27. Nuclear Materials — Radiation Damage and Effects in Matter
27. Nuclear Materials — Radiation Damage and Effects in Matter
MIT 22.01 Introduction to Nuclear Engineering and Ionizing Radiation, Fall 2016
Instructor: Michael Short
View the complete course: https://ocw.mit.edu/22-01F16
YouTube Playlist: • MIT 22.01 Introduction to Nuclear Eng…
Prof. Short uses all the concepts introduced thus far to introduce the study of nuclear materials and radiation damage - his field of study. The concept of ionizing radiation creating nuclear displacements, not just electron ionization, is introduced as the first event in radiation damage. The structural defects produced from these displacements are shown to cluster, move, and evolve, resulting in drastic changes to material properties. Key structural material properties and their formal definitions are introduced and demystified by watching a pair of Finnish scientists smash various items with a 50 ton hydraulic press.
License: Creative Commons BY-NC-SA
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25.789 -> got much more than one request to do
27.619 -> some stuff on nuclear materials and I
29.93 -> think it's just about the right time you
31.789 -> guys know enough about radiation and
34.399 -> driving would matter and everything and
35.809 -> stopping power with prop sections after
37.909 -> excess of nuclear materials and
40.25 -> radiation damage this is my whole field
42.32 -> so happy to come talk to you guys about
44.54 -> this and show you why can you finish
45.71 -> because it all of yours it starts out
47.72 -> this this slide kind of gets on to it it
51.29 -> starts off with the single atom atomic
53.479 -> defects that make up the basic building
56.09 -> blocks of damage and ends up with things
58.489 -> that break in nuclear reactors under
60.17 -> radiation and so to understand the whole
62.42 -> thing you've got to know everything from
64.55 -> the single atoms on the sort of
65.869 -> femtosecond scale all the way up to the
68.179 -> engineering scale where things evolve
69.619 -> over years or even decades so I'll be
73.25 -> talking first probably today we're gonna
74.99 -> go over a material science primer so who
76.94 -> here's how many courses in material
78.35 -> science no one that's good because I'm
81.86 -> assuming that there's a let's see no one
84.74 -> knows anything here I know there's a
85.82 -> couple material scientists in the class
87.619 -> and I apologize ahead of time if it's a
89.119 -> bit of a review but we'll be going
90.979 -> mostly through what are materials and
93.35 -> what are the defects that change their
95.149 -> material properties and how do they
96.95 -> behave that'll take us through about
98.659 -> today so then tomorrow we can see how
100.85 -> radiation causes those defects and
102.71 -> actually changes material properties so
107.39 -> there's a whole laundry list of
108.77 -> different ways that materials fail and
110.899 -> most folks are concerned with all of
113.6 -> these everyone everything from simple
115.399 -> overload which means you stress
117.68 -> something too much and it just breaks to
119.99 -> all the different forms of corrosion
121.549 -> that's a whole field in itself and then
124.1 -> there's the things that just we have to
125.869 -> worry about because there are only
127.009 -> activated with radiation damage and in
130.25 -> this case this isn't quite ionization by
132.5 -> radiation but it's actual radiation
133.97 -> slamming into nuclei and moving atoms
136.7 -> out of their place and we've got one
139.37 -> figure that we had recently in a paper
141.2 -> that sums up the entire multi scale
143.39 -> picture of radiation damage from the
145.64 -> femtosecond
146.63 -> to let's say the mega second scale or i
149.209 -> think it's more than that maybe Giga
150.68 -> second would be the right word for that
152 -> and all the way down from the angstrom
154.1 -> to the meters
154.83 -> scale and I want to pine WA walk you
158.07 -> through sort of a lens scale by lens
160.11 -> scale depiction of radiation damage it
162.87 -> all starts with knocking atoms out of
165.27 -> place we've mentioned this a little bit
167.22 -> when we talked about nuclear stopping
168.75 -> power and this is where it actually
170.28 -> comes into play sometimes an incoming
172.53 -> Neutron or photon or ion can displace an
176.4 -> atom from its original site and we call
179.07 -> that a physical that's a displacement
180.96 -> and then that atom comes off with quite
183.72 -> a bit of kinetic energy and can knock
185.88 -> into a whole bunch of other atoms now
189.15 -> this loss of the solid crystalline
191.1 -> structure you can't really tell what the
194.19 -> original structure looked like right it
196.709 -> actually comprises a very small
198.6 -> localized zone of melting called the
200.52 -> thermal spike if you think about all
202.89 -> these atoms are vibrating at fractions
205.32 -> of an AV at thermal energies like the
207.54 -> thermal neutrons we talked about in the
208.95 -> reactor then you hit them with an MeV
211.14 -> neutron they might transfer a hundred ke
213.66 -> v of energy and a bunch of these atoms
216.27 -> will then be moving about at let's say a
218.28 -> few hundred Evie that's way beyond
221.33 -> liquid temperature so actually it's it's
224.28 -> been theorized that there's a little
225.9 -> pocket of atoms around three to five
227.73 -> nanometers wide that reaches like ten
229.92 -> thousand Kelvin for a very very short
232.56 -> amount of time less than a picosecond
234.98 -> because almost instantly those atoms
237.57 -> knock into the ones around them and this
240.03 -> is how the process of heat transfer
242.07 -> occurs and so very quickly you get
244.17 -> what's called the quench where most of
246.45 -> those atoms very quickly knock into
248.25 -> other ones slowing down finding their
250.709 -> equilibrium positions again but not
252.75 -> everyone you can see there's a few
255 -> places where the atoms are still out of
257.37 -> their original location and it's those
259.47 -> residual defects that actually comprise
261.87 -> radiation damage and as those defects
265.05 -> build up they start to move they can
268.11 -> diffuse they can be transported
270.15 -> ballistically by more radiation damage
272.31 -> they can move by all sorts of different
274.35 -> mechanisms and eventually find each
276.06 -> other forming what's called clusters so
279.15 -> a bunch of those missing atoms could
280.68 -> find each other and make a hole which we
282.27 -> call a void a bunch of the extra atoms
284.82 -> shoved in between the other ones can
286.89 -> form things called interstitial
288.72 -> clusters we say interstitial because
291.09 -> it's like in the space in between where
294 -> you'd normally find some atoms so let's
296.34 -> say you had a whole bunch of those
298.5 -> missing atoms come together forming a
300.45 -> void this is an actual transmission
302.67 -> electron microscope or TEM image of a
305.85 -> void pockets of vacuum in materials no
309.45 -> does anything interesting about its
310.5 -> shape it's rounded but what's the most
316.65 -> striking to me is it isn't actually
318.27 -> round so you would expect a void or a
320.94 -> bubble to be kind of spherical right
323.46 -> that's the minimum energy configuration
324.99 -> of most things not so when you have a
329.07 -> little pocket of vacuum
330.45 -> it's where crystallinity comes into play
332.31 -> and these voids can end up forming super
335.13 -> structures what kind of what curious
337.53 -> thing do you notice here for this whole
341.76 -> ensemble of voids yeah they are all in
349.08 -> the same direction kind of funny that's
351.48 -> that's definitely not an accident right
353.28 -> it's not like they're randomly aligned
354.78 -> there's a reason for this that will go
356.04 -> into in a couple slides yeah the size
360.33 -> scale I think these are on the order of
363.3 -> 20 nanometers or so yeah I crop these
365.88 -> images just to get points across let's
367.65 -> see if it says in the other one not
369.69 -> quite yeah but these voids can get
371.55 -> upwards of tens of nanometers as
374.16 -> smallest single atoms yeah this is the
379.32 -> accumulation of radiation defects into
382.05 -> what's called voids yeah don't worry
384.69 -> we'll go over in more detail again and
387.56 -> if you get little pockets of vacuum in
389.97 -> your material where you yeah you're not
391.38 -> creating or destroying mass you're just
393.15 -> moving it so those voids where that mass
396.15 -> was has to go somewhere else and you
398.13 -> actually get things that swell in the
399.93 -> reactor on their own they don't change
401.85 -> mass but they change volume they just
404.01 -> kind of puff up like Swiss cheese
405.15 -> sometimes upwards of twenty or thirty
407.88 -> percent changes in diameter and length
410.25 -> for some tubing now if you're depending
412.56 -> on these fuel rods being a certain space
414.87 -> apart in a reactor and they start to
417.39 -> swell squeezing out the coolant you lose
420.15 -> the ability to cool the reactor
421.96 -> because then how can you get water
423.37 -> around something where the tubes are
424.72 -> then swell together there's lots of
427.09 -> other bad things that can happen which
428.38 -> we'll get into and so then that's the
430.96 -> origin of void swelling from single
433.15 -> missing atoms called vacancies they can
435.729 -> cluster into voids which then caused
438.28 -> physical dimensional changes of
440.53 -> materials on the scale of centimeters to
442.509 -> meters and that's why we say it's this
444.52 -> full multi scale picture of radiation
447.1 -> damage but to understand what is damage
450.16 -> you have to know what is an undamaged
451.66 -> structure to begin with so it doesn't
454.3 -> make sense to say how does a structure
455.68 -> change if you don't know how it behaves
458.199 -> so I want to give a very quick primer to
461.32 -> material science and apologies to any
463.09 -> material scientists in the room because
464.919 -> this is gonna seem really basic but this
467.169 -> is a very quick intro to this whole
468.849 -> field I want to go over quickly what is
471.19 -> a crystalline solid and perfectly
473.83 -> undamaged material would be a set of
476.02 -> atoms lined up in a very regular lattice
479.44 -> in a regular array where you move over a
481.75 -> certain distance and you find another
483.25 -> atom and this extends forever and ever
485.77 -> and ever all the way out to when you
487.599 -> reach the free surface and so this what
489.88 -> we would call an undamaged material a
491.8 -> pristine perfect single crystal by
495.43 -> crystal I mean an arrangement of atoms
497.74 -> in a certain direction so notice here
500.26 -> all the atoms are lined up in let's say
502.99 -> some cubic XYZ way that's what we would
506.289 -> call one crystal or one green you'll
509.05 -> hear this you'll hear both of those and
510.94 -> you'll notice also that the arrangement
513.94 -> of the atoms tends to determine what the
516.49 -> physical objects look like or we like to
519.159 -> say that for that was it formed follows
522.01 -> structure in material science so for
525.22 -> materials like pyrite which follows a
527.86 -> simple cubic structure that's the
530.079 -> crystals you pull out of the ground they
532.029 -> mimic their atomic configurations in
534.04 -> physical centimeter sized space for gold
537.49 -> atoms they adopt a slightly different
539.41 -> structure it's still cubic but there's
542.32 -> atoms shoved into the cube faces so what
545.079 -> we call a face centered cubic or FCC and
547.209 -> you start to see Cube looking structures
550 -> all over single crystals of gold another
553.63 -> one gypsum it's got a very different
555.85 -> a structure called monoclinic where none
558.49 -> of the sides of this parallelogram are
560.589 -> the same and there's some funny angles
562.329 -> but if you look at the arrangement of
563.98 -> the atoms and the actual crystals a bit
566.139 -> of gypsum that grow you sees a striking
568.509 -> similarity which I find pretty neat I
571.259 -> also want to mention what is the absence
574.72 -> of structure and material science we
576.19 -> call that something that's amorphous
577.69 -> amorphous means without form so for
581.29 -> example crystal and indium phosphide
583.269 -> would have this regular structure like
585.49 -> this you move over a certain distance
587.139 -> you see another green atom and so on and
589.389 -> so on and so on in an amorphous material
591.94 -> it can still be a solid but there's no
593.8 -> fixed distance between any certain types
596.74 -> of atoms and radiation can cause a lot
599.529 -> of this amorphous a ssin by knocking the
601.899 -> atoms about and having them freeze in
603.399 -> random configurations this is one of the
606.069 -> ways that radiation damage can in
608.23 -> brittle materials because well we'll get
611.17 -> into that so now let's talk about the
614.009 -> defects that can be created in a perfect
616.75 -> crystal the simplest ones we call point
619.18 -> defects or zero dimensional because
621.31 -> they're just single atoms out of place
623.11 -> you can have what's called a vacancy
625.149 -> where if you had let's say a face
628 -> centered cubic lattice of atoms or you
630.25 -> have atoms on every Q corner and every
632.17 -> face if you just pull one out somewhere
634.36 -> we refer to that as a vacancy a missing
637.3 -> atom it had to go somewhere though and
639.819 -> we'll get to where it is in just a sec
641.339 -> so it might be kind of hard to
643.42 -> conceptualize how do we know that there
645.31 -> are missing atoms in all these little
647.829 -> cubes or lattices we do have direct
650.86 -> evidence they're what's called quenching
654.1 -> studies where you can measure the
655.81 -> resistance or resistivity of a piece of
658.569 -> material after heating it to a certain
660.399 -> temperature because it turns out that
662.29 -> the hotter you make something the more
664.36 -> of those vacancies just naturally occur
666.55 -> you won't actually ever find an
669 -> absolutely perfect single crystal
671.35 -> anywhere in nature unless you go to 0
674.199 -> Kelvin for infinite time and the atoms
676.66 -> arrange themselves thusly there's always
678.67 -> some amount of atomic vibration going on
681.13 -> and there's actually some thermodynamic
683.949 -> energy gain to having a few defects in
687.519 -> your structure and that game
689.8 -> say that number of defects increases
692.05 -> with increasing temperature once you get
694.329 -> to the melting point of a material like
695.829 -> right before something melts you can
697.959 -> have up to one in 10,000 atoms just
699.97 -> missing moved somewhere else we call
702.85 -> that the thermal equilibrium vacancy
704.769 -> concentration and we can measure that
706.6 -> using these resistivity measurements
708.879 -> where you heat materials up to higher
711.009 -> and higher temperatures cool them down
713.049 -> suddenly and like liquid nitrogen or
714.91 -> liquid helium and measure the change in
717.1 -> resistivity the more defects there are
719.799 -> the harder it is for electrons to flow
721.689 -> through and the only thing that can
723.369 -> really be responsible there in a single
725.679 -> element would be vacancies so we do know
729.91 -> that these really exist
731.249 -> they can also cluster up it turns out
734.529 -> that every time you have a vacancy in a
736.449 -> material the other atoms move in a
738.399 -> little bit towards it relaxing the
740.589 -> pressure they feel from the atoms nearby
742.329 -> and one way for a whole bunch of
744.73 -> vacancies to lower the stress of the
746.649 -> whole atomic configuration is to cluster
749.139 -> together so if you have a whole bunch of
751.66 -> vacancies they may not allow as much
754.749 -> stress accommodation as if they were
757.269 -> separate when they're together now you
760.48 -> might ask what happened to the original
762.509 -> atoms you can't just take atoms away and
765.639 -> then go nowhere because you can't just
766.959 -> destroy matter right unless you turn it
769.269 -> into energy which is what we do in
770.529 -> nuclear engineering so in the material
773.35 -> science world they end up as what's
775.779 -> called interstitials where you know you
777.879 -> have a vacancy created from somewhere
779.679 -> that knocks that atom out and it gets
782.35 -> stuck in the next biggest space between
785.049 -> some other atoms and we refer to those
787.779 -> as interstitials and those can cluster
790.389 -> up to to reduce their total stress in
793.629 -> the lattice they can cluster up into
796.209 -> what's called split dumbbell
797.679 -> interstitials instead of having one
799.809 -> extra atom shoved in here you might
802.029 -> rearrange a couple so there's two atoms
804.009 -> in the center of a cube instead of one
805.449 -> and that tends to be a lower energy or a
808.54 -> more stable configuration so let's look
812.799 -> a little bit at the energetics of these
814.419 -> point defects because understanding how
816.339 -> they move and why will tell us a lot
818.379 -> about how radiation damage happens so it
821.11 -> turns out that interstitials are very
822.579 -> hard to make
823.57 -> it's really hard to shove an atom where
825.79 -> it doesn't want to be but once you get
828.01 -> it there it moves very easily let's draw
832.75 -> a quick simple cubic lattice to do a
836.5 -> little thought experiment and explore
837.85 -> why that might be let's say I want to
842.17 -> shove an interstitial atom in here
845.56 -> between these other atoms well their
847.57 -> electron clouds are going to repel and
849.43 -> it's going to push all the nearby atoms
851.829 -> away by just a little bit and these ones
854.31 -> might push the other atoms away by just
856.93 -> a little bit stretching out the lattice
860.949 -> or adding some compressive stress
863.17 -> wherever that interstitial is but then
866.079 -> how would it move what's the biggest
868.93 -> barrier it has to overcome to get to the
870.76 -> next adjacent location well which
879.399 -> direction would it go would it go this
880.57 -> way probably not there's an atom in the
883.6 -> way so it's gonna find the path of least
886.269 -> resistance to try to get over here
889.889 -> because like we've talked about before
891.67 -> all atoms are always in motion vibrating
894.699 -> some of them will be energetic enough to
896.86 -> squeeze through these two atoms and get
899.5 -> over to the next site and that turns out
900.97 -> to be a pretty easy process we can look
903.85 -> at the energy required for an
905.86 -> interstitial to move we notice it's
908.319 -> really small fractions of an electron
910.36 -> volt whereas creating them takes 2 or 3
913.389 -> electron volts in an atomic land that's
916.089 -> a very high energy penalty now let's
918.76 -> look at vacancies they're quite the
920.319 -> opposite
920.92 -> they're rather easy to make but they're
923.829 -> very hard to move compared to
925.87 -> interstitials notice that the energy of
928.42 -> movement is about the same as the energy
929.98 -> of formation for vacancies to take an
932.68 -> atom outer - pluck it out you have to
934.329 -> break every bond between nearby atoms so
937.93 -> you actually have to put energy in to
940.089 -> break those bonds and then remove the
943.959 -> atom somewhere else now these things are
947.17 -> usually made in pairs so if you think
949.899 -> about how much energy would it take to
952.12 -> cause a single radiation damage event
954.37 -> where you have one vacancy which let's
957.04 -> say
957.25 -> would have been right here and one
959.17 -> interstitial it takes the sum of these
961.6 -> two energies usually about four electron
964.39 -> volts that's not something that tends to
966.88 -> happen chemically or from stress or from
969.94 -> something like that but radiation coming
972.4 -> in with hundreds of kV or even MeV
975.04 -> neutrons anything's on the table because
977.95 -> it's high enough energy so it would take
984.01 -> about three or four evey to make a pair
986.11 -> of a vacancy and an interstitial if you
988.87 -> just add these two up it comes usually
990.67 -> to about three or four evey or electron
993.01 -> volts and that's a very difficult thing
995.26 -> to do in sort of chemical world where
997.63 -> you know reactions might proceed with
1000.09 -> fractions of an electron volt but when
1002.49 -> you have MeV neutrons coming in they can
1005.25 -> do whatever they want
1006.15 -> well they deal what they'll do whatever
1007.53 -> they will so someone actually asked me
1010.14 -> yesterday what sort of materials can you
1012.06 -> put in the way of neutrons to stop them
1014.76 -> from doing damage and the answer is
1016.98 -> pretty much nothing they test neutrons
1020.43 -> tend to travel about 10 centimeters even
1023.19 -> in things like steel or water and
1024.89 -> they're gonna hit what they're gonna hit
1026.819 -> there's not much you can do but put more
1029.31 -> things in the way and we can only get to
1031.709 -> a certain density with regular matter I
1033.569 -> think osmium has upwards of like 22
1036.72 -> grams per cubic centimeter density
1038.459 -> that's not enough to stop neutrons even
1041.28 -> over a considerable distance unless you
1043.47 -> had like liquid neutrons star that you
1045.99 -> could pack nuclei in and away higher
1047.67 -> number density not much you can do so
1052.38 -> I'm moving up in the dimensions there's
1054.78 -> another type of defect called a
1056.13 -> dislocation where it's actually
1058.32 -> energetically favorable to slide an
1060.99 -> extra half plane of atoms in between two
1064.38 -> sets in here in the crystal lattice
1066.42 -> creating a sort of bulged out structure
1069.18 -> like you see right here and dislocations
1071.79 -> are one of the most important defects in
1074.01 -> material science and radiation damage
1075.78 -> there what I like to call the agents of
1077.79 -> plasticity
1078.51 -> if you deform a material enough that it
1081.6 -> doesn't just spring back then low most
1084.63 -> likely you were creating and moving
1087.32 -> dislocations in the material do you
1090 -> think about a couple of diff
1091.11 -> ways to cause deformation let's bring
1094.17 -> our perfect lattice back without all
1095.76 -> these extra notations if you want to
1100.95 -> slide or shear two planes of atoms
1103.23 -> across and they're all bonded to each
1106.32 -> other
1112.34 -> what do you physically have to do how
1120.12 -> can you get these atoms to slide across
1121.65 -> each other what what sort of energy do
1124.11 -> you have to put into it yeah yep because
1130.679 -> all these atoms are bonded to each other
1132.33 -> if you want them to move you have to
1134.73 -> break every bond on that plane that's a
1138.33 -> lot of atomic bonds to break and it's
1140.79 -> extremely unlikely that that would
1142.32 -> happen in fact if you broke an entire
1144.75 -> plane of bonds in some material like
1147.54 -> this what would you physically do to it
1150.71 -> it snap it in half
1152.96 -> that would be fracture so if you broke
1156.179 -> every bond down this plane you would
1158.88 -> then have two pieces of this fuel rod
1160.58 -> that's usually a pretty high-energy
1162.69 -> thing to try to do so instead if you
1167.79 -> shove an extra half plane of atoms in
1170.94 -> there and the bonds are kind of funny
1177.45 -> like so right at that extra half plane
1180.09 -> location then what you can actually do
1183.929 -> is break one let's say you break this
1187.71 -> one form the next one then break this
1189.66 -> one and form the next one and for a few
1192.419 -> atoms to move over you only have to
1194.91 -> break a line of bonds not a plane so
1198.03 -> it's much less energy-intensive to get a
1201.45 -> dislocation to move than to just break
1204.03 -> something in half now you might ask well
1206.46 -> then why do things actually break
1207.98 -> whether or not things deform or break is
1210.72 -> a balance between this process which we
1213.36 -> call slip and breaking an entire plane
1218.22 -> of atoms which we call fracture so this
1221.55 -> one's called slip
1224.82 -> the other mode is fracture
1227.7 -> we would rather materials to form in
1231.159 -> systems like reactors by slip just
1233.379 -> moving a little bit than just breaking
1234.999 -> all together
1235.809 -> unfortunately when enough radiation hits
1238.059 -> materials you can fracture things in a
1240.669 -> brittle manner and we'll see what
1242.409 -> happens then there's a couple kinds of
1244.899 -> dislocations one of them is called a
1247.929 -> screw dislocation so imagine you had a
1250.21 -> whole bunch of sheets of atoms and you
1251.799 -> made a cut halfway through that sheet
1253.96 -> and then moved every plane up by one
1256.929 -> position you then got what's called a
1258.969 -> screw dislocation kind of a spiral
1261.399 -> parking-garage of atoms surrounding that
1263.83 -> core right there
1265.32 -> you can also have what's called an edge
1267.46 -> dislocation which is like the one I've
1269.049 -> got here on the board right here where
1271.69 -> you just have an extra half plane of
1273.129 -> atoms shoved in right there so there's
1276.759 -> two types and they move in two different
1278.889 -> ways the edge dislocation may behaves
1281.739 -> like you may physically expect if you
1284.289 -> kind of push like this on two planes of
1287.889 -> atoms it moves in the direction you push
1289.57 -> it screw dislocations are kind of screwy
1292.389 -> if you push like this it moves
1296.609 -> perpendicular not gonna get into why but
1299.589 -> just remember screw dislocations are
1301.059 -> fairly screwy in the way that they
1302.469 -> behave not quite intuitive well that's
1305.979 -> okay we don't have to worry about those
1307.349 -> and the way that they actually move like
1311.349 -> we showed right here is by what's called
1313.059 -> glide or slip where dislocations can
1316.45 -> slide just by one plane of atoms or one
1319.96 -> atomic position in a mechanism that
1322.869 -> looks something like this whereas that
1325.839 -> dislocation moves you only have to break
1327.789 -> a line of bonds and then reform a line
1329.799 -> of bonds which is a much easier process
1331.809 -> than breaking an entire plane at once
1333.779 -> it's like you have to break the square
1335.979 -> root of the same number of bonds when a
1341.2 -> skip head from some of that there's one
1342.759 -> other mechanism of dislocation movement
1344.619 -> that's important to us in radiation
1346.33 -> damage and that's called climb this is
1349.419 -> when you start to think about what
1350.589 -> happens if you have a dislocation which
1353.679 -> will give this symbol right here
1356.08 -> and you also have a vacancy let's say
1360.58 -> created by radiation damage if that
1363.159 -> vacancy can move it's going to find the
1365.74 -> most stressed-out part of this lattice
1367.539 -> most likely the vacancy will move here
1371.2 -> in other words the atom will move over
1373.84 -> there leaving this vacancy over there
1379.049 -> it's kind of funny to think like what
1381.22 -> does it mean that a vacancy moves has
1383.38 -> anyone ever done anything with
1384.46 -> semiconductors and talked about electron
1386.26 -> and hole movement okay yeah so what does
1389.59 -> it really mean for a hole to move right
1391.24 -> it's a holes not a thing a vacancy is
1393.76 -> also not a thing it's an absence of an
1395.44 -> atom but here we can say that the
1397.779 -> vacancy moves in this direction when the
1400.51 -> corresponding atom moves in the exact
1402.34 -> opposite direction and then what you've
1404.2 -> actually done is moved your dislocation
1406.99 -> up instead of moving in the slip
1411.25 -> direction you've now moved it in a
1413.74 -> perpendicular direction this is usually
1416.529 -> not possible without things like
1418.99 -> radiation damage or very high
1420.519 -> temperature and then to make things even
1424.84 -> crazier you can also have what's called
1426.549 -> loops of dislocations some videos of
1428.559 -> which I'll actually get to show you you
1430.419 -> can have a dislocation that has part
1432.37 -> edge character part screw character if
1434.679 -> you look at how the atoms are arranged
1436.419 -> here you're looking from sort of the top
1438.07 -> down you can see that there's an extra
1440.26 -> half plane of these white atoms shoved
1442.69 -> in in the black ones and this right here
1444.7 -> would be a completely edge dislocation
1447.519 -> you can have a gradual transition or
1450.279 -> about ninety degrees later it looks like
1452.919 -> a spiral and that's a screw dislocation
1454.899 -> and the net effect of that is when you
1457.899 -> push in this direction on an edge
1459.85 -> dislocation it moves that way when you
1462.549 -> push this direction on a screw
1464.049 -> dislocation it moves that way so when
1467.799 -> you stress out a dislocation loop it
1469.809 -> just grows you're not actually creating
1472.51 -> or destroying matter but what you're
1474.669 -> doing is causing this small loop of
1477.34 -> extra half plane of atoms to grow
1479.169 -> further and further until it actually
1480.789 -> reaches some obstacle or the outside of
1483.85 -> a crystal and these dislocations can
1487.149 -> actually feel the force from each other
1489.88 -> if I draw a clean one because I think
1492.37 -> it'll be easier to see if I draw a small
1497.62 -> lattice of atoms here and then a
1503.74 -> dislocation core right there let's say
1507.64 -> that's our dislocation core this region
1510.28 -> of space right here is compressively
1514.69 -> stressed there's more atoms in that
1516.37 -> space than there want to be and so it's
1518.559 -> kind of crammed in there
1519.669 -> well this region right here is in what's
1524.2 -> called tensile stress there's almost
1526.39 -> some space like right here where there's
1529.39 -> notice to few atoms and they kind of
1531.789 -> want there to be more and these
1533.44 -> dislocations can feel neighboring stress
1536.169 -> fields let's say there was another one
1538.179 -> right over here that had its own
1540.429 -> compressive stress field they'll
1543.64 -> actually repel each other because you
1546.61 -> don't want to add even more compressive
1548.77 -> stress to anywhere in this group of
1550.419 -> atoms so they'll actually repel each
1552.309 -> other to the point where if you get two
1554.02 -> dislocations too close to each other
1556.77 -> they'll uh they'll what's called pileup
1559.15 -> or they'll refuse to move a bit so I
1562.299 -> want to show you some videos we can
1563.74 -> actually see these dislocations in this
1565.78 -> one you see that faint line right there
1568.45 -> originating from this area that's
1571.09 -> actually a dislocation loop under stress
1574.15 -> and that's actually growing so what
1575.77 -> you're seeing here is an image of
1578.02 -> electrons passing through material and
1580.539 -> looking at regions of differing contrast
1582.28 -> so wherever there's more atoms or fewer
1584.559 -> atoms it looks darker or lighter and
1586.71 -> that can tell you what sort of defects
1588.94 -> there are you guys all see that faint
1590.53 -> line right there
1591.57 -> notice how the loop just growing it's
1593.679 -> not like you're moving a line but you're
1595.179 -> literally growing the line out of what
1596.95 -> looks like nothing there's other one
1601.78 -> what we call a a Frank Reed source it's
1604.21 -> a source of dislocation loop so what
1606.73 -> you're seeing here each of these lines
1608.08 -> is a single dislocation and then right
1610.659 -> there you see that loop suddenly form
1613.63 -> let's let's show you that one again I'll
1616.15 -> point on where to look by stressing out
1619.72 -> materials you can actually create
1621.07 -> additional dislocation loops
1623.23 -> around here and there it is you guys see
1627.19 -> that one yeah out of what looks like
1629.74 -> nothing but is actually just a couple of
1631.48 -> atomic defects you can create a
1633.4 -> dislocation loop and allow more plastic
1635.89 -> deformation to take place which i think
1638.5 -> is awesome
1640.74 -> see this one another dislocation source
1643.33 -> in germanium it's a little easier to see
1645.1 -> also because it's making this sort of
1648.1 -> spiral set of dislocations a little
1650.049 -> slower so you can track its motion a
1652.45 -> little easier notice how they all kind
1656.11 -> of line up on certain atomic planes yeah
1663.179 -> the topology will change let's say if it
1666.4 -> hits another optical or another
1667.75 -> dislocation yeah they can slam into each
1669.46 -> other and change topology all sorts of
1674.919 -> things yeah that's a subject for a whole
1677.47 -> nother class I'd say I want to skip
1681.7 -> ahead to the pileup because I think this
1685.15 -> kind of gets the point across but
1687.309 -> there's actually we can see direct
1688.87 -> evidence that dislocations feel each
1690.549 -> other's stress fields when you get
1692.23 -> enough of them lined up they won't
1694.24 -> overlap they actually push each other in
1696.73 -> a kind of dislocation traffic jam
1699.419 -> because that what's happening on the
1701.41 -> atomic level is they feel each other
1703.48 -> stress fields there might be a source of
1705.25 -> dislocations further away but when they
1707.38 -> get too close to each other it literally
1709.39 -> is a dislocation traffic jam I mean if
1711.79 -> you try and hit the car in front of you
1713.559 -> the repulsion of the electrons between
1715.66 -> your and their bumper will prevent the
1717.91 -> cars from getting a certain distance
1719.35 -> closer to each other same kind of thing
1722.08 -> here
1725.1 -> moving on to grain boundaries a
1727.14 -> two-dimensional defect anytime you have
1729.91 -> a perfect crystal of atoms that meets
1732.82 -> another perfect crystal at a different
1735.61 -> orientation where the atoms are arranged
1737.44 -> in a different direction you end up with
1739.33 -> a boundary between them that we refer to
1741.64 -> as a grain boundary so you can actually
1744.28 -> see this is a direct physical image of
1747.12 -> atoms of two different crystals meaning
1749.65 -> at the grain boundary again taken in the
1751.87 -> transmission electron microscope so for
1753.7 -> those who didn't know yes we can see
1755.049 -> individual atoms and the defect
1757.06 -> between them I definitely didn't know
1760.24 -> that in high school they didn't even
1761.29 -> mention that whatsoever did you guys
1762.88 -> ever seen images like this anyone yes
1766.18 -> raise your hand just one okay so yeah
1769.39 -> it's important for you guys to know that
1771.16 -> we can have direct evidence for all this
1772.63 -> blackboard stuff because you can see
1774.28 -> atoms in the transmission electron
1776.41 -> microscope and see what happens when the
1778.72 -> two of them meet you see this kind of
1780.52 -> regular structure of empty space where
1783.34 -> this grain boundary meets right you can
1786.43 -> actually model it as a line of 1d
1788.41 -> dislocations because you take a line of
1791.13 -> 1d lines you end up with a 2d boundary
1794.08 -> which you can see very clearly here it's
1796.21 -> almost like there's an extra half plane
1797.41 -> right there another one there another
1799.87 -> one there and another one there and we
1802.57 -> call that a tilt grain boundary grain
1805.36 -> boundaries are nice in that they can
1806.95 -> accommodate lots of these little zero
1809.53 -> dimensional defects moving to them
1811.57 -> without getting destroyed so grain
1814.3 -> boundaries are one of those ways that
1815.53 -> radiation damage can be removed and
1818.34 -> that's one of the reasons why most small
1820.81 -> grain materials are really nano grain
1823.03 -> materials are more resistant to
1824.5 -> radiation damage than large grain ones
1826.9 -> because they act as what's called sinks
1829.12 -> or destroyers of radiation damage
1832.47 -> there's another kind of 2d defect called
1836.59 -> a twin where you can actually get a
1838.54 -> little chunk of atoms sort of switch
1841.57 -> orientation and you can see these very
1843.43 -> clearly in again TEM micrographs and the
1847.42 -> evidence actually that the twin actually
1850.69 -> is a different physical arrangement of
1852.28 -> atoms even though you can't see the
1853.54 -> atoms in this little bar in this little
1856.12 -> band right there look at the way the
1858.31 -> dislocations line up those dislocations
1861.34 -> tend to line up in energetically
1863.44 -> favorable directions and in this screen
1865.09 -> they're all this way and in the twin
1868.15 -> they're all lined up like that and then
1873.1 -> finally there's the most intuitive
1874.3 -> defect inclusions a 3d piece of some
1877.54 -> other material inside what would
1879.85 -> otherwise be a pure material this one I
1882.34 -> actually pulled out of the rotor that
1885.04 -> powers the Alka Tour fusion reactor I
1887.05 -> was asked to do some analysis to find
1889.48 -> out is the structure
1890.71 -> of that road are changing because
1893.02 -> General Electric who was ensuring this
1896.14 -> rotor said we don't want to insure it
1897.669 -> anymore thanks for the premiums but
1899.23 -> we're not insuring it anymore and we
1901.809 -> said why and they said oh it's
1902.799 -> structurally unsound so we said oh yeah
1905.26 -> we'll be back in a year and we'll talk
1907.24 -> about it and we did a lot of this work
1909.909 -> to find out that actually the structure
1911.409 -> hadn't really changed since 1954 when it
1913.69 -> was made but what we did also see is we
1916.899 -> could pop out little precipitates of
1918.61 -> manganese sulfide so there's always
1921.88 -> sulfur and iron and sulfur tends to be a
1924.73 -> bad actor when it comes to material
1926.26 -> properties you throw manganese into iron
1928.929 -> to scoop up that sulfur in the form of
1931.51 -> these little precipitates or inclusions
1933.73 -> which were able to see perfectly when we
1936.37 -> did an x-ray map just like the one we
1938.11 -> did after the first exam it's like we
1940.75 -> were looking at chris's copper silver
1943.09 -> alloy mapping out where's the copper and
1945.01 -> silver
1945.429 -> I made this image the same way mapping
1947.62 -> out where is there iron manganese and
1949.09 -> sulfur that's how you can tell what it
1950.799 -> is
1952.11 -> and so dislocations and defects can
1956.08 -> actually interact let's say this is the
1957.97 -> interaction of a 1d defect a dislocation
1960.399 -> with a 3d defect avoid if you have a
1965.649 -> material that's deforming plastically
1967.69 -> very smoothly and isn't going to undergo
1969.58 -> fracture you want the dislocations to be
1971.649 -> able to move if you put anything in
1973.84 -> their way they tend to get stuck it's
1977.049 -> not easy for that dislocation to shear
1979.299 -> through a whole bunch of extra atoms and
1981.25 -> in some cases you can stop the flood
1983.74 -> that motion and favor fracture over slip
1989.039 -> so anytime you make slip harder it means
1992.649 -> that you're making fracture more likely
1994.51 -> didn't say you're making it easier but
1996.34 -> you're making it more likely and you
1998.23 -> would prefer for materials to deform a
2000.09 -> little bit by a slip then just break by
2003.39 -> fracture so I think now's a good point
2007.289 -> to go over a few key material properties
2010.1 -> all of these are sometimes used to
2013.5 -> describe the same thing in colloquial
2015.63 -> speech that is wrong does anyone hear
2019.559 -> that anyone here thought that lets say
2020.82 -> stiffness or toughness or strength meant
2022.649 -> the same thing
2024.769 -> no okay got a few people it's okay
2027.96 -> because it's used wrong all the time in
2030.539 -> colloquial speech these actually refer
2032.879 -> to different material properties with
2034.559 -> different units and we're gonna go into
2036.629 -> a little bit about what they are and
2038.279 -> then show you a few videos to test your
2041.22 -> intuition about the differences between
2042.6 -> them so first I want to mention what
2046.739 -> you're seeing right here is called a
2048.03 -> stress-strain curve stress is simple
2051.629 -> stress is just a force divided by an
2053.429 -> area and usually the criterion for wheel
2059.579 -> and material deform or will it break is
2061.679 -> does it reach a certain stress it
2064.349 -> doesn't matter just how much force you
2066.179 -> put on it but it's like how much force
2067.74 -> per atom or how much force per area
2069.71 -> determines whether bonds are gonna break
2071.669 -> and so on the y axis is stress let's say
2074.76 -> the amount of force per area we're
2077.73 -> putting in and strain is the amount of
2079.799 -> deformation so that's stress and strain
2084.99 -> is let's say the change in length over
2089.46 -> the original length of some material in
2092.159 -> what's called the engineering or
2093.419 -> simplified notation and so something
2096.119 -> that is stiff means you can put a lot of
2098.67 -> force into it but it won't deform very
2101.369 -> much that's kind of the easiest property
2103.17 -> to understand is something that's very
2105.24 -> stiff will have what's called a high
2107.46 -> Young's modulus or a high slope right
2110.25 -> here something that's super stiff like a
2112.829 -> ceramic you can really push on it quite
2116.16 -> a bit but you won't get it to deform
2118.5 -> like you would this metal so the
2120.9 -> opposite of stiff I would call compliant
2123.03 -> not soft this is one of those tricky
2126.059 -> things right there something that stiff
2127.77 -> you try and flex it and it won't flex
2129.839 -> something that's compliant you put a
2132.15 -> little bit of force into it and it
2134.16 -> undergoes some amount of strain and that
2138.03 -> slope right there between the stress and
2140.49 -> the strain we call the Youngs modulus we
2144.359 -> also note that this part right here is
2146.309 -> what's called the elastic region of
2148.74 -> deformation by elastic we mean
2150.75 -> reversible or it snaps right back so
2153.18 -> right here when I bend this bar and it
2155.46 -> snaps right back
2156.42 -> that's called elastic
2158.39 -> formation and it's reversible because
2160.069 -> you can bend one way and it snaps right
2162.71 -> back if I bent it more which I don't
2165.41 -> want to do because this is a nice
2166.76 -> zirconium fuel cladding rod you would
2169.519 -> deform it irreversibly you to bend it
2171.47 -> permanently and what the undergo what's
2173.839 -> called plastic deformation when you
2176.329 -> deviate from the slope and then a little
2178.97 -> bit more stress can cause a lot more
2180.65 -> deformation if any guys ever tried
2182.96 -> pulling copper wire apart before that's
2187.46 -> something I'd recommend you try for thin
2189.109 -> wire so you don't cut your hands well
2191.329 -> you may notice is that it's awfully hard
2193.789 -> to get the copper deforming in the first
2195.71 -> place but as soon as it starts to
2198.079 -> stretch it gets really easy so this is
2200.75 -> something I recommend go to the
2201.829 -> electronics shop or wherever and try it
2203.809 -> out on some really thin copper wire if
2206.059 -> it's thick you'll slice through your
2207.619 -> fingers and you don't want to do that
2209.71 -> strength however that's a different and
2212.69 -> that's a different metric where a
2214.16 -> stiffness describes this slope here
2216.549 -> strength describes the height or the
2220.13 -> stress at which you start to plastically
2222.319 -> deform they're in different units
2224.95 -> stiffness is in stress over strain
2228.38 -> whereas strength is given as a stress so
2232.97 -> when you hear things like the yield
2234.529 -> stress or the ultimate tensile stress
2236.329 -> that's referring to how strong something
2237.89 -> is which may have nothing to do with how
2240.019 -> stiff it is toughness is another
2243.47 -> property toughness is actually kind of
2246.079 -> like the area under this curve because
2248.809 -> if you do a force and apply it over a
2252.109 -> distance that's like putting work into
2253.819 -> the material and it ends up being a unit
2256.039 -> of energy so toughness will tell you how
2258.92 -> much energy you have to put into
2260.329 -> something before creating a new free
2262.7 -> surface otherwise known as fracture and
2265.539 -> ductility is how much can you deform it
2269.24 -> before it breaks so it would be like
2270.47 -> this point right here on the strain axis
2273.789 -> so I'll so give a little bit more
2275.779 -> examples of what this is all about
2279.039 -> toughness again is actually measured as
2282.2 -> an energy required to form a free
2284.99 -> surface or propagate a crack let's say
2288.4 -> where as something that's ductile it
2290.93 -> doesn't necessarily mean
2292.16 -> it's tough like if you have a piece of
2293.42 -> chewed chewing gum you can stretch it
2296.059 -> quite a lot with very little energy and
2298.01 -> then it's excu can say it's extremely
2299.99 -> ductile but not very strong a piece of
2303.049 -> copper wire you can also stretch at an
2305.359 -> extremely far distance but it takes more
2307.94 -> energy to do so so that's both ductile
2310.339 -> and strong and then if you apply that
2313.64 -> force over a certain distance stretching
2315.65 -> out the wire you can also reveal some of
2317.93 -> its toughness and how much energy it
2319.67 -> takes to stretch that wire before it
2322.039 -> breaks hardness is the last material
2325.789 -> property I want to mention which is not
2327.41 -> any of the ones that I showed on the
2329.839 -> stress-strain curve hardness is the
2333.049 -> resistance to a little bit of plastic
2334.819 -> deformation so assuming that you're
2337.069 -> already here how much more energy do you
2340.16 -> have to put in to get the material to
2341.66 -> deform plastically so very different
2346.22 -> material properties all try and mention
2349.01 -> all what they are so if we have a
2351.799 -> stress-strain curve like so and it
2357.26 -> follows the elastic region and then
2359.359 -> deforms plastically this point what here
2362.75 -> is what we call the yield strength
2370.539 -> whatever that point on the stress axis
2373.069 -> is this point right here our strain to
2378.26 -> failure we can use as a measure of
2380.329 -> ductility this slope right here refers
2388.7 -> to the stiffness and finally this energy
2393.65 -> right here is something like the
2395.779 -> toughness and the hardness isn't quite
2400.13 -> on this plot so I want to see if you
2403.279 -> guys intuitively understand this because
2405.89 -> the next lecture I'm going to be
2407.15 -> throwing around the words like stiffness
2409.339 -> toughness ductility hardness compliance
2411.92 -> hard soft whatever and I want to make
2414.529 -> sure that you just at least intuitively
2415.88 -> understand so there's a few videos you
2418.309 -> may have seen before anyone here watch
2419.9 -> the hydraulic press Channel
2421.39 -> there we go finally something that half
2423.77 -> the class does we are going to predict
2426.11 -> what's going to happen in each of these
2427.85 -> cases based on these material properties
2430.67 -> so in this case this is a compressor
2433.49 -> eyes cylinder of co2 it's made of
2435.8 -> aluminum
2436.43 -> which is a very ductile material it's
2439.46 -> also a very tough material what do you
2441.77 -> how do you think it will deform when
2443.24 -> smashed anyone ever tried this squishing
2452.03 -> aluminum stuff what happens you compress
2456.77 -> it and then what happens
2461.77 -> will it fracture after a while okay if
2466.27 -> you put a lot of energy into it
2467.47 -> eventually when you reach this strain to
2469.63 -> failure it should fracture but in your
2472.21 -> personal hands-on experience does
2474.46 -> aluminum tend to fracture when you bend
2476.02 -> it a little bit
2477.48 -> so then what words would you use to
2479.92 -> describe it based on this curve right
2485.11 -> here yep ductile I say yep ductile and
2491.8 -> not brittle because you can bend it
2494.14 -> quite a bit or stretch it quite a bit
2495.76 -> before it fractures
2496.84 -> how about stiffness is it really hard or
2499.42 -> really easy to get aluminum bending it's
2505.15 -> fairly easy so would you call that stiff
2506.86 -> or compliant compliant okay what about
2510.34 -> strength how hard is it to start
2512.59 -> deforming aluminum irreversibly compared
2515.83 -> to something like steel not very
2518.5 -> especially pure aluminum you can almost
2520.42 -> you can't you can chew through it if you
2522.46 -> guys ever get a 1 yen coin from Japan
2524.23 -> you can chew through it not very strong
2528.55 -> and again you know your bite force is
2530.89 -> also incredibly strong but anyway let's
2533.05 -> see what actually happens when you
2534.85 -> compress a rather ductile compliant and
2538.6 -> not that strong aluminum canister is it
2544.78 -> actually going oh it actually just get
2546.97 -> the head that's what I wanted was there
2550.21 -> sound
2557.32 -> it was also pressurized with co2 but
2562.58 -> notice what's left so actually watch in
2564.92 -> slow-mo look how much you can compress
2566.78 -> that even after the explosion no
2569.33 -> fracture if you had done that with let's
2573.05 -> say a glass canister what do you guys
2574.43 -> think would have happened would have
2576.65 -> shattered yeah we'll see that in a bit
2578.27 -> with a material that may surprise you it
2584.63 -> will fracture eventually but the
2586.88 -> hydraulic press can't get it that far in
2588.62 -> compression so that would be something
2590.48 -> that's extremely ductile not that strong
2592.97 -> cuz it wasn't that hard to deform
2594.5 -> certainly we know it wasn't stronger
2597.17 -> than the steel baseplate that they use
2598.88 -> to do the Smashing because whatever's
2601.64 -> the softer material is going to deform
2603.44 -> more so here he's gonna have well I'll
2605.57 -> let him describe it and then I'll let
2606.83 -> you guess what's going to happen
2616.22 -> you
2632.33 -> what are you guys things gonna happen
2635.65 -> we've got what looks like brass and
2637.85 -> copper coins on a steel baseplate anyone
2641.66 -> have any idea copy yeah everyone's
2647.03 -> making this motion which means
2648.29 -> everything's gonna flatten out right
2651.04 -> let's find out
2663.589 -> not nearly as much as you might have
2665.849 -> expected
2673.29 -> you
2685.26 -> is anyone surprised by this what
2689.28 -> happened there what actually happened
2692.579 -> there was already described up here when
2695.43 -> you get enough dislocations piling up
2697.53 -> against each other during plastic
2698.849 -> deformation you can undergo a process
2701.339 -> called work hardening that process can
2709.14 -> be physically described by a lot of
2710.73 -> those dislocations piling up and making
2712.77 -> it more and more difficult to continue
2715.41 -> that deformation so what happened here
2717.75 -> is the brass in the copper which started
2719.339 -> out quite soft not that hard
2721.92 -> quite ductile as you can see and not
2724.68 -> that strong actually got stronger as
2728.49 -> they were deformed interesting huh did
2732.18 -> anyone expect this to happen okay
2735.68 -> let's go to one that I think everyone
2737.82 -> can guess what's gonna happen a lead
2739.92 -> ball so has anyone ever tried playing
2743.25 -> with lead before hopefully not I have
2746.579 -> quite it ok good I'm not alone how would
2748.98 -> you describe lead in terms of the
2750.54 -> material properties here
2756.89 -> yep it's not very stiff it doesn't take
2759.9 -> much energy to start deforming it how
2761.819 -> else
2764.72 -> well is it hard or soft okay
2768.48 -> you think it's ductile or brittle yeah
2772.88 -> you think it's brittle so by that you
2775.98 -> mean it's just gonna break apart right
2777.779 -> if you deform it okay cool and would you
2783.66 -> say is tough or not tough not a lot of
2788.76 -> folks have hands-on experience with lead
2790.2 -> it's probably good for your brains let's
2793.079 -> find out
2800.72 -> you
2817.68 -> led pancake so what words would you use
2820.859 -> to describe what just happened
2823.039 -> ductile indeed I don't know what sort of
2826.349 -> brittle led was it an alloy that you had
2827.94 -> been playing with maybe uh-huh okay so
2833.279 -> it was a sheet of lead that was easy to
2834.9 -> snap so I would not call lead as a very
2837.72 -> tough material because you didn't have
2839.339 -> to put a lot of energy into it but did
2841.77 -> it deform quite a bit before you snapped
2843.93 -> it or did it just crumble apart okay so
2848.369 -> in that case I would call it ductile
2850.41 -> because it deformed a lot before
2852.72 -> breaking but I would not call it tough
2855.21 -> because it took very little energy to
2857.339 -> get it to that breaking point and it
2859.68 -> wasn't that stiff because it was quite
2861.089 -> easy to get it let's say it's the amount
2863.43 -> of energy what it's the amount of stress
2865.289 -> you put in versus the strain could be
2867.359 -> quite low and it would not be very
2869.819 -> strong because it didn't take a lot of
2871.349 -> energy or stress to get it moving let's
2874.71 -> look at another ball in this case a
2876.869 -> steel ball bearing what do you guys
2880.47 -> think is gonna happen here
2883.08 -> it's gonna shatter so you're guessing
2885.36 -> that the steel is brittle right what
2889.11 -> else
2897.59 -> probably stiff quite stiff and strong
2899.82 -> yeah I think so too but I don't think
2902.61 -> the guy that did this expected that
2911.89 -> you
2931.15 -> let's surprise anybody
2940.76 -> you
2963.109 -> yeah quite a surprise right so in this
2966.109 -> case materials like hardened steel
2968.059 -> aren't necessarily that brittle in fact
2970.279 -> you wouldn't want a ball bearing to be
2972.049 -> brittle if you get a some small chip in
2974.93 -> it or a little bit of grit or sand and
2977.45 -> the bearings you would shot are the ball
2978.92 -> bearing and cause instantaneous failure
2981.229 -> of the rotating component so what you
2983.299 -> actually want out of a high-strength
2985.16 -> ball bearing is something that's
2986.63 -> extremely hard resists deformation so it
2989.269 -> doesn't undergo let's say change of
2991.609 -> shape that would prevent it from rolling
2993.44 -> without friction or with very little
2995.359 -> friction you want it to be quite stiff
2997.339 -> because you don't want the load of
2998.839 -> whatever you're loading onto it to
3000.099 -> deform it but you also don't want it to
3002.65 -> be brittle so it's got to be somewhat
3004.809 -> tough and ductile to prevent sudden
3007.209 -> failure you'd rather it compress a tiny
3009.549 -> bit than just cracking in half so you
3012.4 -> can make things like ceramic ball
3014.079 -> bearings which are very brittle very
3017.049 -> stiff not that tough but also very
3020.44 -> strong and you just have to make sure
3022.869 -> that whatever part you make is not going
3024.64 -> to reach any sort of yield strength
3027.88 -> criterion or crack or anything now the
3031.539 -> last one that's probably the most
3033.099 -> surprising they bought a $4,000 diamond
3037.199 -> it's a diamond like that big what do you
3040.539 -> know about diamonds as a material in
3042.309 -> terms of these properties yep both is
3046.66 -> right they're extremely stiff it's the
3048.91 -> hardest material that we know of almost
3051.4 -> we've made slightly harder ones
3052.719 -> artificially it's the hardest natural
3054.759 -> material we know of what else do you
3057.16 -> know whether they're strong or tough
3060.15 -> they're not tough why do you say that
3064.24 -> and have you seen the video oh wow
3067.55 -> okay what else do we have yeah so you're
3071.21 -> saying it's not tough you can cut
3074.54 -> diamonds with other diamonds so the
3077.24 -> cutting action usually depends on the
3078.89 -> hardness of the relative hardness of the
3080.66 -> material so if you want to polish or cut
3083.15 -> something abrasively you need to use a
3085.25 -> harder material because then the grit
3086.84 -> itself won't wear away before the
3089.66 -> material it's trying to cut but what's
3091.85 -> going to happen here is we're gonna put
3093.26 -> a diamond and try compressing it and
3096.14 -> we'll see what its stress-strain curve
3097.94 -> looks like so votes are what's gonna
3100.01 -> happen who says like Monica it's gonna
3101.54 -> shatter who think it's gonna break the
3104.93 -> tools who think it's going to deform
3108.23 -> plastically yeah I've never seen a
3111.14 -> diamond deform plastically
3119.559 -> you
3163.32 -> oh yeah I could probably still sell
3168.87 -> those absolutely no deformation it just
3173.49 -> rotates and explodes yeah this would be
3177.24 -> a material that we would have say has
3178.98 -> almost zero ductility despite being
3182.82 -> extremely hard I don't know if there
3184.95 -> would even been enough deformation to
3187.23 -> have a slight dent in the tool itself
3189.21 -> there's probably a little hole where the
3190.71 -> point of the diamond poked in but once
3192.78 -> there was enough stress on that diamond
3194.15 -> its stress-strain curve would look
3196.95 -> something like that maybe like that you
3200.6 -> yeah so it's important that you
3203.7 -> intuitively understand the differences
3206.25 -> between strength ductility hardness
3208.2 -> toughness and stiffness because then
3211.35 -> next class we can explain how radiation
3213.15 -> changes them so any questions on the
3217.41 -> materials and properties from today yeah
3228.8 -> mm-hmm so the reason something would be
3231.54 -> ductile versus brittle is whether or not
3233.67 -> you can plastically deform it and that
3235.86 -> means whether or not it's more
3236.88 -> energetically favorable for dislocations
3238.98 -> to keep moving versus just breaking a
3241.14 -> plane of atoms in any irregular
3243.39 -> direction and causing fracture so again
3246.45 -> ductile ductility versus embrittlement
3248.31 -> is that is the interplay between slip
3252.06 -> and fracture slip it's normally done by
3255.06 -> dislocation movement any defects created
3257.88 -> by anything especially radiation damage
3260.07 -> will make slip harder so that any
3262.26 -> continued energy you put in will not
3264 -> move dislocations but move towards
3266.1 -> fracture so there's no other questions
3271.05 -> we'll look at the stress-strain curves
3273.3 -> of some other familiar materials anyway
3276.78 -> it is ten o'clock in case you guys have
3278.31 -> to go to other classes
3285.089 -> yes if you guys have things for nuclear
3287.92 -> activation analysis handed in do you
3290.77 -> guys bring stuff in we're running out of
3294.19 -> opportunities to do this all right in
3298.78 -> that case entry for the quiz and with
3301.93 -> the entry fee for the quiz will be your
3303.7 -> nuclear activation analysis sample
3312.7 -> you
Source: https://www.youtube.com/watch?v=CjZjVUWMEz0