1 00:00:16,082 --> 00:00:21,021 So today, we're going on from zero dimensions 2 00:00:21,021 --> 00:00:24,958 to one dimensions, and we're talking about line defects 3 00:00:24,958 --> 00:00:29,496 in order to understand what they mean. 4 00:00:29,496 --> 00:00:32,232 We're going to contextualize it in terms 5 00:00:32,232 --> 00:00:33,933 of stress/strain curve. 6 00:00:33,933 --> 00:00:37,470 So we'll talk about that. 7 00:00:37,470 --> 00:00:40,073 And all of this is called the dislocation. 8 00:00:40,073 --> 00:00:41,741 So that's our goal for today. 9 00:00:41,741 --> 00:00:45,111 Now, on Monday, after lecture actually, 10 00:00:45,111 --> 00:00:49,015 a student came up to me and asked a really good question. 11 00:00:49,015 --> 00:00:52,986 And it was related to the Hume-Rothery rule. 12 00:00:52,986 --> 00:00:56,156 Remember, on Monday, we talked about point defects, 13 00:00:56,156 --> 00:00:58,258 zero dimensional. 14 00:00:58,258 --> 00:01:01,761 So you have a localized disturbance 15 00:01:01,761 --> 00:01:03,096 in the lattice-- of vacancy. 16 00:01:03,096 --> 00:01:05,965 You take it out. 17 00:01:05,965 --> 00:01:11,304 You can have that occur in a material that's ionic. 18 00:01:11,304 --> 00:01:13,139 Not a metal, but maybe an ionic material. 19 00:01:13,139 --> 00:01:15,608 And then you got to think about charge neutrality, Schottky 20 00:01:15,608 --> 00:01:17,377 and Frenkel. 21 00:01:17,377 --> 00:01:20,547 And then we talked about substitutional defects, which 22 00:01:20,547 --> 00:01:22,949 would be where you take an atom out of the lattice 23 00:01:22,949 --> 00:01:24,283 and you put another one in. 24 00:01:24,283 --> 00:01:27,287 And Hume-Rothery, I mentioned, had come up 25 00:01:27,287 --> 00:01:33,393 with some empirical observations and so I listed these. 26 00:01:33,393 --> 00:01:36,029 So he studied a class of metals and came up 27 00:01:36,029 --> 00:01:37,597 with these general guidelines. 28 00:01:37,597 --> 00:01:40,500 And as with so many things that we've learned so far, 29 00:01:40,500 --> 00:01:45,170 these are general rules that get broken sometimes. 30 00:01:45,170 --> 00:01:47,407 So the question the student asked was a very good one. 31 00:01:47,407 --> 00:01:50,543 Said, well, if the valence of something 32 00:01:50,543 --> 00:01:52,812 you're trying to substitute into something else 33 00:01:52,812 --> 00:01:57,817 has to be the same or higher, how does p-doping work? 34 00:01:57,817 --> 00:02:00,587 How does a p-type semiconductor work? 35 00:02:00,587 --> 00:02:02,122 That's a really good question, right? 36 00:02:02,122 --> 00:02:05,158 Because the valence is lower, by definition, 37 00:02:05,158 --> 00:02:06,893 that's how you make it p-type. 38 00:02:06,893 --> 00:02:07,894 A-ha. 39 00:02:07,894 --> 00:02:10,729 So my response was, well, because Hume-Rothery 40 00:02:10,729 --> 00:02:14,567 was designed around metal, I'm not sure how much of these 41 00:02:14,567 --> 00:02:16,002 apply to semiconductors. 42 00:02:16,002 --> 00:02:18,238 Although for semiconductors, you do want to make sure 43 00:02:18,238 --> 00:02:21,074 the sizes are similar, at the very least. 44 00:02:21,074 --> 00:02:22,275 But that got me thinking. 45 00:02:22,275 --> 00:02:26,579 And so I went and I read some papers last night. 46 00:02:26,579 --> 00:02:29,015 Here's one on the valence effects 47 00:02:29,015 --> 00:02:31,684 and relative stabilities-- valency effects. 48 00:02:31,684 --> 00:02:33,720 So this is from the '80s and I just 49 00:02:33,720 --> 00:02:35,588 wanted to cite something here. 50 00:02:35,588 --> 00:02:39,192 So here they say almost a half century ago-- 51 00:02:39,192 --> 00:02:43,463 so that's Hume-Rothery, so it's the '80s or the '30s-- 52 00:02:43,463 --> 00:02:45,698 pioneered the study of the effect of valency 53 00:02:45,698 --> 00:02:47,333 on metal alloy phase equilibria. 54 00:02:47,333 --> 00:02:50,270 They found that a lower valence metal, like silver, 55 00:02:50,270 --> 00:02:53,973 had only a very small solubility in a higher valence element, 56 00:02:53,973 --> 00:02:56,476 like antimony, but that the solubility of the reverse 57 00:02:56,476 --> 00:02:57,076 was large. 58 00:02:57,076 --> 00:02:58,878 So that's where that rule came from. 59 00:02:58,878 --> 00:03:01,981 And here's the next part of this paper I want to share. 60 00:03:01,981 --> 00:03:04,317 "Although the relative valency works well 61 00:03:04,317 --> 00:03:06,119 for that select group of alloys, it 62 00:03:06,119 --> 00:03:08,454 does not apply to the periodic table as a whole." 63 00:03:08,454 --> 00:03:12,892 So this other guy looked at 607 different systems 64 00:03:12,892 --> 00:03:15,695 and found that 22% of the time, the vacancy rule 65 00:03:15,695 --> 00:03:19,532 works, and 27% of the time, it violated the rule. 66 00:03:19,532 --> 00:03:21,935 And honestly, I'm not sure we can call it a rule 67 00:03:21,935 --> 00:03:25,204 anymore if more times than not, it's violated. 68 00:03:25,204 --> 00:03:26,673 This is a new thing for me. 69 00:03:26,673 --> 00:03:34,847 So that is just to say that this rule probably 70 00:03:34,847 --> 00:03:39,285 is really conditional on the specific elements you're 71 00:03:39,285 --> 00:03:40,119 talking about. 72 00:03:40,119 --> 00:03:44,390 I think the much more important ones are these three here. 73 00:03:44,390 --> 00:03:47,327 The valency rule certainly depends on the system. 74 00:03:47,327 --> 00:03:51,164 And all of the Hume-Rothery observations were empirical-- 75 00:03:51,164 --> 00:03:52,632 it's important to keep in mind. 76 00:03:52,632 --> 00:03:56,668 General guidelines meant to be broken more than half 77 00:03:56,668 --> 00:03:57,570 the time, apparently. 78 00:03:57,570 --> 00:03:58,238 [CHUCKLES] 79 00:03:59,939 --> 00:04:01,608 So where were we? 80 00:04:01,608 --> 00:04:02,942 But thank you for that question. 81 00:04:02,942 --> 00:04:03,943 It was a great question. 82 00:04:03,943 --> 00:04:06,779 It gave me some great reading last night-- materials. 83 00:04:09,716 --> 00:04:11,618 Zero dimensional, Monday. 84 00:04:11,618 --> 00:04:13,219 One dimensional, today. 85 00:04:13,219 --> 00:04:14,587 There we are. 86 00:04:14,587 --> 00:04:15,788 So that's the focus. 87 00:04:15,788 --> 00:04:19,792 Now, the story today starts at MIT. 88 00:04:19,792 --> 00:04:22,862 It starts at MIT in the 1940s. 89 00:04:22,862 --> 00:04:24,831 Because there were these two students-- 90 00:04:24,831 --> 00:04:30,236 there they are, Harold Hindman and George Burr. 91 00:04:30,236 --> 00:04:34,540 MIT students back then were a little bit older, I guess. 92 00:04:34,540 --> 00:04:37,510 So those guys, they were in an MIT lab 93 00:04:37,510 --> 00:04:42,048 and they were trying to study how parachutes break. 94 00:04:42,048 --> 00:04:43,716 How does a parachute break? 95 00:04:43,716 --> 00:04:46,286 Because why is that an important question? 96 00:04:46,286 --> 00:04:48,855 Because you don't want it to break. 97 00:04:48,855 --> 00:04:49,489 Right? 98 00:04:49,489 --> 00:04:50,990 And it's one of these things where 99 00:04:50,990 --> 00:04:53,393 you want it to never break. 100 00:04:53,393 --> 00:04:56,562 Like, you really want almost 100-- 101 00:04:56,562 --> 00:04:58,464 so they were studying why it breaks. 102 00:04:58,464 --> 00:05:00,900 So they were taking a parachute and pulling it apart, 103 00:05:00,900 --> 00:05:02,235 and there it broke. 104 00:05:02,235 --> 00:05:05,004 Take another kind of material, pull that apart. 105 00:05:05,004 --> 00:05:05,905 It broke. 106 00:05:05,905 --> 00:05:08,775 This one didn't, but it's too heavy. 107 00:05:08,775 --> 00:05:11,110 They were studying the relationship 108 00:05:11,110 --> 00:05:16,783 of the mechanical breaking to the material of the parachute. 109 00:05:16,783 --> 00:05:18,384 I love that logo there. 110 00:05:18,384 --> 00:05:19,919 That's the two of them in the lab 111 00:05:19,919 --> 00:05:22,689 at MIT pulling on a parachute. 112 00:05:22,689 --> 00:05:25,358 They just have a tug of war with a parachute. 113 00:05:25,358 --> 00:05:28,261 But the problem was that there was 114 00:05:28,261 --> 00:05:32,298 no instrument at the time that could give them 115 00:05:32,298 --> 00:05:36,569 the sensitivity to the force that they were pulling. 116 00:05:36,569 --> 00:05:40,606 There was no instrument that could capture what they needed 117 00:05:40,606 --> 00:05:42,442 to make a better parachute. 118 00:05:42,442 --> 00:05:46,279 So being MIT students, they built their own. 119 00:05:46,279 --> 00:05:47,747 They built their own and they added 120 00:05:47,747 --> 00:05:50,850 a whole bunch of electronics into this instrument. 121 00:05:50,850 --> 00:05:52,452 And that is where the name comes from. 122 00:05:52,452 --> 00:05:54,454 It's an instrument that had a lot of electronics 123 00:05:54,454 --> 00:05:55,021 for the time. 124 00:05:55,021 --> 00:05:57,423 Instron is the name of their company. 125 00:05:57,423 --> 00:06:02,362 Instron is now synonymous with stress/strain curves. 126 00:06:02,362 --> 00:06:04,697 So you go into any university, any company, 127 00:06:04,697 --> 00:06:07,300 you look at how-- if they need to measure something, 128 00:06:07,300 --> 00:06:12,772 like whether an egg breaks or not at what force, 129 00:06:12,772 --> 00:06:15,274 it's an Instron machine. 130 00:06:15,274 --> 00:06:16,275 It's an Instron machine. 131 00:06:16,275 --> 00:06:19,545 There it is-- same logo. 132 00:06:19,545 --> 00:06:22,048 And because those machines give you 133 00:06:22,048 --> 00:06:28,654 the most accurate and well-calibrated description 134 00:06:28,654 --> 00:06:32,592 of the force that you're applying on this material 135 00:06:32,592 --> 00:06:34,327 and you can apply it very, very slowly-- 136 00:06:34,327 --> 00:06:41,367 so these are very common machines to look at the force 137 00:06:41,367 --> 00:06:45,405 you apply, to take it how much distance. 138 00:06:45,405 --> 00:06:47,507 So we're going to talk about that today. 139 00:06:47,507 --> 00:06:52,044 Now, we're not going to talk about it in terms of eggs. 140 00:06:52,044 --> 00:06:54,247 We're going talk about it in terms of a wire. 141 00:06:54,247 --> 00:06:55,047 So there's a wire. 142 00:06:55,047 --> 00:06:56,382 It's a little hard to see there. 143 00:06:56,382 --> 00:06:56,883 There it is. 144 00:06:56,883 --> 00:06:58,418 There's another Instron machine-- 145 00:06:58,418 --> 00:07:00,353 this is a red one. 146 00:07:00,353 --> 00:07:02,655 And here they are and they're looking at this wire. 147 00:07:02,655 --> 00:07:04,957 And this wire is some material-- maybe it's copper, 148 00:07:04,957 --> 00:07:06,659 maybe it's aluminum. 149 00:07:06,659 --> 00:07:10,029 And they're saying, what happens to this thing 150 00:07:10,029 --> 00:07:14,634 as I just pull it in small increments? 151 00:07:14,634 --> 00:07:16,836 What is the force that I have to put on this 152 00:07:16,836 --> 00:07:18,004 to pull it a certain amount? 153 00:07:18,004 --> 00:07:23,443 Now, that is an important graph, and it's so important 154 00:07:23,443 --> 00:07:29,549 that I really want you guys to understand this graph. 155 00:07:29,549 --> 00:07:32,919 Because it is the way that mechanical properties 156 00:07:32,919 --> 00:07:37,223 of materials are first often analyzed 157 00:07:37,223 --> 00:07:41,227 and it's a plot of the stress, which 158 00:07:41,227 --> 00:07:48,501 is equal to the force per area versus the strain, which 159 00:07:48,501 --> 00:07:52,638 is equal to the change in the length divided 160 00:07:52,638 --> 00:07:53,873 by the original length. 161 00:07:53,873 --> 00:07:54,941 Now, what I mean by that? 162 00:07:54,941 --> 00:07:55,942 Well, let's take a wire. 163 00:07:55,942 --> 00:07:57,243 Here's the wire. 164 00:07:57,243 --> 00:07:58,945 There's that wire in there. 165 00:07:58,945 --> 00:07:59,745 OK. 166 00:07:59,745 --> 00:08:01,514 So here's a wire. 167 00:08:01,514 --> 00:08:04,884 And I'm going to apply a force this way. 168 00:08:04,884 --> 00:08:07,153 That's what the Instron is doing. 169 00:08:07,153 --> 00:08:09,522 So there's some force being applied to it 170 00:08:09,522 --> 00:08:12,191 and there's some area of the wire 171 00:08:12,191 --> 00:08:15,928 and there was some original length. 172 00:08:15,928 --> 00:08:17,730 OK. 173 00:08:17,730 --> 00:08:20,032 This is called a stress/strain curve. 174 00:08:20,032 --> 00:08:24,470 It's the force divided by the area 175 00:08:24,470 --> 00:08:29,775 that you apply to get the thing to stretch a certain amount. 176 00:08:29,775 --> 00:08:35,481 Now, those MIT students were also very smart. 177 00:08:35,481 --> 00:08:39,986 They knew that many materials, as you first 178 00:08:39,986 --> 00:08:42,822 start stretching them, the bonds between the atoms 179 00:08:42,822 --> 00:08:46,325 are kind of elastic. 180 00:08:46,325 --> 00:08:47,226 What does that mean? 181 00:08:47,226 --> 00:08:50,329 Well, it means that if I stretch it a little bit, it goes back. 182 00:08:50,329 --> 00:08:52,398 Might stretch it a little bit more-- it goes back. 183 00:08:52,398 --> 00:08:54,600 They're like springs. 184 00:08:54,600 --> 00:08:58,404 So when you think about the deformation 185 00:08:58,404 --> 00:09:02,375 of that wire and many other materials, 186 00:09:02,375 --> 00:09:07,613 you've got this elastic regime. 187 00:09:07,613 --> 00:09:10,917 And that means that it's reversible, as I just said. 188 00:09:10,917 --> 00:09:11,751 Reversible. 189 00:09:15,955 --> 00:09:20,092 And the displacement-- so it's reversible displacement. 190 00:09:20,092 --> 00:09:22,828 If I stretch it, it goes back. 191 00:09:22,828 --> 00:09:28,568 Reversible displacement only occurring 192 00:09:28,568 --> 00:09:31,237 under some applied force. 193 00:09:31,237 --> 00:09:36,008 Applied force. 194 00:09:38,544 --> 00:09:41,647 This is an elastic stretch. 195 00:09:41,647 --> 00:09:43,449 If I've stretched this material elastically 196 00:09:43,449 --> 00:09:45,718 and I let it go and it goes back, it's like a spring. 197 00:09:45,718 --> 00:09:48,054 Hooke's law is going to apply. 198 00:09:48,054 --> 00:09:49,422 So Hooke's law applies. 199 00:09:49,422 --> 00:09:55,194 So F equals kx or minus kx. 200 00:09:55,194 --> 00:09:56,596 You can press it or-- 201 00:09:56,596 --> 00:09:57,096 OK. 202 00:09:57,096 --> 00:09:58,664 Hooke's law. 203 00:09:58,664 --> 00:10:03,035 So that means that if I start pulling on this 204 00:10:03,035 --> 00:10:05,438 and I start here-- 205 00:10:05,438 --> 00:10:09,909 so there's no stress and there's no strain, 206 00:10:09,909 --> 00:10:13,512 and if I start pulling on this, then I'm 207 00:10:13,512 --> 00:10:19,352 going to get a straight line, like that. 208 00:10:19,352 --> 00:10:27,460 So that's going to be the elastic regime. 209 00:10:27,460 --> 00:10:29,195 But these MIT students-- 210 00:10:29,195 --> 00:10:31,897 the thing is, they knew something else. 211 00:10:31,897 --> 00:10:35,968 They knew that for a lot of materials, 212 00:10:35,968 --> 00:10:38,537 it didn't keep going like this. 213 00:10:38,537 --> 00:10:44,076 It didn't keep going linearly in this nice, elastic spring way. 214 00:10:44,076 --> 00:10:49,482 They knew that either the thing broke completely 215 00:10:49,482 --> 00:10:51,450 or it deformed-- 216 00:10:51,450 --> 00:10:52,585 or it started deforming. 217 00:10:52,585 --> 00:10:56,722 And that is called the plastic regime. 218 00:10:56,722 --> 00:10:59,191 We're coming to plastics later. 219 00:10:59,191 --> 00:11:03,529 And this is a permanent shape change. 220 00:11:03,529 --> 00:11:06,032 Permanent shape change. 221 00:11:11,971 --> 00:11:14,340 And you can imagine, if I'm going 222 00:11:14,340 --> 00:11:17,109 to permanently change the shape of a material, 223 00:11:17,109 --> 00:11:18,978 it's got to be ductile. 224 00:11:18,978 --> 00:11:23,049 So if it's something that's going to break-- 225 00:11:23,049 --> 00:11:25,451 remember, we talked about ionic versus metallic. 226 00:11:29,088 --> 00:11:35,261 The sea of electrons allows those metals to be ductile. 227 00:11:35,261 --> 00:11:36,996 And that has to do with the bonding 228 00:11:36,996 --> 00:11:38,764 with the electronic structure. 229 00:11:38,764 --> 00:11:41,000 If you want a material to be able to change shape 230 00:11:41,000 --> 00:11:43,135 permanently and not crack, you need 231 00:11:43,135 --> 00:11:45,504 that ability to be ductile. 232 00:11:45,504 --> 00:11:48,307 So there's a plastic regime. 233 00:11:48,307 --> 00:11:54,180 And that makes this change dramatically. 234 00:11:54,180 --> 00:11:57,717 So what happens now is, if I keep stretching it 235 00:11:57,717 --> 00:12:00,953 beyond this elastic regime, the stress/strain curve 236 00:12:00,953 --> 00:12:02,254 is no longer linear. 237 00:12:02,254 --> 00:12:06,525 And in fact, it might look something like this 238 00:12:06,525 --> 00:12:10,730 until, at some point-- so this would be the plastic part where 239 00:12:10,730 --> 00:12:15,000 it's literally deforming, it's changing its shape. 240 00:12:15,000 --> 00:12:17,103 And then finally, I've stretched it 241 00:12:17,103 --> 00:12:18,904 to the point where it fractures. 242 00:12:18,904 --> 00:12:20,573 So that's called the fracture point-- 243 00:12:20,573 --> 00:12:22,475 you just can't pull it anymore. 244 00:12:25,077 --> 00:12:27,146 You just can't pull it anymore. 245 00:12:27,146 --> 00:12:29,215 And they're studying parachutes, but they 246 00:12:29,215 --> 00:12:32,084 knew that there were a lot of surprises out there, 247 00:12:32,084 --> 00:12:34,220 there's a lot of complexity in this. 248 00:12:34,220 --> 00:12:37,723 And they needed to understand this regime and this regime 249 00:12:37,723 --> 00:12:41,694 in order to really engineer the parachute to make it better. 250 00:12:41,694 --> 00:12:44,597 They had to get that data and there were no instruments 251 00:12:44,597 --> 00:12:48,000 that could move it and measure it and calibrate it 252 00:12:48,000 --> 00:12:50,603 carefully enough. 253 00:12:50,603 --> 00:12:58,644 By the way, this plastic deformation that happens-- 254 00:12:58,644 --> 00:13:03,883 this permanent shape change, but without breaking, that 255 00:13:03,883 --> 00:13:07,219 happens very early on. 256 00:13:07,219 --> 00:13:08,954 So there were people working, for example, 257 00:13:08,954 --> 00:13:13,325 with aluminum, saying, well, given the bond 258 00:13:13,325 --> 00:13:18,430 strength of aluminum, I predict a certain mechanical strength 259 00:13:18,430 --> 00:13:20,199 that aluminum could go to. 260 00:13:20,199 --> 00:13:24,069 And then they find that it starts deforming 100 times 261 00:13:24,069 --> 00:13:25,871 lower than that. 262 00:13:25,871 --> 00:13:27,973 100 times lower. 263 00:13:27,973 --> 00:13:28,841 Why? 264 00:13:28,841 --> 00:13:32,344 Because it enters into this plastic regime. 265 00:13:32,344 --> 00:13:34,180 It doesn't stay elastic. 266 00:13:34,180 --> 00:13:38,317 So what I want to talk about today is how-- 267 00:13:38,317 --> 00:13:40,319 I hope I've convinced you how important this is, 268 00:13:40,319 --> 00:13:44,490 and I'll have a why this matters on that as well. 269 00:13:44,490 --> 00:13:46,292 But what I want to talk about today 270 00:13:46,292 --> 00:13:51,697 is how these one dimensional defects fit in with this. 271 00:13:51,697 --> 00:13:53,399 How these one dimensional defects fit in. 272 00:13:53,399 --> 00:13:56,702 And in fact, they explain it all. 273 00:13:56,702 --> 00:13:57,937 They explain it all. 274 00:13:57,937 --> 00:14:00,472 This is what the wire looks like. 275 00:14:00,472 --> 00:14:04,643 When you stretch a wire and you look at it carefully, 276 00:14:04,643 --> 00:14:08,681 it's not just a little bit longer. 277 00:14:08,681 --> 00:14:14,620 It has these very set features that occur. 278 00:14:14,620 --> 00:14:18,390 It's not just deforming in any random way. 279 00:14:18,390 --> 00:14:21,994 It's deforming in a very specific way. 280 00:14:21,994 --> 00:14:24,997 It's deforming in a very specific way. 281 00:14:24,997 --> 00:14:29,235 And so what I want to talk about is that way. 282 00:14:33,105 --> 00:14:38,177 The plastic deformation mechanism is called slip. 283 00:14:38,177 --> 00:14:44,817 The mechanism is called slip that 284 00:14:44,817 --> 00:14:47,586 leads to plastic deformation. 285 00:14:47,586 --> 00:14:55,327 And it leads to that picture and it leads to the data 286 00:14:55,327 --> 00:14:59,164 that they got from the Instron machine. 287 00:14:59,164 --> 00:15:01,767 So where does it come from? 288 00:15:01,767 --> 00:15:05,571 This is a 2D picture of a lattice. 289 00:15:05,571 --> 00:15:09,575 This is a 2D picture of a lattice. 290 00:15:09,575 --> 00:15:12,278 So you can imagine what I've done-- oh, I'm 291 00:15:12,278 --> 00:15:13,312 going to hand this out. 292 00:15:13,312 --> 00:15:13,846 I love this. 293 00:15:13,846 --> 00:15:15,381 I can just sit and look at this-- 294 00:15:18,217 --> 00:15:19,218 this is so cool. 295 00:15:19,218 --> 00:15:21,320 So this is a cubic-- 296 00:15:21,320 --> 00:15:23,355 you can think about it as a simple cubic lattice. 297 00:15:23,355 --> 00:15:28,928 Here's a plane, here's another plane, there's another one. 298 00:15:28,928 --> 00:15:30,729 Look at that yellow one there. 299 00:15:30,729 --> 00:15:31,630 So many planes. 300 00:15:34,300 --> 00:15:38,237 I got lost there just thinking about it. 301 00:15:38,237 --> 00:15:40,739 I'm going to the other side. 302 00:15:40,739 --> 00:15:44,777 You could get lost, too. 303 00:15:44,777 --> 00:15:46,712 That's one of the planes. 304 00:15:46,712 --> 00:15:47,746 That's one of the planes. 305 00:15:47,746 --> 00:15:48,580 Imagine it. 306 00:15:48,580 --> 00:15:52,584 Now, you notice here, I've got hanging out-- because why not? 307 00:15:52,584 --> 00:15:55,087 Oh, why not? 308 00:15:55,087 --> 00:15:57,957 Because there's no other possibility. 309 00:15:57,957 --> 00:16:02,494 Because vacancies always exist, as we learned Monday. 310 00:16:02,494 --> 00:16:04,596 They cannot not exist. 311 00:16:04,596 --> 00:16:06,098 So there is one right there. 312 00:16:06,098 --> 00:16:06,865 Cool. 313 00:16:06,865 --> 00:16:09,568 There's an interstitial point defect, 314 00:16:09,568 --> 00:16:11,737 there's a substitutional point defect. 315 00:16:11,737 --> 00:16:13,038 That's all from Monday. 316 00:16:13,038 --> 00:16:14,206 They're just hanging out. 317 00:16:14,206 --> 00:16:16,008 But here's what we're talking about now. 318 00:16:16,008 --> 00:16:16,842 Look at this. 319 00:16:16,842 --> 00:16:19,478 I want to take this little symbol-- 320 00:16:19,478 --> 00:16:22,681 this T, and I want to look at the lines that 321 00:16:22,681 --> 00:16:23,615 come down from it. 322 00:16:23,615 --> 00:16:24,883 So there they are. 323 00:16:24,883 --> 00:16:27,920 So these are now planes. 324 00:16:27,920 --> 00:16:29,221 These are planes. 325 00:16:29,221 --> 00:16:30,656 These are crystallographic planes. 326 00:16:30,656 --> 00:16:33,225 And now, look at the other side of it 327 00:16:33,225 --> 00:16:37,663 and notice those same planes are following-- 328 00:16:37,663 --> 00:16:39,131 see to the left there? 329 00:16:39,131 --> 00:16:41,300 You see how that's now a plane, a plane? 330 00:16:41,300 --> 00:16:44,837 So this is actually lined up from the outside in. 331 00:16:44,837 --> 00:16:46,038 It's lined up, it's lined up. 332 00:16:46,038 --> 00:16:46,739 Ah! 333 00:16:46,739 --> 00:16:49,408 Right here, I put an extra one in. 334 00:16:49,408 --> 00:16:51,377 You see that? 335 00:16:51,377 --> 00:16:54,046 That's an extra plane that doesn't go all the way through. 336 00:16:54,046 --> 00:16:57,683 If it went all the way through, it wouldn't be an extra plane. 337 00:16:57,683 --> 00:17:00,786 So it's sandwiched in here. 338 00:17:00,786 --> 00:17:02,521 And then here, it continues over there. 339 00:17:02,521 --> 00:17:05,290 That one kind of goes like that, and that one goes like that, 340 00:17:05,290 --> 00:17:08,193 and then they go back to the normal planes. 341 00:17:08,193 --> 00:17:12,297 I have inserted a half-- if you want to think about it-- 342 00:17:12,297 --> 00:17:15,034 of a plane into-- 343 00:17:15,034 --> 00:17:16,167 right there. 344 00:17:16,167 --> 00:17:17,301 Look at that. 345 00:17:17,301 --> 00:17:20,606 That is called a dislocation. 346 00:17:20,606 --> 00:17:22,307 That is a one dimensional defect. 347 00:17:22,307 --> 00:17:24,977 It's a dislocation that is caused 348 00:17:24,977 --> 00:17:29,448 by putting an extra plane that stopped somewhere, 349 00:17:29,448 --> 00:17:35,054 and where it stops, you draw a little T in the crystal. 350 00:17:35,054 --> 00:17:36,388 Oh, let's go with the mean. 351 00:17:36,388 --> 00:17:36,989 Come on. 352 00:17:36,989 --> 00:17:37,656 There we go. 353 00:17:37,656 --> 00:17:38,757 We'll go back to this. 354 00:17:38,757 --> 00:17:41,894 So that is called a dislocation. 355 00:17:41,894 --> 00:17:44,129 That is called a dislocation. 356 00:17:44,129 --> 00:17:47,499 That is the 1D defect. 357 00:17:47,499 --> 00:17:53,539 It's called a dislocation. 358 00:17:53,539 --> 00:18:04,917 It's a line defect formed by what is essentially 359 00:18:04,917 --> 00:18:09,588 a misregistry of atoms that gives you an extra plane. 360 00:18:19,531 --> 00:18:23,168 Now, you can already see that if I put a line defect in there, 361 00:18:23,168 --> 00:18:26,538 it's messing with the bonds at this place here. 362 00:18:26,538 --> 00:18:28,507 Here, the bonds just go along, they go along. 363 00:18:28,507 --> 00:18:31,777 And then all of a sudden, I've got a misregistry now because 364 00:18:31,777 --> 00:18:34,613 of that dislocation-- 365 00:18:34,613 --> 00:18:36,548 because of that dislocation. 366 00:18:39,418 --> 00:18:41,553 One way to look at it is with the model. 367 00:18:41,553 --> 00:18:45,124 Another way is just get ears of corn. 368 00:18:45,124 --> 00:18:46,425 I did this last night. 369 00:18:46,425 --> 00:18:50,329 I actually-- I got ears of corn. 370 00:18:50,329 --> 00:18:51,763 That's really true. 371 00:18:51,763 --> 00:18:53,232 And I looked inside. 372 00:18:53,232 --> 00:18:54,733 This isn't from there. 373 00:18:54,733 --> 00:18:56,869 This is from the internet. 374 00:18:56,869 --> 00:18:58,871 But there it is. 375 00:18:58,871 --> 00:19:02,875 The corn needed to grow another row and so-- 376 00:19:02,875 --> 00:19:04,443 it's not going to grow a half kernel. 377 00:19:04,443 --> 00:19:05,477 Well, actually, it could. 378 00:19:05,477 --> 00:19:06,278 It does sometimes. 379 00:19:06,278 --> 00:19:08,614 But here, it grew another full row of kernels. 380 00:19:08,614 --> 00:19:09,314 And there it is. 381 00:19:09,314 --> 00:19:11,583 There's a dislocation. 382 00:19:11,583 --> 00:19:14,820 It's very similar to what we're looking at. 383 00:19:14,820 --> 00:19:18,891 Now, there are two types of dislocations. 384 00:19:18,891 --> 00:19:21,927 This one, where you insert an extra plane in this way 385 00:19:21,927 --> 00:19:25,497 is called an edge dislocation. 386 00:19:25,497 --> 00:19:26,965 There's another type of dislocation 387 00:19:26,965 --> 00:19:29,234 that we won't talk about in this class called the screw 388 00:19:29,234 --> 00:19:30,135 dislocation. 389 00:19:30,135 --> 00:19:30,936 You can look it up. 390 00:19:30,936 --> 00:19:34,439 It's a different type of line defect. 391 00:19:34,439 --> 00:19:36,241 But the one that we're concerned with 392 00:19:36,241 --> 00:19:38,343 is called an edge dislocation and it's 393 00:19:38,343 --> 00:19:39,912 described exactly in this way. 394 00:19:39,912 --> 00:19:43,982 And the way you note it in an atomic scale 395 00:19:43,982 --> 00:19:44,950 drawing like this-- 396 00:19:44,950 --> 00:19:50,756 by putting that upside down T where the edge is. 397 00:19:50,756 --> 00:19:53,025 An edge dislocation. 398 00:19:53,025 --> 00:19:54,660 Edge dislocations in corn. 399 00:19:54,660 --> 00:19:56,962 Now, you can actually see these things. 400 00:19:56,962 --> 00:19:59,498 And on these models, the dislocations 401 00:19:59,498 --> 00:20:03,835 look very nice and uniform and you can hold it and look at it. 402 00:20:03,835 --> 00:20:10,275 But in reality, here's an actual video of dislocations. 403 00:20:10,275 --> 00:20:12,411 These are groups of dislocations. 404 00:20:12,411 --> 00:20:14,980 Notice, these are 1D line defects. 405 00:20:14,980 --> 00:20:19,451 They're places where you've got this extra plane. 406 00:20:19,451 --> 00:20:22,154 Notice, that they're all over the place. 407 00:20:22,154 --> 00:20:27,059 They're not just straight, they bend around in the crystal 408 00:20:27,059 --> 00:20:28,126 and they move. 409 00:20:28,126 --> 00:20:30,529 And the movement is critical. 410 00:20:30,529 --> 00:20:32,197 The movement is critical. 411 00:20:32,197 --> 00:20:36,768 That's what's going to get us back to this, the movement. 412 00:20:36,768 --> 00:20:41,773 This is the defect, and then the movement is what gets us this. 413 00:20:41,773 --> 00:20:42,641 So let's take a look. 414 00:20:42,641 --> 00:20:44,743 So here's an here's an example. 415 00:20:44,743 --> 00:20:47,179 This is a piece of material that under the microscope 416 00:20:47,179 --> 00:20:48,413 is being pulled. 417 00:20:48,413 --> 00:20:50,549 Watch what happens. 418 00:20:50,549 --> 00:20:52,317 Well, those things are moving. 419 00:20:52,317 --> 00:20:55,187 They're forming, they're ending, they're reforming, 420 00:20:55,187 --> 00:20:57,022 they're interacting with each other. 421 00:20:57,022 --> 00:20:59,024 That one didn't, but whatever. 422 00:20:59,024 --> 00:21:01,627 And they're spreading out, they're growing. 423 00:21:01,627 --> 00:21:07,399 Those dislocations and their movement 424 00:21:07,399 --> 00:21:09,568 is why this doesn't crack. 425 00:21:09,568 --> 00:21:13,405 It's why this can move, this can bend. 426 00:21:13,405 --> 00:21:17,743 It's why this point is 100 times lower 427 00:21:17,743 --> 00:21:18,910 than you might have thought. 428 00:21:21,513 --> 00:21:26,318 And that can be understood by thinking about the bonds. 429 00:21:26,318 --> 00:21:29,921 That can be understood by thinking about the bonds. 430 00:21:29,921 --> 00:21:31,223 Here's a sequence of pictures. 431 00:21:34,359 --> 00:21:38,297 So this is the dislocation. 432 00:21:38,297 --> 00:21:40,632 You see it there. 433 00:21:40,632 --> 00:21:44,169 Now, what I'm doing is, I'm pushing-- 434 00:21:44,169 --> 00:21:46,571 or let's not push yet. 435 00:21:46,571 --> 00:21:48,340 We'll do that on the next slide. 436 00:21:48,340 --> 00:21:50,409 For now, I'm just watching it move. 437 00:21:50,409 --> 00:21:52,844 Here you see it moving. 438 00:21:52,844 --> 00:21:53,345 There it is. 439 00:21:53,345 --> 00:21:54,746 How does it move? 440 00:21:54,746 --> 00:21:56,581 Well, here's a model. 441 00:21:56,581 --> 00:22:00,619 There is a dislocation and you can see that if I take this 442 00:22:00,619 --> 00:22:05,324 bonding area around here and I connect this atom to there 443 00:22:05,324 --> 00:22:07,759 and then you start pushing-- there's a strain field around 444 00:22:07,759 --> 00:22:08,293 there-- 445 00:22:08,293 --> 00:22:10,062 and then you push it over, you might be 446 00:22:10,062 --> 00:22:13,699 able to move it over to there. 447 00:22:13,699 --> 00:22:16,034 And maybe you could keep on doing that 448 00:22:16,034 --> 00:22:18,270 and keep on moving it over and over until it 449 00:22:18,270 --> 00:22:20,272 gets all the way to that end-- 450 00:22:20,272 --> 00:22:21,907 all the way to the end of the material. 451 00:22:25,444 --> 00:22:30,549 Here is a super polished, high detail 452 00:22:30,549 --> 00:22:33,518 animation of that process. 453 00:22:33,518 --> 00:22:34,286 Here it is. 454 00:22:34,286 --> 00:22:37,923 I'm starting over here and watch the dislocation. 455 00:22:37,923 --> 00:22:38,423 There it is. 456 00:22:38,423 --> 00:22:40,625 There it is! 457 00:22:40,625 --> 00:22:44,429 And it's moving and it's moving, and those bonds are breaking. 458 00:22:44,429 --> 00:22:46,431 Let's watch that again. 459 00:22:46,431 --> 00:22:47,032 Here it is. 460 00:22:47,032 --> 00:22:48,233 There, it formed. 461 00:22:48,233 --> 00:22:50,435 But why did it form? 462 00:22:50,435 --> 00:22:55,907 Because, you see, what happened is, I took this material 463 00:22:55,907 --> 00:22:58,777 and I applied a force to it. 464 00:22:58,777 --> 00:23:04,416 I went back to the wire and I did this or maybe I did this. 465 00:23:04,416 --> 00:23:06,718 And the material said, OK, hold on, how 466 00:23:06,718 --> 00:23:08,720 can I respond to this thing? 467 00:23:08,720 --> 00:23:10,956 Well, either I can respond elastically 468 00:23:10,956 --> 00:23:12,958 and all my bonds could stretch, but I'm 469 00:23:12,958 --> 00:23:17,462 getting to this uncomfortable place here. 470 00:23:17,462 --> 00:23:21,233 Or a dislocation could come in and allow 471 00:23:21,233 --> 00:23:25,837 me to translate a whole set of bonds over by one. 472 00:23:25,837 --> 00:23:29,474 So I'm applying a force to this material. 473 00:23:29,474 --> 00:23:34,613 You can think about it as a force maybe on the bottom g-- 474 00:23:34,613 --> 00:23:37,416 here, let's watch what happens. 475 00:23:37,416 --> 00:23:37,949 Look at this. 476 00:23:37,949 --> 00:23:40,352 I've got a little extra room here. 477 00:23:40,352 --> 00:23:41,353 Now watch what happens. 478 00:23:41,353 --> 00:23:45,056 There it is, there it is. 479 00:23:45,056 --> 00:23:48,126 And I have moved the whole top row 480 00:23:48,126 --> 00:23:51,596 of atoms over-- the whole top three rows over by one. 481 00:23:51,596 --> 00:23:54,566 So I have transformed this so that the force-- 482 00:23:54,566 --> 00:23:56,468 if you think about it, the force on the bottom 483 00:23:56,468 --> 00:23:59,337 could be applied that way and the force on the top that way. 484 00:23:59,337 --> 00:24:01,139 The atoms get to a certain point where it's 485 00:24:01,139 --> 00:24:03,675 like, no, my elasticnesss-- 486 00:24:03,675 --> 00:24:05,710 I'm not sure I want to go anymore. 487 00:24:05,710 --> 00:24:09,481 But a dislocation comes in and saves the day 488 00:24:09,481 --> 00:24:10,982 and it allows that to happen. 489 00:24:10,982 --> 00:24:13,885 Because here's the alternative. 490 00:24:13,885 --> 00:24:17,456 if I want that same translation to happen, that same slip, 491 00:24:17,456 --> 00:24:20,358 I would have had to break all the bonds at once. 492 00:24:20,358 --> 00:24:23,028 If I didn't have a dislocation, this is what would-- ah, 493 00:24:23,028 --> 00:24:24,729 there's the dislocation. 494 00:24:24,729 --> 00:24:27,799 One bond at a time, one bond at a time. 495 00:24:27,799 --> 00:24:32,571 Or the much, much harder ask, especially 496 00:24:32,571 --> 00:24:35,474 since there's 10 to the 20 something of these, 497 00:24:35,474 --> 00:24:36,708 is to break them all at once. 498 00:24:36,708 --> 00:24:37,576 There it is. 499 00:24:37,576 --> 00:24:39,110 Bam. 500 00:24:39,110 --> 00:24:42,414 That's why that doesn't happen. 501 00:24:42,414 --> 00:24:44,916 It just takes too much energy. 502 00:24:44,916 --> 00:24:48,553 But if I can just call up a dislocation to the rescue, 503 00:24:48,553 --> 00:24:52,090 I can translate an entire set of atoms 504 00:24:52,090 --> 00:24:54,626 with much, much, much less energy. 505 00:24:54,626 --> 00:24:55,527 Did you see that? 506 00:24:55,527 --> 00:24:58,930 So the dislocation moving is what 507 00:24:58,930 --> 00:25:02,133 allows plastic deformation, it's what 508 00:25:02,133 --> 00:25:06,705 allows these atoms to slide over one another. 509 00:25:06,705 --> 00:25:11,176 It is what enables this region to be there 510 00:25:11,176 --> 00:25:14,045 and it's what defines it. 511 00:25:14,045 --> 00:25:16,615 A very important point here-- 512 00:25:16,615 --> 00:25:19,317 aw, you got a goody bag. 513 00:25:19,317 --> 00:25:20,919 That's OK. 514 00:25:20,919 --> 00:25:21,920 Was he in the class? 515 00:25:21,920 --> 00:25:22,754 It doesn't matter. 516 00:25:22,754 --> 00:25:23,522 We share. 517 00:25:23,522 --> 00:25:24,022 [LAUGHTER] 518 00:25:24,022 --> 00:25:26,491 If you guys have friends-- 519 00:25:26,491 --> 00:25:27,726 just came in and took one. 520 00:25:27,726 --> 00:25:29,628 If you guys have friends that need goody bags, 521 00:25:29,628 --> 00:25:31,029 we are here to help. 522 00:25:31,029 --> 00:25:33,265 I will never turn down a goody bag. 523 00:25:36,568 --> 00:25:40,939 There is a plane here that's moving on another plane. 524 00:25:40,939 --> 00:25:43,241 There is a plane that's moving on another plane 525 00:25:43,241 --> 00:25:46,811 and it's doing it only because of this location, that one 526 00:25:46,811 --> 00:25:49,614 dimensional defect. 527 00:25:49,614 --> 00:25:53,318 Now, that is slipping, so it's called a slip plane. 528 00:25:58,189 --> 00:26:00,025 It's called a slip plane. 529 00:26:00,025 --> 00:26:08,967 And it's the plane along which the dislocation moves. 530 00:26:13,538 --> 00:26:15,273 [PHONE RINGING] 531 00:26:15,273 --> 00:26:15,774 OK. 532 00:26:15,774 --> 00:26:18,610 We got a phone going on there. 533 00:26:18,610 --> 00:26:21,313 Now, I want to make a really important point here. 534 00:26:21,313 --> 00:26:24,049 Because what the dislocation is allowing, 535 00:26:24,049 --> 00:26:31,022 what this defect is allowing you to do is resolve forces. 536 00:26:31,022 --> 00:26:32,357 That's what you're doing. 537 00:26:32,357 --> 00:26:35,093 I'm putting this force on-- 538 00:26:35,093 --> 00:26:36,695 it's again, the Instron. 539 00:26:36,695 --> 00:26:40,332 I'm pulling the wire and the material is like, hold on, 540 00:26:40,332 --> 00:26:42,534 I got to respond to this. 541 00:26:42,534 --> 00:26:44,035 How can I do-- 542 00:26:44,035 --> 00:26:44,769 yeah? 543 00:26:44,769 --> 00:26:48,073 [INAUDIBLE] 544 00:26:48,073 --> 00:26:48,640 Yes. 545 00:26:48,640 --> 00:26:49,908 And I will come back to that. 546 00:26:49,908 --> 00:26:52,277 It's this plane. 547 00:26:52,277 --> 00:26:53,078 Where's the model? 548 00:26:53,078 --> 00:26:54,412 Who's got the model? 549 00:26:54,412 --> 00:26:56,381 Is that the dislocation model? 550 00:26:56,381 --> 00:26:59,317 It's the plane where the dislocation 551 00:26:59,317 --> 00:27:03,021 comes in and can move. 552 00:27:03,021 --> 00:27:03,855 Why? 553 00:27:03,855 --> 00:27:07,559 Because that's the plane that is-- 554 00:27:07,559 --> 00:27:10,328 by motion of the dislocation, that's 555 00:27:10,328 --> 00:27:13,131 the plane that's slipping along another one 556 00:27:13,131 --> 00:27:16,001 by exactly this animation. 557 00:27:16,001 --> 00:27:17,769 I'll talk about this slip plane in a minute 558 00:27:17,769 --> 00:27:18,837 also and more about it. 559 00:27:22,407 --> 00:27:26,077 So what this motion, what this slip plane, 560 00:27:26,077 --> 00:27:29,714 what these movements of the planes allow is is it 561 00:27:29,714 --> 00:27:32,617 allows you to resolve an applied force. 562 00:27:32,617 --> 00:27:40,225 So I come at this material with a force and I resolve it. 563 00:27:40,225 --> 00:27:51,336 It's resolved at the atomic level-- 564 00:27:51,336 --> 00:27:55,407 and I'll finish this-- at the atomic level along that slip 565 00:27:55,407 --> 00:27:58,143 plane. 566 00:27:58,143 --> 00:28:00,178 That's what it does. 567 00:28:00,178 --> 00:28:08,219 The Instron pulls or I pull on the parachute, like the logo, 568 00:28:08,219 --> 00:28:13,091 and then either the material is going to break 569 00:28:13,091 --> 00:28:16,227 or maybe it's just in that elastic region 570 00:28:16,227 --> 00:28:19,431 and then it will go all the way back or it's going to deform. 571 00:28:19,431 --> 00:28:20,965 And if it deforms, what's it doing? 572 00:28:20,965 --> 00:28:24,769 It's feeling the force and it's trying to resolve it. 573 00:28:24,769 --> 00:28:27,539 And it resolves it by those dislocations moving. 574 00:28:27,539 --> 00:28:31,476 The planes move on top of each other. 575 00:28:31,476 --> 00:28:34,245 Well, so let's go back to this plane. 576 00:28:34,245 --> 00:28:36,181 What plane is it? 577 00:28:36,181 --> 00:28:42,153 If I look at the planes, which are somewhere out there-- 578 00:28:42,153 --> 00:28:46,825 you can imagine that if I've got all those planes 579 00:28:46,825 --> 00:28:49,561 and they're looking at the Instron and the force being 580 00:28:49,561 --> 00:28:54,199 applied to me, which one is going to do the slipping? 581 00:28:54,199 --> 00:28:58,103 Well, you can imagine this with a very simple analogy. 582 00:28:58,103 --> 00:29:01,639 If I push on something-- if I push on a rope, 583 00:29:01,639 --> 00:29:02,874 what does it do? 584 00:29:02,874 --> 00:29:04,409 Well, it kind of curls up, it doesn't 585 00:29:04,409 --> 00:29:07,679 do too much-- it doesn't move. 586 00:29:07,679 --> 00:29:10,949 But if I really want to try to resolve force, 587 00:29:10,949 --> 00:29:12,250 I got to move those atoms. 588 00:29:14,986 --> 00:29:17,689 But if I push on a stick and not a rope, 589 00:29:17,689 --> 00:29:21,025 then the stick just all moves. 590 00:29:21,025 --> 00:29:26,030 So the plane that is going to move across another plane 591 00:29:26,030 --> 00:29:28,233 is going to be the most dense-- 592 00:29:28,233 --> 00:29:30,335 the closest packed. 593 00:29:30,335 --> 00:29:32,170 It's going to be the closest packed. 594 00:29:35,240 --> 00:29:38,243 So the slip plane-- 595 00:29:38,243 --> 00:29:39,711 let's write it here. 596 00:29:47,819 --> 00:29:52,891 Slip plane-- and I'll give you an example in a minute. 597 00:29:52,891 --> 00:30:03,635 Slip plane, closest packed or highest planar density. 598 00:30:09,808 --> 00:30:11,910 Because then I'm pushing on the stick. 599 00:30:11,910 --> 00:30:16,247 In the material, I've got stick and ropes defined by the plane. 600 00:30:16,247 --> 00:30:17,315 You can see it over there. 601 00:30:17,315 --> 00:30:20,518 Some of those planes, they're beautiful-- the yellow one, 602 00:30:20,518 --> 00:30:24,422 but it doesn't have that many atoms per area. 603 00:30:24,422 --> 00:30:27,292 So if I try to move that one, it's 604 00:30:27,292 --> 00:30:31,930 not as strong of a plane to move and to resolve my forces 605 00:30:31,930 --> 00:30:32,964 at the atomic scale with. 606 00:30:32,964 --> 00:30:38,069 So I'm always going to go for that strongest plane. 607 00:30:38,069 --> 00:30:39,737 So that's a slip plane. 608 00:30:39,737 --> 00:30:46,444 Now, a great analogy for this is the rug. 609 00:30:46,444 --> 00:30:49,948 So if you're helping people move or you're setting up 610 00:30:49,948 --> 00:30:51,916 your own rug, and you get it and you put down 611 00:30:51,916 --> 00:30:55,153 the rug base like that, and then you put it down, 612 00:30:55,153 --> 00:30:55,753 and you're, ah! 613 00:30:55,753 --> 00:30:58,156 I missed it by a foot. 614 00:30:58,156 --> 00:30:59,490 I got it wrong. 615 00:30:59,490 --> 00:31:03,895 This rug weighs-- rugs are really heavy. 616 00:31:03,895 --> 00:31:06,831 You pick it up, you're like, oh, I'll just move it. 617 00:31:06,831 --> 00:31:09,434 You call a friend and you both pick it up, and you're like, 618 00:31:09,434 --> 00:31:11,336 this is 1,000 pounds. 619 00:31:11,336 --> 00:31:13,371 Who knew? 620 00:31:13,371 --> 00:31:15,106 It's really a pain. 621 00:31:15,106 --> 00:31:19,377 And so then you think about dislocations 622 00:31:19,377 --> 00:31:23,448 and you think about how a material would 623 00:31:23,448 --> 00:31:25,016 handle this situation. 624 00:31:25,016 --> 00:31:28,319 And you say, well, if I just make a little dislocation, 625 00:31:28,319 --> 00:31:30,188 like a little crinkle there, and I 626 00:31:30,188 --> 00:31:33,691 move that, think about how much effort it 627 00:31:33,691 --> 00:31:35,827 takes to move that instead. 628 00:31:35,827 --> 00:31:36,461 Not much. 629 00:31:36,461 --> 00:31:38,663 You make a little crinkle, and then you move it, 630 00:31:38,663 --> 00:31:39,530 and then you move it. 631 00:31:39,530 --> 00:31:41,699 It doesn't take much force to do that. 632 00:31:41,699 --> 00:31:46,537 It takes a lot less than picking up the whole rug. 633 00:31:46,537 --> 00:31:48,973 That's the motion of a dislocation. 634 00:31:48,973 --> 00:31:51,943 We just talked about the plane that it is, 635 00:31:51,943 --> 00:31:53,378 but which direction? 636 00:31:53,378 --> 00:31:57,415 Which way should you move it? 637 00:31:57,415 --> 00:31:58,783 And so this is the second part. 638 00:31:58,783 --> 00:32:02,020 So you have the slip plane, the closest packed 639 00:32:02,020 --> 00:32:04,055 or highest planar density, and then 640 00:32:04,055 --> 00:32:06,991 you also have the slip direction. 641 00:32:06,991 --> 00:32:10,495 The slip direction is the other part. 642 00:32:10,495 --> 00:32:11,963 Which way does it go? 643 00:32:11,963 --> 00:32:14,365 I've got the closest packed plane. 644 00:32:14,365 --> 00:32:15,466 Which way should it go? 645 00:32:15,466 --> 00:32:20,104 Well, for this, you can see from the experiments, 646 00:32:20,104 --> 00:32:23,708 and when you look at the wire-- again, if I take a copper wire 647 00:32:23,708 --> 00:32:26,844 and I pull it and I look at it with my eyes, 648 00:32:26,844 --> 00:32:29,047 it just looks like the uniform wire's been stretched. 649 00:32:29,047 --> 00:32:30,515 But now I magnify it. 650 00:32:30,515 --> 00:32:31,416 This is what you see. 651 00:32:34,118 --> 00:32:39,023 So you can see that there are very definite directions. 652 00:32:39,023 --> 00:32:41,726 This is actually all the same direction. 653 00:32:41,726 --> 00:32:45,163 So that's a slip plane and there's a direction 654 00:32:45,163 --> 00:32:45,763 that it slides. 655 00:32:48,766 --> 00:32:52,070 I think this can be understood-- 656 00:32:52,070 --> 00:32:55,740 I love this picture, because I love ping pong. 657 00:32:55,740 --> 00:32:57,308 These are ping pong balls and you guys 658 00:32:57,308 --> 00:33:00,878 can do these experiments yourself. 659 00:33:00,878 --> 00:33:02,747 You put a bunch of ping pong balls in a row, 660 00:33:02,747 --> 00:33:04,549 and then measure the force that it 661 00:33:04,549 --> 00:33:08,052 takes to slide it depending on whether they're closely packed 662 00:33:08,052 --> 00:33:09,754 or not. 663 00:33:09,754 --> 00:33:11,889 This is a little counterintuitive sometimes 664 00:33:11,889 --> 00:33:14,492 when you first see it, but it makes sense. 665 00:33:14,492 --> 00:33:17,128 The more densely packed the plane 666 00:33:17,128 --> 00:33:21,265 is, the easier it is for one set of atoms 667 00:33:21,265 --> 00:33:23,568 to slide across the other. 668 00:33:23,568 --> 00:33:24,769 The easier it is. 669 00:33:24,769 --> 00:33:26,604 So you can feel it. 670 00:33:26,604 --> 00:33:28,506 If I have to let this one-- 671 00:33:28,506 --> 00:33:30,341 and they showed it just by angle at which 672 00:33:30,341 --> 00:33:31,309 it would slide or roll. 673 00:33:31,309 --> 00:33:34,579 I like the idea of two people going in there-- 674 00:33:34,579 --> 00:33:38,116 and here, you'd have to apply more force to make that one 675 00:33:38,116 --> 00:33:40,752 slide than this one. 676 00:33:40,752 --> 00:33:45,123 You can see it, again, with this super precise animation. 677 00:33:45,123 --> 00:33:46,524 There it is. 678 00:33:46,524 --> 00:33:48,126 One of them and there's the other one. 679 00:33:48,126 --> 00:33:50,862 Look at how much more work you got to do to get that thing 680 00:33:50,862 --> 00:33:53,564 to go over and over. 681 00:33:53,564 --> 00:33:54,098 No. 682 00:33:54,098 --> 00:33:56,801 If you've got a high density of atoms, 683 00:33:56,801 --> 00:33:59,670 it's actually easier to slide. 684 00:33:59,670 --> 00:34:04,409 It's easier to slide because you've got more bonds. 685 00:34:04,409 --> 00:34:06,978 And so I'm not having to fully break a bond 686 00:34:06,978 --> 00:34:08,279 before I get to the next one. 687 00:34:11,549 --> 00:34:22,226 So the slip direction then is going to be the highest density 688 00:34:22,226 --> 00:34:25,897 direction, so it's also the close-packed-- 689 00:34:25,897 --> 00:34:28,399 I've got an example there-- close-packed direction. 690 00:34:32,270 --> 00:34:37,975 So notice, this would be like a direction-- a vector and this 691 00:34:37,975 --> 00:34:39,310 is a plane. 692 00:34:39,310 --> 00:34:41,946 Well, that's jogging some memories 693 00:34:41,946 --> 00:34:45,016 of understanding crystals by their Miller planes 694 00:34:45,016 --> 00:34:45,917 and directions. 695 00:34:48,619 --> 00:34:51,389 We say, well, what would this be for, say, FCC? 696 00:34:51,389 --> 00:34:54,992 There's an FCC crystal and there's an FCC lattice. 697 00:34:54,992 --> 00:34:59,464 So if I had, say, FCC as an example-- 698 00:34:59,464 --> 00:35:03,000 well, OK, so let's see if I can really quickly-- 699 00:35:03,000 --> 00:35:05,870 oh, boy, here we go. 700 00:35:05,870 --> 00:35:08,206 There it is. 701 00:35:08,206 --> 00:35:11,275 And here are the face atoms. 702 00:35:11,275 --> 00:35:13,377 There we go. 703 00:35:13,377 --> 00:35:14,679 OK. 704 00:35:14,679 --> 00:35:17,548 That's the outer part of FCC-- three of the faces. 705 00:35:17,548 --> 00:35:23,221 And now, OK, but if I take a 100 plane-- 706 00:35:23,221 --> 00:35:26,124 so let's take the 100 plane. 707 00:35:26,124 --> 00:35:28,359 Then I've got that. 708 00:35:28,359 --> 00:35:30,728 And you'll remember, I've got these atoms on the corner. 709 00:35:30,728 --> 00:35:33,131 Why did I draw that so much bigger? 710 00:35:33,131 --> 00:35:34,632 Let's not do that. 711 00:35:34,632 --> 00:35:36,300 That would be like the 100 plane. 712 00:35:39,137 --> 00:35:44,775 And this one might be the usual lattice constant A. You could 713 00:35:44,775 --> 00:35:48,679 also look at the 110 plane. 714 00:35:48,679 --> 00:35:49,647 Here it is. 715 00:35:49,647 --> 00:35:52,216 The 110 plane. 716 00:35:52,216 --> 00:35:56,654 That would look like this and this. 717 00:35:56,654 --> 00:36:00,124 That would be the 110 This is just bringing back memories-- 718 00:36:00,124 --> 00:36:01,192 I know good ones. 719 00:36:03,995 --> 00:36:05,930 Here is the 111 plane. 720 00:36:11,569 --> 00:36:15,139 So here, we've got this one, this one, and this one. 721 00:36:18,476 --> 00:36:23,314 In an FCC metal or crystal, which plane 722 00:36:23,314 --> 00:36:26,651 is going to slip and along which direction? 723 00:36:26,651 --> 00:36:28,786 You can get that now. 724 00:36:28,786 --> 00:36:32,690 Because you can calculate the planar densities. 725 00:36:32,690 --> 00:36:38,296 In which case is the packing of atoms-- in which 726 00:36:38,296 --> 00:36:41,632 of these planes is it the highest? 727 00:36:41,632 --> 00:36:43,968 Well, you've got to know the number of atoms. 728 00:36:43,968 --> 00:36:46,103 So how many atoms are in this? 729 00:36:46,103 --> 00:36:48,139 2. 730 00:36:48,139 --> 00:36:49,674 How many atoms are in this? 731 00:36:49,674 --> 00:36:50,374 Oh, why? 732 00:36:50,374 --> 00:36:56,881 1/4, 1/4-- I'm just counting in the unit cell of the plane. 733 00:36:56,881 --> 00:36:58,416 So 1/4, 1/4, 1/4, 1/4-- 734 00:36:58,416 --> 00:37:00,051 1. 735 00:37:00,051 --> 00:37:04,188 It only counts in the plane if the plane goes right 736 00:37:04,188 --> 00:37:04,889 through the atom. 737 00:37:07,725 --> 00:37:12,396 So here in the 110 I've got 1/4, 1/4, 1/4, 1/4. 738 00:37:12,396 --> 00:37:12,897 OK. 739 00:37:12,897 --> 00:37:15,299 That's 1, plus 1/2, 1/2. 740 00:37:15,299 --> 00:37:18,135 Two. 741 00:37:18,135 --> 00:37:20,271 I've got two atoms in that plane-- 742 00:37:20,271 --> 00:37:23,207 effectively, two atoms in that plane. 743 00:37:23,207 --> 00:37:24,041 But what about here? 744 00:37:24,041 --> 00:37:25,276 Oh, boy. 745 00:37:25,276 --> 00:37:25,776 OK. 746 00:37:25,776 --> 00:37:29,714 1/6, 1/6, 1/6. 747 00:37:29,714 --> 00:37:32,450 You see it from the geometry it's a triangle. 748 00:37:32,450 --> 00:37:36,320 So that's 1/2 from those corners, plus 1/2, 1/2, 1/2. 749 00:37:36,320 --> 00:37:38,155 2. 750 00:37:38,155 --> 00:37:42,260 I've got the same number of atoms 751 00:37:42,260 --> 00:37:46,564 in those planes of the unit cell-- of those planes drawn 752 00:37:46,564 --> 00:37:49,734 within these unit cell boxes. 753 00:37:49,734 --> 00:37:51,636 But they're different planar densities. 754 00:37:51,636 --> 00:37:53,871 They're different planar densities. 755 00:37:53,871 --> 00:38:03,147 Because the area here is equal to a squared in this case, 756 00:38:03,147 --> 00:38:06,784 it's equal to root 2 a squared in this case. 757 00:38:06,784 --> 00:38:07,985 So it's larger. 758 00:38:07,985 --> 00:38:10,221 Same atoms, larger area. 759 00:38:10,221 --> 00:38:14,025 That's not going to have a higher packing than that. 760 00:38:14,025 --> 00:38:16,727 And now I get to have really a lot of fun with triangles 761 00:38:16,727 --> 00:38:19,664 and thinking about the height here and stuff like that. 762 00:38:19,664 --> 00:38:23,301 And the area of this one is equal to root 763 00:38:23,301 --> 00:38:32,877 3 over 2 a squared less than 1. 764 00:38:32,877 --> 00:38:37,315 That's going to have the highest planar density. 765 00:38:37,315 --> 00:38:39,283 You know it. 766 00:38:39,283 --> 00:38:41,352 That's going to have the highest planar density. 767 00:38:41,352 --> 00:38:43,187 So that's a connection now. 768 00:38:43,187 --> 00:38:45,690 So you can connect now what we've 769 00:38:45,690 --> 00:38:47,658 learned about Miller planes, about 770 00:38:47,658 --> 00:38:54,465 crystallography to this simple calculations of density, 771 00:38:54,465 --> 00:38:58,803 to this incredibly important behavior of materials-- 772 00:38:58,803 --> 00:39:00,471 plastic deformation. 773 00:39:00,471 --> 00:39:05,576 Because now you know how that's going to plastically deform. 774 00:39:05,576 --> 00:39:11,115 It's going to deform along its closest packed plane. 775 00:39:11,115 --> 00:39:13,184 And you can also look at the directions. 776 00:39:13,184 --> 00:39:15,419 And it's shown here that the slip direction 777 00:39:15,419 --> 00:39:17,521 will be along one of those-- 778 00:39:17,521 --> 00:39:22,293 that's the highest density direction, the 110. 779 00:39:22,293 --> 00:39:26,597 So it's going to slip along one of those directions. 780 00:39:26,597 --> 00:39:27,465 Good. 781 00:39:27,465 --> 00:39:33,938 Slip plane plus this together combined-- 782 00:39:33,938 --> 00:39:35,306 these are called slip systems. 783 00:39:45,549 --> 00:39:49,320 The slip plane and the direction. 784 00:39:49,320 --> 00:39:51,355 That's called the slip system. 785 00:39:51,355 --> 00:39:52,022 OK. 786 00:39:52,022 --> 00:39:56,427 Now, there's another thing that happens 787 00:39:56,427 --> 00:39:59,997 with this location that's so cool and so important 788 00:39:59,997 --> 00:40:06,070 to materials and to the things we want to do with materials. 789 00:40:06,070 --> 00:40:08,072 It has to do with the fact that, like I said, 790 00:40:08,072 --> 00:40:13,611 the dislocations come in and they can be 791 00:40:13,611 --> 00:40:17,415 a total mess and entanglement. 792 00:40:17,415 --> 00:40:19,350 By the way, how did it come in the first place? 793 00:40:19,350 --> 00:40:21,519 If I started with a perfect crystal, 794 00:40:21,519 --> 00:40:24,121 where did it come from? 795 00:40:24,121 --> 00:40:27,825 How could there be a dislocation? 796 00:40:27,825 --> 00:40:30,928 It can come in from the edge. 797 00:40:30,928 --> 00:40:32,430 You've got an edge there. 798 00:40:32,430 --> 00:40:35,800 There can be a mismatch and I call it up-- 799 00:40:36,200 --> 00:40:39,270 I need a dislocation or I'm going to break. 800 00:40:39,270 --> 00:40:39,870 Boom. 801 00:40:39,870 --> 00:40:42,606 Comes in from the edge, comes in from the edge. 802 00:40:42,606 --> 00:40:45,543 They come to the rescue. 803 00:40:45,543 --> 00:40:49,180 This is a simulation of exactly that done 804 00:40:49,180 --> 00:40:54,485 by the chair of the Department of Civil Engineering, Marcus 805 00:40:54,485 --> 00:40:56,821 Beuhler-- 806 00:40:56,821 --> 00:40:58,722 a simulation he did some years ago. 807 00:40:58,722 --> 00:41:00,724 But he's modeling the dislocations-- 808 00:41:00,724 --> 00:41:02,326 that's what you're going to see here-- 809 00:41:02,326 --> 00:41:04,995 as a piece of metal is cracked. 810 00:41:04,995 --> 00:41:05,763 Watch this. 811 00:41:05,763 --> 00:41:06,897 Watch them come in. 812 00:41:06,897 --> 00:41:09,266 It's saying, I need you, help me. 813 00:41:09,266 --> 00:41:10,968 And there they go. 814 00:41:10,968 --> 00:41:12,369 Now, look at what happen-- there's 815 00:41:12,369 --> 00:41:15,673 so many of them forming and forming, and now tangling 816 00:41:15,673 --> 00:41:17,408 and tangling and tangling. 817 00:41:17,408 --> 00:41:18,542 Then he's going to zoom in. 818 00:41:18,542 --> 00:41:19,210 It's very cool. 819 00:41:19,210 --> 00:41:22,746 Those are dislocations like we saw in the experiments, 820 00:41:22,746 --> 00:41:24,515 but this is a computer simulation of them. 821 00:41:24,515 --> 00:41:25,783 They're still kind of tangling. 822 00:41:25,783 --> 00:41:26,750 Here they go. 823 00:41:26,750 --> 00:41:32,623 They come in, they get called in to relieve the atomic force 824 00:41:32,623 --> 00:41:35,125 and let the material slip. 825 00:41:35,125 --> 00:41:37,595 But there's something that happens now. 826 00:41:37,595 --> 00:41:39,864 There's something that happens. 827 00:41:39,864 --> 00:41:43,200 Because now we go back to the rug-- 828 00:41:43,200 --> 00:41:44,502 we go back to the rug. 829 00:41:44,502 --> 00:41:45,870 And look at what's happened. 830 00:41:45,870 --> 00:41:47,638 I'm going along and I did the calculations 831 00:41:47,638 --> 00:41:49,473 and I had no one to lift it, it's too heavy. 832 00:41:49,473 --> 00:41:52,376 But I'm going to create a little dislocation and move it along. 833 00:41:52,376 --> 00:41:55,779 And I come across someone else's dislocation here. 834 00:41:55,779 --> 00:41:56,447 Look at this. 835 00:41:56,447 --> 00:41:59,416 I'm trying to roll mine and there's another one there. 836 00:41:59,416 --> 00:42:00,684 What's going to happen? 837 00:42:03,521 --> 00:42:05,422 I can't go anymore. 838 00:42:05,422 --> 00:42:07,992 I'm blocked. 839 00:42:07,992 --> 00:42:09,193 I'm blocked. 840 00:42:09,193 --> 00:42:13,464 So when dislocations tangle up, you 841 00:42:13,464 --> 00:42:18,769 can imagine now it prevents dislocations from moving. 842 00:42:18,769 --> 00:42:24,642 But dislocations moving is what gives me the plastic regime. 843 00:42:24,642 --> 00:42:28,412 So as I introduce dislocations, I 844 00:42:28,412 --> 00:42:32,816 make it so that it's harder for dislocations to move. 845 00:42:32,816 --> 00:42:34,451 There's actually a name for that. 846 00:42:34,451 --> 00:42:37,588 There's a name for that because it really changes 847 00:42:37,588 --> 00:42:39,023 the mechanical properties. 848 00:42:39,023 --> 00:42:41,759 It's called cold hardening or work hardening or strain 849 00:42:41,759 --> 00:42:43,260 hardening-- sorry, cold work. 850 00:42:43,260 --> 00:42:44,228 Cold working. 851 00:42:44,228 --> 00:42:56,574 So work hardening or sometimes strain hardening. 852 00:42:56,574 --> 00:42:59,677 Why is it called hardening? 853 00:42:59,677 --> 00:43:05,149 Because you're literally making this material harder. 854 00:43:05,149 --> 00:43:09,587 What's happened is, I've taken away ductility, 855 00:43:09,587 --> 00:43:11,589 so I've made it more brittle. 856 00:43:11,589 --> 00:43:15,793 I've taken away the plastic deformation. 857 00:43:15,793 --> 00:43:17,795 It can't do as much plastic deformation, 858 00:43:17,795 --> 00:43:20,331 because that only happens by motion of dislocations 859 00:43:20,331 --> 00:43:22,299 and I'm locking them in. 860 00:43:22,299 --> 00:43:26,637 But I'm letting it maybe go a little further in how much 861 00:43:26,637 --> 00:43:28,639 it can elastically deform. 862 00:43:28,639 --> 00:43:31,909 Maybe now, once it gets to here, it says, 863 00:43:31,909 --> 00:43:36,880 you know what, I've got all these dislocations in there, 864 00:43:36,880 --> 00:43:39,083 It's not letting me plastically deform, 865 00:43:39,083 --> 00:43:43,120 so I'll keep elastically deforming. 866 00:43:43,120 --> 00:43:46,190 But that gets you up to much higher stresses. 867 00:43:46,190 --> 00:43:47,992 It gets you up to much higher stresses 868 00:43:47,992 --> 00:43:50,494 with elastic deformation. 869 00:43:50,494 --> 00:43:51,729 And so this is a plot-- 870 00:43:51,729 --> 00:43:52,296 look at that. 871 00:43:52,296 --> 00:43:54,865 That's called the-- oh, I should have put that down there. 872 00:43:54,865 --> 00:43:57,167 Let me label that because it's a pretty important point. 873 00:43:57,167 --> 00:43:58,502 I labeled the fracture point. 874 00:43:58,502 --> 00:44:00,270 This is the yield point. 875 00:44:00,270 --> 00:44:03,807 There it is, the yield strength. 876 00:44:03,807 --> 00:44:07,478 That's called the yield because it's where it 877 00:44:07,478 --> 00:44:11,782 yields to plastic deformation. 878 00:44:11,782 --> 00:44:12,483 So here you go. 879 00:44:12,483 --> 00:44:15,919 You've got some steel, you've got some brass, 880 00:44:15,919 --> 00:44:18,255 and you've got some copper. 881 00:44:18,255 --> 00:44:19,757 This is plotting-- look at that. 882 00:44:19,757 --> 00:44:21,759 This is the percent cold work. 883 00:44:21,759 --> 00:44:23,927 Cold work is work hardening. 884 00:44:23,927 --> 00:44:27,931 Work hardening is adding dislocations on purpose-- 885 00:44:27,931 --> 00:44:29,199 on purpose. 886 00:44:29,199 --> 00:44:30,401 So that what? 887 00:44:30,401 --> 00:44:32,970 So that I increase the yield strength 888 00:44:32,970 --> 00:44:36,073 so that this material can elastically 889 00:44:36,073 --> 00:44:42,079 deform now up to higher and higher and higher strengths. 890 00:44:42,079 --> 00:44:43,947 But the ductility goes down. 891 00:44:43,947 --> 00:44:46,083 When you bend a paperclip, don't you 892 00:44:46,083 --> 00:44:49,286 go and tell anybody it's because of heat that it breaks. 893 00:44:49,286 --> 00:44:51,321 That's not why a paperclip breaks. 894 00:44:51,321 --> 00:44:53,123 It gets hot-- or warm. 895 00:44:53,123 --> 00:44:55,159 You bend it back and forth, it breaks 896 00:44:55,159 --> 00:44:58,328 because you are putting dislocations into it 897 00:44:58,328 --> 00:45:00,130 and you're making it more brittle. 898 00:45:00,130 --> 00:45:01,899 In fact, if you heated it up, you 899 00:45:01,899 --> 00:45:06,637 would anneal those out and make it more ductile again. 900 00:45:06,637 --> 00:45:08,772 That's how you get rid of the dislocations-- you've 901 00:45:08,772 --> 00:45:10,741 got to heat it back up. 902 00:45:10,741 --> 00:45:15,145 It makes it brittle, the ductility goes down. 903 00:45:15,145 --> 00:45:16,013 Why does this matter? 904 00:45:16,013 --> 00:45:17,548 Well, this is one reason. 905 00:45:17,548 --> 00:45:22,086 If you don't plan your cold work carefully, 906 00:45:22,086 --> 00:45:24,354 you might make the material too brittle. 907 00:45:24,354 --> 00:45:26,256 You wanted it to be so hard because this 908 00:45:26,256 --> 00:45:30,060 was such an important ship and it was going to be a big deal 909 00:45:30,060 --> 00:45:31,528 and the launch was really exciting. 910 00:45:31,528 --> 00:45:35,199 And then it cracks in half, the entire ship. 911 00:45:35,199 --> 00:45:35,799 Why? 912 00:45:35,799 --> 00:45:39,570 Because you didn't look up what dislocations mean. 913 00:45:39,570 --> 00:45:45,075 You didn't take 3.091, that's why. 914 00:45:45,075 --> 00:45:46,176 That's a pretty big crack. 915 00:45:49,213 --> 00:45:50,614 The main why this matters-- 916 00:45:50,614 --> 00:45:52,416 oh, I couldn't help it. 917 00:45:52,416 --> 00:45:56,220 I am a big fan of wind. 918 00:45:56,220 --> 00:45:58,422 Wind energy is growing and growing 919 00:45:58,422 --> 00:46:02,726 and it's such a great national resource. 920 00:46:02,726 --> 00:46:05,295 Here's the global capacity. 921 00:46:05,295 --> 00:46:10,601 This is install capacity for wind turbines. 922 00:46:10,601 --> 00:46:15,472 But see, this is a mechanical materials problem 923 00:46:15,472 --> 00:46:22,679 that you are now equipped to think about more deeply. 924 00:46:22,679 --> 00:46:27,317 Because, you see, you can do a lot of different experiments 925 00:46:27,317 --> 00:46:28,085 on those turbines. 926 00:46:28,085 --> 00:46:31,054 So the blades here are critical. 927 00:46:31,054 --> 00:46:34,124 You can imagine that you want them to be light, 928 00:46:34,124 --> 00:46:36,860 but if they're too light, they may not be strong enough. 929 00:46:36,860 --> 00:46:38,495 And then how do they need to be strong? 930 00:46:38,495 --> 00:46:43,367 Because you've got huge amounts of wind coming at them. 931 00:46:43,367 --> 00:46:46,937 And it turns out, you need to hit just the right balance 932 00:46:46,937 --> 00:46:52,876 of elastic deformation before it goes into some plastic regime. 933 00:46:52,876 --> 00:46:55,412 You need to hit just the right balance of ductility. 934 00:46:55,412 --> 00:46:56,980 So here's, for example-- 935 00:46:56,980 --> 00:46:58,148 these are some simulations. 936 00:46:58,148 --> 00:47:01,585 Here are some experiments on a new material for a wind turbine 937 00:47:01,585 --> 00:47:02,519 blade. 938 00:47:02,519 --> 00:47:06,857 And then you put it out there and look at what happens-- ice. 939 00:47:06,857 --> 00:47:10,460 By the way, this ice comes off at hundreds of miles an hour 940 00:47:10,460 --> 00:47:11,495 in chunks. 941 00:47:11,495 --> 00:47:14,498 These farmers are not happy about that. 942 00:47:14,498 --> 00:47:15,933 Seriously. 943 00:47:15,933 --> 00:47:16,767 And those are bugs. 944 00:47:19,369 --> 00:47:22,639 Actually, bugs in wind turbine blades is a serious problem. 945 00:47:22,639 --> 00:47:25,008 How do you clean bugs off of it? 946 00:47:25,008 --> 00:47:27,544 Because it dramatically changes the aerodynamics 947 00:47:27,544 --> 00:47:28,579 and the efficiency. 948 00:47:28,579 --> 00:47:31,849 It also can damage the blade itself. 949 00:47:31,849 --> 00:47:33,650 So there's all sorts of work going on. 950 00:47:33,650 --> 00:47:36,887 How do you make bug-proof wind turbine blades? 951 00:47:36,887 --> 00:47:37,387 OK. 952 00:47:37,387 --> 00:47:38,989 Well, now just spray it with something. 953 00:47:38,989 --> 00:47:41,859 Ah, but then does it have the right plastic deform-- 954 00:47:41,859 --> 00:47:43,493 does it have the right yield point 955 00:47:43,493 --> 00:47:44,695 or is it just going to crack? 956 00:47:47,431 --> 00:47:51,468 And by the way, it's got to have 5 times 10 to the 9 cycles 957 00:47:51,468 --> 00:47:52,803 before it can fail. 958 00:47:52,803 --> 00:47:54,271 That's the metric. 959 00:47:54,271 --> 00:47:56,473 So that's a pretty big ask of a material. 960 00:47:56,473 --> 00:47:59,509 It all comes down to understanding this curve. 961 00:47:59,509 --> 00:48:03,780 And in the broader sense of materials, this to me 962 00:48:03,780 --> 00:48:06,683 is a very exciting ask. 963 00:48:06,683 --> 00:48:08,118 Why? 964 00:48:08,118 --> 00:48:13,557 Because if you look at a plot of the density of materials-- 965 00:48:13,557 --> 00:48:14,992 heavy, light. 966 00:48:14,992 --> 00:48:15,826 Good. 967 00:48:15,826 --> 00:48:17,127 Kilograms per meter cubed. 968 00:48:17,127 --> 00:48:19,062 And the Young's modulus-- 969 00:48:19,062 --> 00:48:23,767 now, this is a measure of the strength of the material. 970 00:48:23,767 --> 00:48:27,037 It's a measure of how much strain 971 00:48:27,037 --> 00:48:30,140 you could put on the material before it breaks or goes 972 00:48:30,140 --> 00:48:30,974 through deformation. 973 00:48:30,974 --> 00:48:32,309 But look at this. 974 00:48:32,309 --> 00:48:34,745 Different materials are here-- 975 00:48:34,745 --> 00:48:36,613 rubbers, foams. 976 00:48:36,613 --> 00:48:40,517 OK, foams have relatively low Young's modulus, 977 00:48:40,517 --> 00:48:41,585 but they're really light. 978 00:48:41,585 --> 00:48:43,053 That could be good. 979 00:48:43,053 --> 00:48:47,024 Up here you've got metals and alloys, ceramics, 980 00:48:47,024 --> 00:48:48,926 you've got polymers in here. 981 00:48:48,926 --> 00:48:52,829 But notice, I've got so many different applications 982 00:48:52,829 --> 00:48:55,432 and needs in the applications and I've 983 00:48:55,432 --> 00:48:59,603 got this plot where I've got nothing here and nothing here, 984 00:48:59,603 --> 00:49:04,174 even though, if I could fill this out and dial up 985 00:49:04,174 --> 00:49:09,947 any stress/strain curve for any density or Young's modulus, 986 00:49:09,947 --> 00:49:11,515 you can make a big difference in a lot 987 00:49:11,515 --> 00:49:12,582 of different applications. 988 00:49:12,582 --> 00:49:15,218 So I think this is a great challenge. 989 00:49:15,218 --> 00:49:15,986 Have a good night. 990 00:49:15,986 --> 00:49:18,255 See you guys on Friday.