFESSOR: Yep.

440
00:18:50,670 --> 00:18:52,890
Because all these atoms
are bonded to each other,

441
00:18:52,890 --> 00:18:54,780
if you want them
to move, you have

442
00:18:54,780 --> 00:18:58,080
to break every
bond on that plane.

443
00:18:58,080 --> 00:19:00,600
That's a lot of
atomic bonds to break

444
00:19:00,600 --> 00:19:03,180
and it's extremely unlikely
that that would happen.

445
00:19:03,180 --> 00:19:06,600
In fact, if you broke
an entire plane of bonds

446
00:19:06,600 --> 00:19:11,164
in some material like this, what
would you physically do to it?

447
00:19:11,164 --> 00:19:13,860
You'd snap it in half.

448
00:19:13,860 --> 00:19:15,590
That would be fracture.

449
00:19:15,590 --> 00:19:18,720
So if you broke every
bond down this plane,

450
00:19:18,720 --> 00:19:21,480
you would then have two
pieces of this fuel rod.

451
00:19:21,480 --> 00:19:24,270
That's usually a pretty
high-energy thing to try to do.

452
00:19:24,270 --> 00:19:29,980
So instead, if you
shove an extra half

453
00:19:29,980 --> 00:19:34,850
plane of atoms in
there, and the bonds

454
00:19:34,850 --> 00:19:40,550
are kind of funny like so,
right at that extra half-plane

455
00:19:40,550 --> 00:19:46,930
location, then what you can
actually do is break one.

456
00:19:46,930 --> 00:19:49,030
Let's say you break this
one, form the next one,

457
00:19:49,030 --> 00:19:51,880
then break this one
and form the next one.

458
00:19:51,880 --> 00:19:54,190
And for a few
atoms to move over,

459
00:19:54,190 --> 00:19:57,970
you only have to break a
line of bonds, not a plane.

460
00:19:57,970 --> 00:20:01,450
So it's much less
energy-intensive to get

461
00:20:01,450 --> 00:20:05,800
a dislocation to move than to
just break something in half.

462
00:20:05,800 --> 00:20:08,810
Now you might ask, well, then
why do things actually break?

463
00:20:08,810 --> 00:20:10,670
Whether or not things
deform or break

464
00:20:10,670 --> 00:20:16,180
is a balance between this
process, which we call slip,

465
00:20:16,180 --> 00:20:21,300
and breaking an entire plane of
atoms, which we call fracture.

466
00:20:21,300 --> 00:20:22,470
So this one's called slip.

467
00:20:25,750 --> 00:20:28,640
The other mode is fracture.

468
00:20:28,640 --> 00:20:32,600
We would rather materials to
form in systems like reactors

469
00:20:32,600 --> 00:20:34,280
by slip, just
moving a little bit,

470
00:20:34,280 --> 00:20:36,200
then just breaking altogether.

471
00:20:36,200 --> 00:20:39,200
Unfortunately, when enough
radiation hits materials,

472
00:20:39,200 --> 00:20:41,390
you can fracture things
in a brutal manner,

473
00:20:41,390 --> 00:20:43,920
and we'll see what happens then.

474
00:20:43,920 --> 00:20:46,950
There's a couple
kinds of dislocations.

475
00:20:46,950 --> 00:20:49,480
One of them is called
a screw dislocation.

476
00:20:49,480 --> 00:20:52,080
So imagine you had a whole
bunch of sheets of atoms,

477
00:20:52,080 --> 00:20:54,630
and you made a cut
halfway through that sheet

478
00:20:54,630 --> 00:20:58,080
and then moved every
plane up by one position.

479
00:20:58,080 --> 00:21:00,150
You then got what's called
a screw dislocation--

480
00:21:00,150 --> 00:21:04,080
kind of a spiral parking
garage of atoms surrounding

481
00:21:04,080 --> 00:21:05,993
that core right there.

482
00:21:05,993 --> 00:21:08,410
You can also have what's called
an edge dislocation, which

483
00:21:08,410 --> 00:21:11,110
is like the one I've
got here on the board

484
00:21:11,110 --> 00:21:14,170
right here, where you just have
an extra half plane of atoms

485
00:21:14,170 --> 00:21:16,420
shoved in right there.

486
00:21:16,420 --> 00:21:20,120
So there's two types, and they
move in two different ways.

487
00:21:20,120 --> 00:21:24,100
The edge dislocation behaves
like you may physically expect.

488
00:21:24,100 --> 00:21:28,270
If you kind of push like
this on two planes of atoms,

489
00:21:28,270 --> 00:21:30,710
it moves in the
direction you push it.

490
00:21:30,710 --> 00:21:33,170
Screw dislocations
are kind of screwy.

491
00:21:33,170 --> 00:21:38,225
If you push like this,
it moves perpendicular.

492
00:21:38,225 --> 00:21:40,100
Not going to get into
why, but just remember,

493
00:21:40,100 --> 00:21:42,200
screw dislocations are
fairly screwy in the way

494
00:21:42,200 --> 00:21:44,060
that they behave.

495
00:21:44,060 --> 00:21:45,740
Not quite intuitive.

496
00:21:45,740 --> 00:21:46,410
But that's OK.

497
00:21:46,410 --> 00:21:49,540
We don't have to
worry about those.

498
00:21:49,540 --> 00:21:51,400
And the way that
they actually move,

499
00:21:51,400 --> 00:21:54,550
like we showed right here, is
by what's called glide, or slip,

500
00:21:54,550 --> 00:21:59,680
where dislocations can slide
just by one plane of atoms

501
00:21:59,680 --> 00:22:02,920
or one atomic position
in a mechanism that

502
00:22:02,920 --> 00:22:04,880
looks something like this.

503
00:22:04,880 --> 00:22:06,970
Where, as that
dislocation moves,

504
00:22:06,970 --> 00:22:08,890
you only have to
break a line of bonds

505
00:22:08,890 --> 00:22:10,810
and then reform a
line of bonds, which

506
00:22:10,810 --> 00:22:12,460
is a much easier
process than breaking

507
00:22:12,460 --> 00:22:14,458
an entire plane at once.

508
00:22:14,458 --> 00:22:16,250
It's like you have to
break the square root

509
00:22:16,250 --> 00:22:17,417
of the same number of bonds.

510
00:22:20,545 --> 00:22:22,420
I'm going to skip ahead
through some of that.

511
00:22:22,420 --> 00:22:24,820
There's one other mechanism
of dislocation movement

512
00:22:24,820 --> 00:22:27,340
that's important to
us in radiation damage

513
00:22:27,340 --> 00:22:28,847
and that's called climb.

514
00:22:28,847 --> 00:22:30,430
This is when you
start to think about,

515
00:22:30,430 --> 00:22:33,910
what happens if you have
a dislocation, which we'll

516
00:22:33,910 --> 00:22:40,300
give this symbol right here,
and you also have a vacancy,

517
00:22:40,300 --> 00:22:42,910
let's say created
by radiation damage.

518
00:22:42,910 --> 00:22:44,590
If that vacancy can
move, it's going

519
00:22:44,590 --> 00:22:47,650
to find the most stressed-out
part of this lattice.

520
00:22:47,650 --> 00:22:51,710
Most likely, the
vacancy will move here.

521
00:22:51,710 --> 00:22:56,550
In other words, the atom
will move over there,

522
00:22:56,550 --> 00:22:59,472
leaving this vacancy over there.

523
00:22:59,472 --> 00:23:01,430
It's kind of funny to
think, like, what does it

524
00:23:01,430 --> 00:23:03,168
mean that a vacancy moves?

525
00:23:03,168 --> 00:23:05,210
Has anyone ever done
anything with semiconductors

526
00:23:05,210 --> 00:23:08,280
and talked about electron
and hole movement?

527
00:23:08,280 --> 00:23:08,790
OK, yeah.

528
00:23:08,790 --> 00:23:12,060
So what does it really mean
for a hole to move, right?

529
00:23:12,060 --> 00:23:13,240
A hole's not a thing.

530
00:23:13,240 --> 00:23:14,590
A vacancy's also not a thing.

531
00:23:14,590 --> 00:23:16,660
It's an absence of an atom.

532
00:23:16,660 --> 00:23:18,690
But here, we can say
that the vacancy moves

533
00:23:18,690 --> 00:23:21,660
in this direction when the
corresponding atom moves

534
00:23:21,660 --> 00:23:23,760
in the exact opposite direction.

535
00:23:23,760 --> 00:23:25,650
And then what
you've actually done

536
00:23:25,650 --> 00:23:29,240
is moved your dislocation up.

537
00:23:29,240 --> 00:23:31,790
Instead of moving in
the slip direction,

538
00:23:31,790 --> 00:23:36,040
you've now moved it in a
perpendicular direction.

539
00:23:36,040 --> 00:23:38,980
This is usually not
possible without things

540
00:23:38,980 --> 00:23:41,518
like radiation damage or
very high temperature.

541
00:23:43,907 --> 00:23:45,490
And then, to make
things even crazier,

542
00:23:45,490 --> 00:23:47,060
you can also have
what's called loops

543
00:23:47,060 --> 00:23:49,143
of dislocation, some videos
of which I'll actually

544
00:23:49,143 --> 00:23:50,360
get to show you.

545
00:23:50,360 --> 00:23:53,690
You can have a dislocation that
has part edge character, part

546
00:23:53,690 --> 00:23:54,732
screw character.

547
00:23:54,732 --> 00:23:56,690
If you look at how the
atoms are arranged here,

548
00:23:56,690 --> 00:23:59,037
you're looking from
sort of the top-down.

549
00:23:59,037 --> 00:24:00,620
You can see that
there's an extra half

550
00:24:00,620 --> 00:24:04,040
plane of these white atoms
shoved in in the black ones,

551
00:24:04,040 --> 00:24:08,210
and this right here would be
a completely edge dislocation.

552
00:24:08,210 --> 00:24:09,950
You can have a
gradual transition,

553
00:24:09,950 --> 00:24:13,640
where about 90 degrees
later, it looks like a spiral

554
00:24:13,640 --> 00:24:15,840
and that's a screw dislocation.

555
00:24:15,840 --> 00:24:17,840
And the net effect
of that is when

556
00:24:17,840 --> 00:24:20,600
you push in this direction
on an edge dislocation,

557
00:24:20,600 --> 00:24:22,370
it moves that way.

558
00:24:22,370 --> 00:24:24,750
When you push this direction
on a screw dislocation,

559
00:24:24,750 --> 00:24:27,520
it moves that way.

560
00:24:27,520 --> 00:24:29,780
So when you stress out
a dislocation loop,

561
00:24:29,780 --> 00:24:31,600
it just grows.

562
00:24:31,600 --> 00:24:34,240
You're not actually creating
or destroying matter,

563
00:24:34,240 --> 00:24:38,170
but what you're doing is causing
this small loop of extra half

564
00:24:38,170 --> 00:24:40,840
plane of atoms to grow further
and further until it actually

565
00:24:40,840 --> 00:24:46,000
reaches some obstacle or
the outside of a crystal.

566
00:24:46,000 --> 00:24:48,400
And these dislocations
can actually

567
00:24:48,400 --> 00:24:50,800
feel the force from each other.

568
00:24:50,800 --> 00:24:54,220
If I draw a clean one because I
think it'll be easier to see--

569
00:24:56,770 --> 00:24:58,580
if I draw a small
lattice of atoms

570
00:24:58,580 --> 00:25:07,540
here and then a dislocation
core right there.

571
00:25:07,540 --> 00:25:09,630
So that's our dislocation core.

572
00:25:09,630 --> 00:25:15,350
This region of space right
here is compressively stressed.

573
00:25:15,350 --> 00:25:18,270
There's more atoms in that
space than there want to be

574
00:25:18,270 --> 00:25:20,060
and so it's kind of
crammed in there.

575
00:25:20,060 --> 00:25:24,250
While this region
right here is in what's

576
00:25:24,250 --> 00:25:26,080
called tensile stress.

577
00:25:26,080 --> 00:25:29,230
There's almost some space,
like right here, where

578
00:25:29,230 --> 00:25:33,220
there's too few atoms and they
kind of want there to be more.

579
00:25:33,220 --> 00:25:37,180
And these dislocations can
feel neighboring stress fields.

580
00:25:37,180 --> 00:25:39,820
Let's say there was
another one right over here

581
00:25:39,820 --> 00:25:43,500
that had its own
compressive stress field.

582
00:25:43,500 --> 00:25:46,300
They'll actually
repel each other

583
00:25:46,300 --> 00:25:49,120
because you don't want to add
even more compressive stress

584
00:25:49,120 --> 00:25:51,160
to anywhere in this
group of atoms.

585
00:25:51,160 --> 00:25:53,447
So they'll actually repel
each other to the point

586
00:25:53,447 --> 00:25:55,030
where, if you get
two dislocations too

587
00:25:55,030 --> 00:25:59,630
close to each other, they'll
what's called pile-up,

588
00:25:59,630 --> 00:26:02,113
or they'll refuse to move a bit.

589
00:26:02,113 --> 00:26:03,530
So I want to show
you some videos.

590
00:26:03,530 --> 00:26:05,540
We can actually see
these dislocations.

591
00:26:05,540 --> 00:26:09,200
In this one, you see that faint
line right there originating

592
00:26:09,200 --> 00:26:10,940
from this area?

593
00:26:10,940 --> 00:26:14,270
That's actually a
dislocation loop under stress

594
00:26:14,270 --> 00:26:15,600
and that's actually growing.

595
00:26:15,600 --> 00:26:17,930
So what you're seeing
here is an image

596
00:26:17,930 --> 00:26:20,450
of electrons passing
through material

597
00:26:20,450 --> 00:26:22,940
and looking at regions
of different contrast.

598
00:26:22,940 --> 00:26:25,010
So wherever there is more
atoms or fewer atoms,

599
00:26:25,010 --> 00:26:28,190
it looks darker or lighter,
and that can tell you

600
00:26:28,190 --> 00:26:29,510
what sort of defects there are.

601
00:26:29,510 --> 00:26:32,152
You guys all see that
faint line right there?

602
00:26:32,152 --> 00:26:33,610
Notice how the
loop's just growing.

603
00:26:33,610 --> 00:26:35,068
It's not like you're
moving a line,

604
00:26:35,068 --> 00:26:36,730
but you're literally
growing a line out

605
00:26:36,730 --> 00:26:37,855
of what looks like nothing.

606
00:26:41,320 --> 00:26:44,110
There's another one we
call a Frank-Read source.

607
00:26:44,110 --> 00:26:46,510
It's a source of
dislocation loop.

608
00:26:46,510 --> 00:26:49,030
So what you're seeing
here, each of these lines

609
00:26:49,030 --> 00:26:50,290
is a single dislocation.

610
00:26:50,290 --> 00:26:54,520
And then right there, you
see that loop suddenly form?

611
00:26:54,520 --> 00:26:56,110
Let's show you that one again.

612
00:26:56,110 --> 00:26:58,987
I'll point on where to look.

613
00:26:58,987 --> 00:27:00,820
By stressing out
materials, you can actually

614
00:27:00,820 --> 00:27:05,161
create additional dislocation
loops, right around here.

615
00:27:05,161 --> 00:27:07,000
And there it is.

616
00:27:07,000 --> 00:27:08,410
You guys see that one?

617
00:27:08,410 --> 00:27:08,923
Yeah.

618
00:27:08,923 --> 00:27:10,840
Out of what looks like
nothing but is actually

619
00:27:10,840 --> 00:27:12,970
just a couple of
atomic defects, you

620
00:27:12,970 --> 00:27:15,910
can create a dislocation
loop and allow more plastic

621
00:27:15,910 --> 00:27:21,355
deformation to take place,
which I think is awesome.

622
00:27:21,355 --> 00:27:22,150
Look at this one.

623
00:27:22,150 --> 00:27:24,160
Another dislocation
source in germanium.

624
00:27:24,160 --> 00:27:26,140
It's a little
easier to see, also

625
00:27:26,140 --> 00:27:29,800
because it's making this sort
of spiral set of dislocations

626
00:27:29,800 --> 00:27:31,330
a little slower.

627
00:27:31,330 --> 00:27:34,500
So you can track its
motion a little easier.

628
00:27:34,500 --> 00:27:38,330
Notice how they all kind of line
up on certain atomic planes.

629
00:27:38,330 --> 00:27:38,830
Yeah?

630
00:27:38,830 --> 00:27:41,163
AUDIENCE: Does the topology
of these things ever change,

631
00:27:41,163 --> 00:27:44,150
or is it always just
a slow [INAUDIBLE]

632
00:27:44,150 --> 00:27:45,877
PROFESSOR: The
topology will change.

633
00:27:45,877 --> 00:27:47,460
Let's say, if it
hits another obstacle

634
00:27:47,460 --> 00:27:48,877
or another
dislocation, yeah, they

635
00:27:48,877 --> 00:27:51,510
can slam into each other
and change topology.

636
00:27:51,510 --> 00:27:54,390
AUDIENCE: Breaking
too [INAUDIBLE]

637
00:27:54,390 --> 00:27:56,447
PROFESSOR: All sorts
of things, yeah.

638
00:27:56,447 --> 00:27:58,530
That's a subject for a
whole other class, I'd say.

639
00:28:01,230 --> 00:28:03,570
I want to skip
ahead to the pile-up

640
00:28:03,570 --> 00:28:07,140
because I think this kind
of gets the point across.

641
00:28:07,140 --> 00:28:09,330
But actually, we can
see direct evidence

642
00:28:09,330 --> 00:28:11,970
that dislocations feel
each other's stress fields.

643
00:28:11,970 --> 00:28:15,000
When you get enough of them
lined up, they won't overlap.

644
00:28:15,000 --> 00:28:18,180
They actually push each other
in a kind of dislocation traffic

645
00:28:18,180 --> 00:28:20,280
jam.

646
00:28:20,280 --> 00:28:22,740
Because what's happening
on the atomic level is,

647
00:28:22,740 --> 00:28:24,300
they feel each
other's stress fields.

648
00:28:24,300 --> 00:28:26,945
There might be a source of
dislocations further away,

649
00:28:26,945 --> 00:28:28,950
but when they get too
close to each other,

650
00:28:28,950 --> 00:28:31,500
it literally is a
dislocation traffic jam.

651
00:28:31,500 --> 00:28:33,660
I mean, if you try and hit
the car in front of you,

652
00:28:33,660 --> 00:28:36,900
the repulsion of the electrons
between your and their bumper

653
00:28:36,900 --> 00:28:39,900
will prevent the cars from
getting a certain distance

654
00:28:39,900 --> 00:28:41,370
closer to each other.

655
00:28:41,370 --> 00:28:42,390
Same kind of thing here.

656
00:28:45,890 --> 00:28:49,100
Moving onto grain boundaries,
a two-dimensional defect.

657
00:28:49,100 --> 00:28:51,950
Any time you have
a perfect crystal

658
00:28:51,950 --> 00:28:54,980
of atoms that meets
another perfect crystal

659
00:28:54,980 --> 00:28:57,110
at a different orientation,
or where the atoms

660
00:28:57,110 --> 00:28:58,880
are arranged in a
different direction,

661
00:28:58,880 --> 00:29:00,920
you end up with a
boundary between them

662
00:29:00,920 --> 00:29:03,990
that we refer to as
a grain boundary.

663
00:29:03,990 --> 00:29:06,950
So you can actually see, this
is a direct physical image

664
00:29:06,950 --> 00:29:10,280
of atoms of two different
crystals meaning at the grain

665
00:29:10,280 --> 00:29:11,120
boundary.

666
00:29:11,120 --> 00:29:13,430
Again, taken in the transmission
electron microscope.

667
00:29:13,430 --> 00:29:14,847
So for those who
didn't know, yes,

668
00:29:14,847 --> 00:29:19,085
we can see individual atoms
and the defects between them.

669
00:29:19,085 --> 00:29:20,960
I definitely didn't know
that in high school.

670
00:29:20,960 --> 00:29:22,668
They didn't even
mention that whatsoever.

671
00:29:22,668 --> 00:29:25,360
Did you guys ever
see images like this?

672
00:29:25,360 --> 00:29:25,990
Anyone?

673
00:29:25,990 --> 00:29:26,490
Yes?

674
00:29:26,490 --> 00:29:27,760
Raise your hand.

675
00:29:27,760 --> 00:29:29,140
Just one, OK.

676
00:29:29,140 --> 00:29:29,830
So yeah.

677
00:29:29,830 --> 00:29:31,060
It's important for
you guys to know

678
00:29:31,060 --> 00:29:33,643
that we can have direct evidence
for all this blackboard stuff

679
00:29:33,643 --> 00:29:36,460
because you can see atoms
in the transmission electron

680
00:29:36,460 --> 00:29:39,880
microscope and see what happens
when the two of them meet.

681
00:29:39,880 --> 00:29:43,240
You see this kind of regular
structure of empty space

682
00:29:43,240 --> 00:29:46,280
where this grain
boundary meets, right?

683
00:29:46,280 --> 00:29:49,830
You can actually model it as
a line of 1-D dislocations,

684
00:29:49,830 --> 00:29:52,640
because if you take
a line of 1-D lines,

685
00:29:52,640 --> 00:29:54,830
you end up with a
2-D boundary, which

686
00:29:54,830 --> 00:29:56,130
you can see very clearly here.

687
00:29:56,130 --> 00:29:58,505
It's almost like there's an
extra half plane right there.

688
00:29:58,505 --> 00:30:02,420
Another one there, another one
there, and another one there.

689
00:30:02,420 --> 00:30:05,190
And we call that a
tilt grain boundary.

690
00:30:05,190 --> 00:30:08,010
Grain boundaries are nice in
that they can accommodate lots

691
00:30:08,010 --> 00:30:10,860
of these little
zero-dimensional defects,

692
00:30:10,860 --> 00:30:13,890
moving to them without
getting destroyed.

693
00:30:13,890 --> 00:30:15,630
So grain boundaries
are one of those ways

694
00:30:15,630 --> 00:30:19,140
that radiation damage
can be removed.

695
00:30:19,140 --> 00:30:21,780
And that's one of the reasons
why most small-grain materials

696
00:30:21,780 --> 00:30:22,560
are really--

697
00:30:22,560 --> 00:30:25,980
nano-grain materials are more
resistant to radiation damage

698
00:30:25,980 --> 00:30:28,020
than large-grain
ones because they

699
00:30:28,020 --> 00:30:33,390
act as what's called sinks or
destroyers of radiation damage.

700
00:30:33,390 --> 00:30:37,860
There's another kind of 2-D
defect called a twin, where

701
00:30:37,860 --> 00:30:41,700
you can actually get a little
chunk of atoms sort of switch

702
00:30:41,700 --> 00:30:42,630
orientation.

703
00:30:42,630 --> 00:30:45,900
And you can see these very
clearly in, again, TEM

704
00:30:45,900 --> 00:30:50,760
micrographs, and the evidence
actually that the twin actually

705
00:30:50,760 --> 00:30:52,680
is a different physical
arrangement of atoms,

706
00:30:52,680 --> 00:30:56,430
even though you can't see
the atoms in this little band

707
00:30:56,430 --> 00:30:57,780
right there.

708
00:30:57,780 --> 00:31:00,540
Look at the way the
dislocations line up.

709
00:31:00,540 --> 00:31:02,190
Those dislocations
tend to line up

710
00:31:02,190 --> 00:31:05,190
in energetically-favorable
directions, and in this grain,

711
00:31:05,190 --> 00:31:08,280
they're all this
way, and in the twin,

712
00:31:08,280 --> 00:31:13,013
they're all lined up like that.

713
00:31:13,013 --> 00:31:15,180
And then finally, there's
the most intuitive defect,

714
00:31:15,180 --> 00:31:16,380
inclusions.

715
00:31:16,380 --> 00:31:19,740
A 3-D piece of some other
material inside what

716
00:31:19,740 --> 00:31:21,880
would otherwise be
a pure material.

717
00:31:21,880 --> 00:31:25,530
This one, I actually pulled
out of the rotor that powers

718
00:31:25,530 --> 00:31:27,240
the Alcator fusion reactor.

719
00:31:27,240 --> 00:31:29,790
I was asked to do some
analysis to find out,

720
00:31:29,790 --> 00:31:32,670
is the structure of
that rotor changing,

721
00:31:32,670 --> 00:31:36,622
because General Electric who
was insuring this rotor said,

722
00:31:36,622 --> 00:31:38,080
we don't want to
insure it anymore.

723
00:31:38,080 --> 00:31:41,400
Thanks for the premiums, but
we're not insuring it anymore.

724
00:31:41,400 --> 00:31:42,400
And we said, why?

725
00:31:42,400 --> 00:31:44,400
And they said, oh, it's
structurally unsound.

726
00:31:44,400 --> 00:31:45,840
So we said, oh yeah?

727
00:31:45,840 --> 00:31:48,150
We'll be back in a year
and we'll talk about it.

728
00:31:48,150 --> 00:31:50,315
And we did a lot of
this work to find out

729
00:31:50,315 --> 00:31:52,440
that, actually, the structure
hadn't really changed

730
00:31:52,440 --> 00:31:55,020
since 1954 when it was made.

731
00:31:55,020 --> 00:31:57,600
But what we did also
see is we could pop out

732
00:31:57,600 --> 00:32:01,320
little precipitates
of manganese sulfide.

733
00:32:01,320 --> 00:32:03,750
So there's always
sulfur in iron,

734
00:32:03,750 --> 00:32:05,550
and sulfur tends to
be a bad actor when it

735
00:32:05,550 --> 00:32:07,590
comes to material properties.

736
00:32:07,590 --> 00:32:10,170
You throw manganese
into iron to scoop up

737
00:32:10,170 --> 00:32:13,050
that sulfur in the form of
these little precipitates

738
00:32:13,050 --> 00:32:16,170
or inclusions, which we
were able to see perfectly

739
00:32:16,170 --> 00:32:18,150
when we did an x-ray
map, just like the one we

740
00:32:18,150 --> 00:32:20,400
did after the first exam.

741
00:32:20,400 --> 00:32:23,910
It's like we were looking at
Chris' copper silver alloy,

742
00:32:23,910 --> 00:32:25,947
mapping out where is
the copper and silver.

743
00:32:25,947 --> 00:32:27,780
I made this image the
same way, mapping out,

744
00:32:27,780 --> 00:32:29,682
where is there iron,
manganese and sulfur.

745
00:32:29,682 --> 00:32:31,140
That's how you can
tell what it is.

746
00:32:34,430 --> 00:32:37,160
And so dislocations and
defects can actually interact.

747
00:32:37,160 --> 00:32:38,540
Let's say this is
the interaction

748
00:32:38,540 --> 00:32:45,200
of a 1-D defect, a dislocation,
with a 3-D defect, a void.

749
00:32:45,200 --> 00:32:47,840
If you have a material
that's deforming plastically,

750
00:32:47,840 --> 00:32:50,330
very smoothly, and isn't
going to undergo fracture,

751
00:32:50,330 --> 00:32:52,910
you want the dislocations
to be able to move.

752
00:32:52,910 --> 00:32:56,990
If you put anything in their
way, they tend to get stuck.

753
00:32:56,990 --> 00:32:58,850
It's not easy for
that dislocation

754
00:32:58,850 --> 00:33:01,260
to shear through a whole
bunch of extra atoms.

755
00:33:01,260 --> 00:33:06,230
And in some cases, you can
stop that motion and favor

756
00:33:06,230 --> 00:33:09,940
fracture over slip.

757
00:33:09,940 --> 00:33:12,370
So any time you
make slip harder,

758
00:33:12,370 --> 00:33:14,668
it means that you're making
fracture more likely.

759
00:33:14,668 --> 00:33:16,210
I didn't say you're
making it easier,

760
00:33:16,210 --> 00:33:18,070
but you're making
it more likely.

761
00:33:18,070 --> 00:33:19,570
And you would
prefer for materials

762
00:33:19,570 --> 00:33:25,940
to deform a little bit by a slip
than just break by fracture.

763
00:33:25,940 --> 00:33:29,320
So I think now is a good point
to go over a few key material

764
00:33:29,320 --> 00:33:31,110
properties.

765
00:33:31,110 --> 00:33:35,040
All of these are sometimes
used to describe the same thing

766
00:33:35,040 --> 00:33:36,750
in colloquial speech.

767
00:33:36,750 --> 00:33:39,253
That is wrong.

768
00:33:39,253 --> 00:33:40,920
Has anyone here thought
that, let's say,

769
00:33:40,920 --> 00:33:45,600
stiffness or toughness or
strength meant the same thing?

770
00:33:45,600 --> 00:33:46,100
No.

771
00:33:46,100 --> 00:33:46,930
OK.

772
00:33:46,930 --> 00:33:47,570
A few people.

773
00:33:47,570 --> 00:33:48,500
It's OK.

774
00:33:48,500 --> 00:33:52,010
Because it's used wrong all
the time in colloquial speech.

775
00:33:52,010 --> 00:33:54,470
These actually refer to
different material properties

776
00:33:54,470 --> 00:33:55,568
with different units.

777
00:33:55,568 --> 00:33:57,860
And we're going to go into
a little bit about what they

778
00:33:57,860 --> 00:34:00,680
are and then show
you a few videos

779
00:34:00,680 --> 00:34:04,610
to test your intuition about
the differences between them.

780
00:34:04,610 --> 00:34:07,690
So first, I want to mention
what you're seeing right here.

781
00:34:07,690 --> 00:34:10,239
It's called a
stress-strain curve.

782
00:34:10,239 --> 00:34:11,739
Stress is simple.

783
00:34:11,739 --> 00:34:13,960
Stress is just a force
divided by an area.

784
00:34:17,920 --> 00:34:19,690
And usually, the
criterion for will

785
00:34:19,690 --> 00:34:22,030
a material deform
or will it break

786
00:34:22,030 --> 00:34:24,340
is does it reach
a certain stress.

787
00:34:24,340 --> 00:34:26,630
It doesn't matter just how
much force you put on it,

788
00:34:26,630 --> 00:34:28,480
but it's like, how
much force per atom

789
00:34:28,480 --> 00:34:31,300
or how much force per area
determines whether bonds

790
00:34:31,300 --> 00:34:32,860
are going to break.

791
00:34:32,860 --> 00:34:34,449
And so on the y-axis is stress.

792
00:34:34,449 --> 00:34:38,530
Let's say the amount of force
per area we're putting in.

793
00:34:38,530 --> 00:34:43,449
And strain is the
amount of deformation.

794
00:34:43,449 --> 00:34:44,420
So that's stress.

795
00:34:44,420 --> 00:34:49,100
And strain is, let's
say, the change in length

796
00:34:49,100 --> 00:34:52,460
over the original length
of some material in what's

797
00:34:52,460 --> 00:34:55,449
called the engineering
or simplified notation.

798
00:34:55,449 --> 00:34:57,740
And so something
that is stiff means

799
00:34:57,740 --> 00:35:00,360
you can put a lot
of force into it

800
00:35:00,360 --> 00:35:01,760
but it won't deform very much.

801
00:35:01,760 --> 00:35:04,295
That's kind of the easiest
property to understand.

802
00:35:04,295 --> 00:35:06,680
Is something that's
very stiff will

803
00:35:06,680 --> 00:35:08,540
have what's called a
high Young's modulus,

804
00:35:08,540 --> 00:35:11,480
or a high slope right here.

805
00:35:11,480 --> 00:35:13,940
Something that's super
stiff, like a ceramic,

806
00:35:13,940 --> 00:35:17,070
you could really push
on it quite a bit,

807
00:35:17,070 --> 00:35:20,660
but you won't get it to deform
like you would this metal.

808
00:35:20,660 --> 00:35:23,720
So the opposite of stiff,
I would call compliant.

809
00:35:23,720 --> 00:35:25,160
Not soft.

810
00:35:25,160 --> 00:35:27,140
This is one of those
tricky things right there.

811
00:35:27,140 --> 00:35:30,410
Something that's stiff, you try
and flex it and it won't flex.

812
00:35:30,410 --> 00:35:33,020
Something that's compliant,
you put a little bit of force

813
00:35:33,020 --> 00:35:37,840
into it and it undergoes
some amount of strain.

814
00:35:37,840 --> 00:35:40,430
And that slope right
there between the stress

815
00:35:40,430 --> 00:35:44,300
and the strain, we call
the Young's modulus.

816
00:35:44,300 --> 00:35:46,340
We also note that
this part right here

817
00:35:46,340 --> 00:35:49,790
is what's called the elastic
region of deformation.

818
00:35:49,790 --> 00:35:53,050
By elastic, we mean reversible,
or it snaps right back.

819
00:35:53,050 --> 00:35:57,290
So right here, when I bend this
bar and it snaps right back,

820
00:35:57,290 --> 00:35:59,030
that's called
elastic deformation.

821
00:35:59,030 --> 00:36:02,090
And it's reversible,
because you can bend one way

822
00:36:02,090 --> 00:36:03,650
and it snaps right back.

823
00:36:03,650 --> 00:36:05,720
If I bent it more,
which I don't want

824
00:36:05,720 --> 00:36:09,290
to do because this is a nice
zirconium fuel cladding rod,

825
00:36:09,290 --> 00:36:10,880
you would deform
it irreversibly.

826
00:36:10,880 --> 00:36:12,950
You'd bend it permanently.

827
00:36:12,950 --> 00:36:16,250
And to undergo what's called
plastic deformation, when

828
00:36:16,250 --> 00:36:19,790
you deviate from the slope, and
then a little bit more stress

829
00:36:19,790 --> 00:36:21,880
can cause a lot
more deformation.

830
00:36:21,880 --> 00:36:24,170
Have any of you guys ever
tried pulling copper wire

831
00:36:24,170 --> 00:36:24,830
apart before?

832
00:36:27,312 --> 00:36:29,520
That's something I'd recommend
you try, for thin wire

833
00:36:29,520 --> 00:36:31,250
so you don't cut your hands.

834
00:36:31,250 --> 00:36:34,220
What you may notice is that
it's awfully hard to get

835
00:36:34,220 --> 00:36:36,740
the copper deforming
in the first place,

836
00:36:36,740 --> 00:36:40,165
but as soon as it starts to
stretch, it gets really easy.

837
00:36:40,165 --> 00:36:41,540
So this is something
I recommend.

838
00:36:41,540 --> 00:36:43,460
Go to the electronics
shop or wherever

839
00:36:43,460 --> 00:36:45,960
and try it out on some
really thin copper wire.

840
00:36:45,960 --> 00:36:47,960
If it's thick, you'll
slice through your fingers

841
00:36:47,960 --> 00:36:50,350
and you don't want to do that.

842
00:36:50,350 --> 00:36:54,090
Strength, however, that's
a different metric.

843
00:36:54,090 --> 00:36:57,070
Whereas stiffness
describes the slope here,

844
00:36:57,070 --> 00:37:01,450
strength describes the height,
or the stress at which you

845
00:37:01,450 --> 00:37:03,130
start to plastically deform.

846
00:37:03,130 --> 00:37:05,420
They're in different units.

847
00:37:05,420 --> 00:37:08,540
Stiffness is in
stress over strain,

848
00:37:08,540 --> 00:37:12,870
whereas strength is
given as a stress.

849
00:37:12,870 --> 00:37:14,610
So when you hear
things like the yield

850
00:37:14,610 --> 00:37:16,440
stress or the ultimate
tensile strength,

851
00:37:16,440 --> 00:37:18,810
that's referring to how
strong something is,

852
00:37:18,810 --> 00:37:22,010
which may have nothing to
do with how stiff it is.

853
00:37:22,010 --> 00:37:24,560
Toughness is another property.

854
00:37:24,560 --> 00:37:28,760
Toughness is actually kind of
like the area under this curve,

855
00:37:28,760 --> 00:37:32,720
because if you do a force
and apply it over a distance,

856
00:37:32,720 --> 00:37:35,000
that's like putting
work into the material

857
00:37:35,000 --> 00:37:37,720
and it ends up being
a unit of energy.

858
00:37:37,720 --> 00:37:39,630
So toughness will tell
you how much energy

859
00:37:39,630 --> 00:37:42,210
you have to put into
something before creating

860
00:37:42,210 --> 00:37:45,930
a new free surface,
otherwise known as fracture.

861
00:37:45,930 --> 00:37:49,940
And ductility is how much can
you deform it before it breaks.

862
00:37:49,940 --> 00:37:54,800
So it would be like this point
right here on the strain axis.

863
00:37:54,800 --> 00:37:56,480
So I'll give a little
bit more examples

864
00:37:56,480 --> 00:37:59,820
of what this is all about.

865
00:37:59,820 --> 00:38:02,160
Toughness, again,
is actually measured

866
00:38:02,160 --> 00:38:06,150
as an energy required
to form a free surface,

867
00:38:06,150 --> 00:38:08,970
or propagate a crack, let's say.

868
00:38:08,970 --> 00:38:10,877
Whereas something
that's ductile,

869
00:38:10,877 --> 00:38:12,710
it doesn't necessarily
mean that it's tough.

870
00:38:12,710 --> 00:38:15,350
Like, if you have a piece
of chewed chewing gum,

871
00:38:15,350 --> 00:38:18,410
you can stretch it quite a
lot with very little energy.

872
00:38:18,410 --> 00:38:21,110
And then you can say it's
extremely ductile but not

873
00:38:21,110 --> 00:38:22,650
very strong.

874
00:38:22,650 --> 00:38:24,830
A piece of copper
wire, you can also

875
00:38:24,830 --> 00:38:27,320
stretch it an
extremely far distance,

876
00:38:27,320 --> 00:38:29,360
but it takes more
energy to do so.

877
00:38:29,360 --> 00:38:32,660
So that's both
ductile and strong.

878
00:38:32,660 --> 00:38:35,150
And then if you apply that
force over a certain distance,

879
00:38:35,150 --> 00:38:37,400
stretching out the
wire, you can also

880
00:38:37,400 --> 00:38:39,590
reveal some of its toughness
and how much energy

881
00:38:39,590 --> 00:38:44,170
it takes to stretch that
wire before it breaks.

882
00:38:44,170 --> 00:38:46,820
Hardness is the last material
property I want to mention,

883
00:38:46,820 --> 00:38:50,200
which is not any of
the ones that I showed

884
00:38:50,200 --> 00:38:52,380
on the stress-strain curve.

885
00:38:52,380 --> 00:38:54,240
Hardness is the
resistance to a little bit

886
00:38:54,240 --> 00:38:56,160
of plastic deformation.

887
00:38:56,160 --> 00:38:59,010
So assuming that
you're already here,

888
00:38:59,010 --> 00:39:00,810
how much more energy
do you have to put in

889
00:39:00,810 --> 00:39:02,455
to get the material
to deform plastic?

890
00:39:05,460 --> 00:39:07,050
So very different
material properties.

891
00:39:07,050 --> 00:39:11,320
I'll try and mention
all what they are.

892
00:39:11,320 --> 00:39:16,660
So if we have a
stress-strain curve like so,

893
00:39:16,660 --> 00:39:19,150
and it follows
the elastic region

894
00:39:19,150 --> 00:39:22,480
and then deforms
plastically, this point

895
00:39:22,480 --> 00:39:24,730
here is what we call
the yield strength.

896
00:39:31,300 --> 00:39:35,200
Whatever that point
on the stress axis is.

897
00:39:35,200 --> 00:39:39,340
This point right here,
our strain to failure,

898
00:39:39,340 --> 00:39:43,940
we can use as a
measure of ductility.

899
00:39:43,940 --> 00:39:50,470
This slope right here
refers to the stiffness.

900
00:39:50,470 --> 00:39:54,910
And finally, this
energy right here

901
00:39:54,910 --> 00:39:56,490
is something like the toughness.

902
00:39:59,160 --> 00:40:02,270
And the hardness isn't
quite on this plot.

903
00:40:02,270 --> 00:40:04,940
So I want to see if
you guys intuitively

904
00:40:04,940 --> 00:40:06,880
understand this, because
the next lecture,

905
00:40:06,880 --> 00:40:09,830
I'm going to be throwing around
the words like stiffness,

906
00:40:09,830 --> 00:40:12,440
toughness, ductility,
hardness, compliance,

907
00:40:12,440 --> 00:40:14,690
hard, soft, whatever,
and I want to make sure

908
00:40:14,690 --> 00:40:17,360
that you just at least
intuitively understand.

909
00:40:17,360 --> 00:40:19,310
There's a few videos you
may have seen before.

910
00:40:19,310 --> 00:40:22,260
Anyone here watch the
hydraulic press channel?

911
00:40:22,260 --> 00:40:22,860
There we go.

912
00:40:22,860 --> 00:40:25,150
Finally, something that
half the class does.

913
00:40:25,150 --> 00:40:26,820
We're going to
predict what's going

914
00:40:26,820 --> 00:40:30,210
to happen in each of these
cases based on these material

915
00:40:30,210 --> 00:40:31,350
properties.

916
00:40:31,350 --> 00:40:35,370
So in this case, this is a
pressurized cylinder of CO2.

917
00:40:35,370 --> 00:40:39,460
It's made of aluminum, which
is a very ductile material.

918
00:40:39,460 --> 00:40:41,685
It's also a very tough material.

919
00:40:41,685 --> 00:40:43,560
How do you think it will
deform when smashed?

920
00:40:50,672 --> 00:40:51,630
Anyone ever tried this?

921
00:40:51,630 --> 00:40:52,990
Squishing aluminum stuff.

922
00:40:52,990 --> 00:40:54,074
What happens?

923
00:40:54,074 --> 00:40:56,035
AUDIENCE: You compress it.

924
00:40:56,035 --> 00:40:57,160
PROFESSOR: You compress it.

925
00:40:57,160 --> 00:40:58,077
And then what happens?

926
00:41:00,772 --> 00:41:02,600
AUDIENCE: Fracture?

927
00:41:02,600 --> 00:41:03,882
PROFESSOR: Will it fracture?

928
00:41:03,882 --> 00:41:05,022
AUDIENCE: After a while.

929
00:41:05,022 --> 00:41:06,230
PROFESSOR: After a while, OK.

930
00:41:06,230 --> 00:41:08,570
If you put a lot of energy
into it, eventually,

931
00:41:08,570 --> 00:41:11,780
when you reach this strain to
failure, it should fracture.

932
00:41:11,780 --> 00:41:14,360
But in your personal
hands-on experience,

933
00:41:14,360 --> 00:41:17,017
does aluminum tend to fracture
when you bend it a little bit?

934
00:41:17,017 --> 00:41:18,510
AUDIENCE: No.

935
00:41:18,510 --> 00:41:23,310
PROFESSOR: So then what words
would you use to describe it?

936
00:41:23,310 --> 00:41:25,410
Based on this curve right here.

937
00:41:28,480 --> 00:41:28,980
Yep?

938
00:41:28,980 --> 00:41:29,790
AUDIENCE: Ductile.

939
00:41:29,790 --> 00:41:30,582
PROFESSOR: Ductile.

940
00:41:30,582 --> 00:41:32,600
I would say ductile
and not brittle

941
00:41:32,600 --> 00:41:35,810
because you can bend it quite
a bit or stretch it quite a bit

942
00:41:35,810 --> 00:41:37,480
before it fractures.

943
00:41:37,480 --> 00:41:38,540
How about stiffness?

944
00:41:38,540 --> 00:41:41,000
Is it really hard or really
easy to get aluminum bending?

945
00:41:43,690 --> 00:41:44,815
AUDIENCE: It's pretty easy.

946
00:41:44,815 --> 00:41:45,982
PROFESSOR: It's fairly easy.

947
00:41:45,982 --> 00:41:47,740
So would you call that
stiff or compliant?

948
00:41:47,740 --> 00:41:48,573
AUDIENCE: Compliant.

949
00:41:48,573 --> 00:41:49,448
PROFESSOR: Compliant.

950
00:41:49,448 --> 00:41:50,080
OK.

951
00:41:50,080 --> 00:41:51,430
What about strength?

952
00:41:51,430 --> 00:41:53,830
How hard is it to start
deforming aluminum

953
00:41:53,830 --> 00:41:56,800
irreversibly, compared
to something like steel?

954
00:41:56,800 --> 00:41:58,060
AUDIENCE: Not very.

955
00:41:58,060 --> 00:41:59,080
PROFESSOR: Not very.

956
00:41:59,080 --> 00:42:00,130
Especially pure aluminum.

957
00:42:00,130 --> 00:42:02,320
You can chew through it.

958
00:42:02,320 --> 00:42:04,990
If you guys ever got a
one yen coin from Japan,

959
00:42:04,990 --> 00:42:07,480
you can chew through it.

960
00:42:07,480 --> 00:42:09,160
Not very strong.

961
00:42:09,160 --> 00:42:12,292
Then again, your bite force
is also incredibly strong.

962
00:42:12,292 --> 00:42:13,750
But anyway, let's
see what actually

963
00:42:13,750 --> 00:42:18,550
happens when you compress a
rather ductile, compliant,

964
00:42:18,550 --> 00:42:21,475
and not that strong
aluminum canister.

965
00:42:24,364 --> 00:42:25,800
Is it actually going?

966
00:42:25,800 --> 00:42:28,770
Oh, it actually skipped ahead.

967
00:42:28,770 --> 00:42:30,750
That's what I wanted,
was their sound.

968
00:42:37,920 --> 00:42:39,685
It was also
pressurized with CO2.

969
00:42:42,460 --> 00:42:44,170
But notice what's left.

970
00:42:44,170 --> 00:42:45,460
So actually watch in slow-mo.

971
00:42:45,460 --> 00:42:49,210
Look how much you can compress
that, even after the explosion.

972
00:42:49,210 --> 00:42:51,858
No fracture.

973
00:42:51,858 --> 00:42:54,150
If you had done that with,
let's say, a glass canister,

974
00:42:54,150 --> 00:42:55,230
what do you guys think
would have happened?

975
00:42:55,230 --> 00:42:56,140
AUDIENCE: It would
have shattered.

976
00:42:56,140 --> 00:42:57,340
PROFESSOR: It would
have shattered.

977
00:42:57,340 --> 00:42:59,280
Yeah, we'll see that in
a bit with a material

978
00:42:59,280 --> 00:43:01,140
that may surprise you.

979
00:43:01,140 --> 00:43:04,615
AUDIENCE: So it basically
doesn't fracture, right?

980
00:43:04,615 --> 00:43:06,240
PROFESSOR: It will
fracture eventually,

981
00:43:06,240 --> 00:43:09,720
but the hydraulic press can't
get it that far in compression.

982
00:43:09,720 --> 00:43:12,480
So that would be something
that's extremely ductile, not

983
00:43:12,480 --> 00:43:13,270
that strong--

984
00:43:13,270 --> 00:43:15,030
so it wasn't that
hard to deform.

985
00:43:15,030 --> 00:43:18,210
Certainly we know it wasn't
stronger than the steel base

986
00:43:18,210 --> 00:43:21,090
plate that they used
to do the smashing.

987
00:43:21,090 --> 00:43:22,680
Because whatever's
the softer material

988
00:43:22,680 --> 00:43:23,972
is going to deform more.

989
00:43:23,972 --> 00:43:26,430
So here he's going to have--
well I'll let him describe it,

990
00:43:26,430 --> 00:43:29,232
and then I'll let you guess
what's going to happen.

991
00:43:52,200 --> 00:43:53,950
What do you guys think
is going to happen?

992
00:43:56,550 --> 00:43:58,930
You've got what looks like
brass and copper coins

993
00:43:58,930 --> 00:44:01,470
on a steel base plate.

994
00:44:01,470 --> 00:44:04,170
Anyone have any idea?

995
00:44:04,170 --> 00:44:05,376
AUDIENCE: [INAUDIBLE]

996
00:44:05,376 --> 00:44:06,670
PROFESSOR: Yeah.

997
00:44:06,670 --> 00:44:08,170
Everyone's making
this motion, which

998
00:44:08,170 --> 00:44:11,620
means everything's going
to flatten out, right?

999
00:44:11,620 --> 00:44:12,610
Let's find out.

1000
00:44:24,423 --> 00:44:26,340
Not nearly as much as
you might have expected.

1001
00:44:45,970 --> 00:44:49,060
Is anyone surprised by this?

1002
00:44:49,060 --> 00:44:51,540
What happened there?

1003
00:44:51,540 --> 00:44:55,320
What actually happened there
was already described up here.

1004
00:44:55,320 --> 00:44:58,290
When you get enough dislocations
piling up against each other

1005
00:44:58,290 --> 00:45:01,410
during plastic deformation,
you can undergo a process

1006
00:45:01,410 --> 00:45:03,011
called work hardening.

1007
00:45:08,420 --> 00:45:10,280
That process can be
physically described

1008
00:45:10,280 --> 00:45:12,830
by a lot of those dislocations
piling up and making

1009
00:45:12,830 --> 00:45:16,717
it more and more difficult
to continue that deformation.

1010
00:45:16,717 --> 00:45:18,800
So what happened here is
the brass and the copper,

1011
00:45:18,800 --> 00:45:23,150
which started out quite soft,
not that hard, quite ductile,

1012
00:45:23,150 --> 00:45:28,400
as you can see, and not that
strong actually got stronger

1013
00:45:28,400 --> 00:45:30,820
as they were deformed.

1014
00:45:30,820 --> 00:45:32,050
Interesting, huh?

1015
00:45:32,050 --> 00:45:34,700
Did anyone expect
this to happen?

1016
00:45:34,700 --> 00:45:36,060
OK.

1017
00:45:36,060 --> 00:45:38,310
Let's go to one that I think
everyone can guess what's

1018
00:45:38,310 --> 00:45:41,330
going to happen, a lead ball.

1019
00:45:41,330 --> 00:45:44,400
So has anyone ever tried
playing with lead before?

1020
00:45:44,400 --> 00:45:46,340
Hopefully not.

1021
00:45:46,340 --> 00:45:47,020
I have quite a--

1022
00:45:47,020 --> 00:45:48,793
OK good, I'm not alone.

1023
00:45:48,793 --> 00:45:50,960
How would you describe lead
in terms of the material

1024
00:45:50,960 --> 00:45:51,710
properties here?

1025
00:45:55,702 --> 00:45:57,700
AUDIENCE: [INAUDIBLE]

1026
00:45:57,700 --> 00:45:58,440
PROFESSOR: Yep.

1027
00:45:58,440 --> 00:45:59,450
It's not very stiff.

1028
00:45:59,450 --> 00:46:01,710
It doesn't take much energy
to start deforming it.

1029
00:46:01,710 --> 00:46:02,310
How else?

1030
00:46:05,710 --> 00:46:07,150
Was it hard or soft?

1031
00:46:07,150 --> 00:46:08,090
AUDIENCE: Soft.

1032
00:46:08,090 --> 00:46:08,720
PROFESSOR: OK.

1033
00:46:08,720 --> 00:46:12,320
Do you think it's
ductile or brittle?

1034
00:46:12,320 --> 00:46:12,872
Yeah?

1035
00:46:12,872 --> 00:46:13,830
AUDIENCE: It's brittle.

1036
00:46:13,830 --> 00:46:15,400
PROFESSOR: You
think it's brittle.

1037
00:46:15,400 --> 00:46:18,460
So by that, you mean it's just
going to break apart, right?

1038
00:46:18,460 --> 00:46:21,550
If you deform it?

1039
00:46:21,550 --> 00:46:22,800
OK, cool.

1040
00:46:22,800 --> 00:46:25,280
And would you say it
is tough or not tough?

1041
00:46:28,470 --> 00:46:30,870
Not a lot of folks have
hands-on experience with lead.

1042
00:46:30,870 --> 00:46:33,000
It's probably good
for your brains.

1043
00:46:33,000 --> 00:46:33,690
Let's find out.

1044
00:46:57,900 --> 00:46:58,950
Lead pancake.

1045
00:46:58,950 --> 00:47:02,703
So what words would you use to
describe what just happened?

1046
00:47:02,703 --> 00:47:03,670
AUDIENCE: It's ductile.

1047
00:47:03,670 --> 00:47:05,233
PROFESSOR: Ductile indeed.

1048
00:47:05,233 --> 00:47:06,900
I don't know what
sort of brittle lead--

1049
00:47:06,900 --> 00:47:09,341
was it an alloy that you had
been playing with, maybe?

1050
00:47:09,341 --> 00:47:10,883
AUDIENCE: It was
like a little sheet.

1051
00:47:10,883 --> 00:47:12,110
It was just easy to snap.

1052
00:47:12,110 --> 00:47:13,290
PROFESSOR: Aha, OK.

1053
00:47:13,290 --> 00:47:15,870
So it was a sheet of lead
that was easy to snap.

1054
00:47:15,870 --> 00:47:18,660
So I would not call lead
as a very tough material

1055
00:47:18,660 --> 00:47:21,450
because you didn't have to
put a lot of energy into it,

1056
00:47:21,450 --> 00:47:24,420
but did it deform quite a bit
before you snapped it or did

1057
00:47:24,420 --> 00:47:25,957
it just crumble apart?

1058
00:47:25,957 --> 00:47:27,420
AUDIENCE: Oh, it deformed.

1059
00:47:27,420 --> 00:47:28,290
PROFESSOR: OK.

1060
00:47:28,290 --> 00:47:31,740
So in that case, I would call
it ductile because it deformed

1061
00:47:31,740 --> 00:47:34,620
a lot before breaking,
but I would not

1062
00:47:34,620 --> 00:47:37,680
call it tough because it took
very little energy to get it

1063
00:47:37,680 --> 00:47:39,540
to that breaking point.

1064
00:47:39,540 --> 00:47:42,310
And it wasn't that stiff because
it was quite easy to get it--

1065
00:47:42,310 --> 00:47:45,360
let's say it's the
amount of stress

1066
00:47:45,360 --> 00:47:47,000
you put in versus the strain.

1067
00:47:47,000 --> 00:47:48,800
It could be quite low.

1068
00:47:48,800 --> 00:47:50,550
And it would not be
very strong because it

1069
00:47:50,550 --> 00:47:54,690
didn't take a lot of energy
or stress to get it moving.

1070
00:47:54,690 --> 00:47:56,340
Let's look at another ball.

1071
00:47:56,340 --> 00:48:00,210
In this case, a
steel ball bearing.

1072
00:48:00,210 --> 00:48:02,262
What do you guys think
is going to happen here?

1073
00:48:02,262 --> 00:48:03,990
AUDIENCE: It's going to shatter.

1074
00:48:03,990 --> 00:48:05,365
PROFESSOR: It's
going to shatter.

1075
00:48:05,365 --> 00:48:09,010
So you're guessing that the
steel is brittle, right?

1076
00:48:09,010 --> 00:48:09,550
What else?

1077
00:48:16,538 --> 00:48:18,330
AUDIENCE: Probably
pretty stiff and strong.

1078
00:48:18,330 --> 00:48:21,220
PROFESSOR: Probably quite
stiff and strong, yeah.

1079
00:48:21,220 --> 00:48:22,970
I think so, too, but
I don't think the guy

1080
00:48:22,970 --> 00:48:24,789
that did this expected that.

1081
00:48:48,025 --> 00:48:50,918
[INTERPOSING VOICES]

1082
00:48:50,918 --> 00:48:51,585
PROFESSOR: Yeah.

1083
00:48:51,585 --> 00:48:53,498
Did that surprise anybody?

1084
00:49:23,770 --> 00:49:24,270
Yeah.

1085
00:49:24,270 --> 00:49:25,800
Quite a surprise, right?

1086
00:49:25,800 --> 00:49:28,290
So in this case, materials
like hardened steel

1087
00:49:28,290 --> 00:49:30,000
aren't necessarily that brittle.

1088
00:49:30,000 --> 00:49:33,480
In fact, you wouldn't want a
ball bearing to be brittle.

1089
00:49:33,480 --> 00:49:37,380
If you get some small chip in it
or a little bit of grit or sand

1090
00:49:37,380 --> 00:49:39,780
in the bearings, you would
shatter the ball bearing

1091
00:49:39,780 --> 00:49:43,020
and cause instantaneous failure
of the rotating component.

1092
00:49:43,020 --> 00:49:46,140
So what you actually want out
of a high-strength ball bearing

1093
00:49:46,140 --> 00:49:47,700
is something that's
extremely hard.

1094
00:49:47,700 --> 00:49:50,730
Resists deformation
so it doesn't undergo,

1095
00:49:50,730 --> 00:49:53,100
let's say, change of shape
that would prevent it

1096
00:49:53,100 --> 00:49:56,032
from rolling without friction
or with very little friction.

1097
00:49:56,032 --> 00:49:57,990
You want it to be quite
stiff because you don't

1098
00:49:57,990 --> 00:50:01,230
want the load of whatever you're
loading onto it to deform it,

1099
00:50:01,230 --> 00:50:03,780
but you also don't
want it to be brittle.

1100
00:50:03,780 --> 00:50:06,960
So it's got to be somewhat
tough and ductile to prevent

1101
00:50:06,960 --> 00:50:08,160
sudden failure.

1102
00:50:08,160 --> 00:50:12,320
You'd rather it compress a tiny
bit than just cracking in half.

1103
00:50:12,320 --> 00:50:15,550
So you can make things like
ceramic ball bearings, which

1104
00:50:15,550 --> 00:50:20,530
are very brittle, very stiff,
not that tough, but also very

1105
00:50:20,530 --> 00:50:22,930
strong, and you just
have to make sure

1106
00:50:22,930 --> 00:50:24,400
that whatever part
you make is not

1107
00:50:24,400 --> 00:50:29,620
going to reach any sort of yield
strength criterion or crack

1108
00:50:29,620 --> 00:50:31,410
or anything.

1109
00:50:31,410 --> 00:50:34,890
Now the last one that's
probably the most surprising.

1110
00:50:34,890 --> 00:50:38,130
They bought a $4,000 diamond.

1111
00:50:38,130 --> 00:50:40,270
It's a diamond like that big.

1112
00:50:40,270 --> 00:50:42,610
What do you know about
diamonds as a material in terms

1113
00:50:42,610 --> 00:50:43,690
of these properties?

1114
00:50:43,690 --> 00:50:45,650
AUDIENCE: They're hard.

1115
00:50:45,650 --> 00:50:46,900
PROFESSOR: Yep, both is right.

1116
00:50:46,900 --> 00:50:48,610
They're extremely stiff.

1117
00:50:48,610 --> 00:50:51,460
It's the hardest material
that we know of, almost.

1118
00:50:51,460 --> 00:50:53,815
We've made slightly
harder ones artificially.

1119
00:50:53,815 --> 00:50:55,690
It's the hardest natural
material we know of.

1120
00:50:55,690 --> 00:50:57,070
What else?

1121
00:50:57,070 --> 00:50:59,756
Do you know whether
they're strong or tough?

1122
00:50:59,756 --> 00:51:01,040
AUDIENCE: They're not tough.

1123
00:51:01,040 --> 00:51:02,248
PROFESSOR: They're not tough.

1124
00:51:02,248 --> 00:51:03,352
Why do you say that?

1125
00:51:03,352 --> 00:51:05,420
AUDIENCE: Because
it will shatter.

1126
00:51:05,420 --> 00:51:06,200
PROFESSOR: Have
you seen the video?

1127
00:51:06,200 --> 00:51:06,980
AUDIENCE: [INAUDIBLE]

1128
00:51:06,980 --> 00:51:07,810
PROFESSOR: Oh wow.

1129
00:51:07,810 --> 00:51:09,260
OK.

1130
00:51:09,260 --> 00:51:10,160
What else do we have?

1131
00:51:10,160 --> 00:51:11,000
Yeah.

1132
00:51:11,000 --> 00:51:12,597
So you're saying it's not tough.

1133
00:51:12,597 --> 00:51:14,180
AUDIENCE: You can
cut diamonds, right?

1134
00:51:14,180 --> 00:51:17,000
PROFESSOR: You can cut
diamonds with other diamonds.

1135
00:51:17,000 --> 00:51:18,800
So the cutting action
usually depends

1136
00:51:18,800 --> 00:51:21,810
on the relative hardness
of the material.

1137
00:51:21,810 --> 00:51:24,680
So if you want to polish or
cut something abrasively,

1138
00:51:24,680 --> 00:51:26,210
you need to use
a harder material

1139
00:51:26,210 --> 00:51:29,180
because then the grit
itself won't wear away

1140
00:51:29,180 --> 00:51:31,420
before the material
it's trying to cut.

1141
00:51:31,420 --> 00:51:34,700
But what's going to happen here
is we're going to put a diamond

1142
00:51:34,700 --> 00:51:36,687
and try compressing
it, and we'll

1143
00:51:36,687 --> 00:51:38,520
see what its stress-strain
curve looks like.

1144
00:51:38,520 --> 00:51:40,460
So votes on what's
going to happen.

1145
00:51:40,460 --> 00:51:43,541
Who says, like Monica,
it's going to shatter?

1146
00:51:43,541 --> 00:51:47,340
Who thinks it's going
to break the tools?

1147
00:51:47,340 --> 00:51:49,804
Who thinks it's going
to deform plastically?

1148
00:51:49,804 --> 00:51:52,765
Yeah, I've never seen a
diamond deform plastically.

1149
00:52:45,133 --> 00:52:47,300
AUDIENCE: They still have
pretty big chunks, though.

1150
00:52:47,300 --> 00:52:50,040
PROFESSOR: Oh yeah, they could
probably still sell those.

1151
00:52:50,040 --> 00:52:53,250
Absolutely no deformation.

1152
00:52:53,250 --> 00:52:56,410
It just rotates and explodes.

1153
00:52:56,410 --> 00:52:56,910
Yeah.

1154
00:52:56,910 --> 00:52:58,620
This would be a
material that we would

1155
00:52:58,620 --> 00:53:02,220
say has almost zero ductility.

1156
00:53:02,220 --> 00:53:04,320
Despite being
extremely hard, I don't

1157
00:53:04,320 --> 00:53:05,820
know if there would
even been enough

1158
00:53:05,820 --> 00:53:08,918
deformation to have a slight
dent in the tool itself.

1159
00:53:08,918 --> 00:53:11,460
There's probably a little hole
where the point of the diamond

1160
00:53:11,460 --> 00:53:15,030
poked in, but once there was
enough stress on that diamond,

1161
00:53:15,030 --> 00:53:19,280
its stress-strain curve would
look something like that.

1162
00:53:19,280 --> 00:53:21,480
Maybe like that.

1163
00:53:21,480 --> 00:53:22,910
Yeah.

1164
00:53:22,910 --> 00:53:24,530
So it's important
that you intuitively

1165
00:53:24,530 --> 00:53:27,920
understand the differences
between strength, ductility,

1166
00:53:27,920 --> 00:53:31,490
hardness, toughness, and
stiffness, because then

1167
00:53:31,490 --> 00:53:35,870
next class, we can explain
how radiation changes them.

1168
00:53:35,870 --> 00:53:38,570
So any questions on the
materials and properties

1169
00:53:38,570 --> 00:53:39,110
from today?

1170
00:53:43,620 --> 00:53:44,720
Yeah?

1171
00:53:44,720 --> 00:53:47,790
AUDIENCE: Can you clarify why
something is, for example,

1172
00:53:47,790 --> 00:53:49,705
ductile versus brittle?

1173
00:53:49,705 --> 00:53:50,330
PROFESSOR: Mhm.

1174
00:53:50,330 --> 00:53:51,997
So the reason something
would be ductile

1175
00:53:51,997 --> 00:53:55,442
versus brittle is whether or not
you can plastically deform it,

1176
00:53:55,442 --> 00:53:57,650
and that means whether or
not it's more energetically

1177
00:53:57,650 --> 00:54:00,200
favorable for dislocations
to keep moving

1178
00:54:00,200 --> 00:54:03,890
versus just breaking a plane
of atoms in any irregular

1179
00:54:03,890 --> 00:54:06,110
direction and causing fracture.

1180
00:54:06,110 --> 00:54:08,480
So again, ductility
versus embrittlement

1181
00:54:08,480 --> 00:54:13,670
is the interplay between
slip and fracture.

1182
00:54:13,670 --> 00:54:16,670
Slip is normally done
by dislocation movement.

1183
00:54:16,670 --> 00:54:20,120
Any defects created by anything,
especially radiation damage,

1184
00:54:20,120 --> 00:54:23,690
will make slip harder so that
any continued energy you put in

1185
00:54:23,690 --> 00:54:26,628
will not move dislocations
but move towards fracture.

1186
00:54:29,496 --> 00:54:32,020
If there's no other
questions, we'll

1187
00:54:32,020 --> 00:54:34,420
look at the
stress-strain curves of

1188
00:54:34,420 --> 00:54:36,743
some other familiar materials.

1189
00:54:36,743 --> 00:54:39,160
It is 10:00, in case you guys
have to go to other classes.

1190
00:54:43,380 --> 00:54:45,880
AUDIENCE: Are you taking any
nuclear activation stuff today?

1191
00:54:45,880 --> 00:54:46,505
PROFESSOR: Yes.

1192
00:54:46,505 --> 00:54:48,940
If you guys have things for a
nuclear activation analysis,

1193
00:54:48,940 --> 00:54:50,720
hand it in.

1194
00:54:50,720 --> 00:54:53,663
You guys bring stuff in?

1195
00:54:53,663 --> 00:54:55,580
We're running out of
opportunities to do this.

1196
00:54:58,260 --> 00:54:58,760
All right.

1197
00:54:58,760 --> 00:55:03,260
In that case, the
entry fee for the quiz

1198
00:55:03,260 --> 00:55:06,790
will be your nuclear
activation analysis sample.