§1. Single-crystal tensile tests and texture [00:03]
We talked about slipping on different crystallographic planes, and how that leads to anisotropy. I came across this in one of the books I was looking at. If you take a bar that is a single crystal and you pull on it, without any bending, it would just start shearing on certain crystallographically preferred planes, and you would actually get a translation off to the side. This is just a demonstration of that. In fact, when you actually pull on a single-crystal bar, you pull in the same tensile direction because of the stiffness of the machine pulling it, and that requires that you also get a bending as well as a shear. You can see that here — it's just a nice drawing to show that even a simple tensile test is not so simple.
I remember Backofen saying once when I was a student — and he'd been looking at tensile tests all his life — he'd concluded that the tensile test, even though it seemingly was the most simplistic test you could imagine, was actually extremely complex when you got into the details. We're going to go through some of that a little bit later. Here's another picture from Hertzberg's book: a Charpy bar that fractured in a steel. The texture we develop because of anisotropic behavior gives you a fiber orientation, just like wood is anisotropic. You can see the fibers in this particular piece of steel.
§2. Superplasticity and SPF/DB [01:57]
We've been talking about sheet metal properties, and we're starting to talk about superplasticity. We have n values, R values, and M values. n is the work hardening coefficient, R is the anisotropy ratio, and M is the strain rate sensitivity. We looked at the forming limit diagrams for certain types of metals. When the biaxial strain in the transverse direction E2 is zero, you might have 30 or 40 or 50% elongation in a regular tensile bar. But in a superplastic bar, you can have 100%, 250%, 300%. When you start stretching or compressing in the transverse direction, you actually can get even more strain. That allows you to do all kinds of cheap metal forming operations when it's superplastic — and it also helps you even when the material is not superplastic.
So to review a little bit of superplasticity: superplastic forming, diffusion bonding. They call it SPF/DB — superplastic form, diffusion bonded. The Air Force put a lot of money into this after they realized they could form titanium this way. You basically form two sheets and weld them together in a hermetic seal. You put them in a die cavity, and since it's hermetically sealed, you put pressure in between the sheets as you heat the whole thing up. It might take ten or twenty minutes, it might take an hour or two depending on the temperature and the alloy you're using. Your dies might be operating at 800 or 900° Centigrade. And you can form a very complex part.
The bond — this is one of the diffusion bonds between those areas — you end up getting an excellent bond. Titanium is one of the easiest materials to diffusion bond, and it also happens to be superplastic. You can see a finer grain structure in the top — the stiffening ribs — and in the outside shell you can see a coarser structure. This is a Rolls-Royce alloy.
§3. Strain rate, grain size, and dislocation speeds [04:46]
Superplastic forming is actually a creep phenomenon in the metal. This is just for aluminum alloys, but these are different strain rates from 10⁻⁶ to 10⁵. So 10⁵ is 10 million percent deformation per second — these are ballistic velocities over here. Over here at 10⁻⁶ to 10⁻⁸ per second, this is pretty slow. Forming and tensile testing would be in the 10⁻³ to 10⁻⁴ range. Your grain size can vary by orders of magnitude. Basically aluminum, zinc, magnesium alloy, some of your 7000 series aerospace aluminum alloys, may have grain sizes on the order of 20 microns.
As you get finer and finer grain, creep becomes easier and easier. In fact, when we want the best high-temperature creep properties, we try to get rid of grains — we go to single crystals to completely eliminate grain boundaries. But if you want to get creep and make things superplastic, you want fine grain. This shows the fine grain size as a function of strain rate for aluminum alloys — powder metallurgy aluminum alloys. You're getting into superplastic forming strain rates of a tenth per second, which means you'll get 100% strain in 100 seconds. So in a couple of minutes you're pulling this thing — it's almost like my taking the clay and pulling it before your very eyes.
If you get very fine grains by physical vapor deposition, or if you go to mechanically alloyed aluminum — which is beating it in a ball mill and then consolidating it by extrusion — you get even finer grain. If you go to amorphous or nanocrystalline and you get grain sizes down in the thousand-angstrom range, you get superplasticity and you get high strain rates. In this region you have negative exponents of the strain rate; here we have positive exponents; and then we have impact superplasticity. You can have superplasticity in all these alloys at different strain rates, but it's basically a creep phenomenon.
Now, a question I get about once a year from someplace: someone will claim that the metal was deformed so rapidly that it was very highly strengthened. Superplasticity basically says you can deform it very fast and you're not necessarily going to be stronger — you're going to be weaker at higher temperatures. But the question is, at what strain rates do metals start to become stronger? It's at the strain rates where you start to compete with dislocation motion speeds. Dislocations go at about a third of the speed of sound in the material. When you're at movements of the metal on the order of a thousand meters per second — which you can be at ballistic velocities — things get stronger because the dislocations don't have time to move. But in general, in an automobile crash or a building collapse, the metal is not stronger. That's on a very slow time scale compared to dislocation movement. You hear this old wives' tale all the time: "oh, it was so fast the metal was much stronger." No — not unless you're talking ballistic velocities.
§4. Conform and dynamic recrystallization [09:27]
I mentioned Conform before. [Tom passes around a section of trolley wire.] This is a commercial Conform machine. Conform is the way they made this trolley wire. It's an extrusion process developed in the late 60s, early 70s at a British national laboratory — an energy laboratory doing work for the nuclear business. Some guys were doing basic research, and you basically pour some powder into a rotating channel, and at the end of the channel, just by friction, all this powder is being consolidated and churned together. The working end is this big circular thing right here. You've got powder filling in here, this is the motor to run it — a big motor. Conform stands for continuous forming. As long as you pour the powder in, you're mechanically working it, and you're getting tremendous redundancy. If you wanted to figure out some delta for this operation, it'd be like a thousand. You're kneading the powders, cold welding them together, and you end up with a tremendous amount of mechanical work in the alloy.
For these alloys, I determined, you end up getting what's called dynamic recrystallization. It comes out hot — you don't want to touch this piece of copper, because all the work that's gone into it brings it out at 400 or 500°. You'll burn your hand. But at that 400 or 500° for this particular alloy, which has a little bit of silver in it, you get to the recrystallization range. So you're mechanically working it, you're also nucleating fresh grains at the same time, but you're continuing to mechanically work it while you're nucleating the grains. That's dynamic recrystallization. You can get fantastic properties: super fine grain size, which you need for superplasticity, and fantastic material strength and toughness. Fine grain size gives you good toughness — the Hall-Petch equation, which hopefully you may have learned somewhere else.
Dynamic recrystallization is used in the steel industry to make sheet and plate, where they have rolling mills now with enough force to roll the steel at lower temperatures. Instead of rolling at 1100° Centigrade, they can roll at 1300 or 1400° Fahrenheit, which is kind of like forging temperatures. You're squeezing the stuff, deforming the grains, and it's recrystallizing while you're deforming, so you get fine grain size and much better properties. That's the basis for most of our high-strength low-alloy steels — being able to get dynamic recrystallization.
When I looked up Conform recently — I actually tried to do Conform to make some superconductors 30 years ago and it didn't work very well — if you look at Conform now on the web, it's used for the production of copper flat strip, copper bus bar, commutator connectors for motors, trolley contact wire, and other copper conductors. You can do it with aluminum, but it's easier to extrude aluminum the old-fashioned way; aluminum is at a low enough temperature. Obviously this is good for copper alloys. I tried to do it for a mixture of copper and niobium alloys, and I just broke the dies. The niobium strengthened things too much; I couldn't come up with a steel die that wouldn't break. When you've got something relatively soft that undergoes dynamic recrystallization, you end up with very good electrical conductors.
The only two countries I could find that have Conform machines now are India and China. Why? Because they still build railroads, and use trolley wire. Most of the rest of the world, we just buy it from them if we need it. So it's a specialized process, but it is a very high redundancy process. The North American company that was trying to sell Conform in the early 70s was located in Rhode Island — close enough that I could go do some research down there.
§5. The 30% rule: why two-phase alloys are superplastic [14:22]
If I'm going to try to get something superplastic, I need something that's going to help me maintain the fine grain size. One of the problems with very fine grain size is that the grains want to grow, and the smaller they are, the bigger the driving force for growth, because it goes as the inverse of the radius. This is a plot for the alpha-beta titanium alloys that are superplastic, showing elongation in percent. We're getting up to 1000%, certainly superplastic for a metal. But you get a peak somewhere around 30 volume percent.
Does anyone know what's significant about 30 volume percent? It comes out of geometry, but it shows up in metals. If you studied solidification of composites — anybody have an idea? It turns out, if a structure is controlled by surface tension, you can either get a bunch of little spheres embedded in a matrix, or you can get it to form plates or fibers embedded in the matrix. [Tom sketches at the board.] Actually cylinders — plates are cylinders, but just think of these little cylinders. The question is, will the alloy form spheres, or will it form cylinders, if you heat treat it, recrystallize it, and let the grains grow?
It turns out that the minimum surface energy for a mixture, if you have less than 30 volume percent of your second phase, is spheres. Above 30% you want to form fibers. You can do a minimization. I had to do this as a student — the method of Lagrange multipliers, variational calculus. You're MIT students, you can go look it up. If you write down the differential equation and do a minimization, you can prove that at 31 volume percent, exactly, the system prefers to have cylinders above 31%; below 31% it prefers a bunch of spheres for the second phase. These can coarsen into something else, but the point is, this is basically where we're getting structures stabilized by two interpenetrating phases in the alpha-beta mixture.
If I look at titanium and the microstructure for a superplastic alpha-beta titanium alloy — they're not exactly spheres in this case, for other reasons; one's hexagonal close packed (the alpha phase) and the beta phase is body-centered cubic. But you get something where the grains can't grow rapidly and coarsen because the beta grains, or the alpha grains, are completely surrounded by the other phase at around just under 30%. It's harder to get superplasticity in a single-phase alloy. It's easiest to get superplasticity when you have about 30% of one phase inside another, because that 30% phase is completely encapsulated. Superplasticity is really a creep phenomenon of grain boundaries sliding against each other. You want as much grain boundary area as possible to get the most superplasticity.
Student: Why isn't there a peak at 70%?
In theory there should be. In this case, in titanium, it has to do with the diffusion of the different elements in one phase versus the other phase. If it were just simple geometry, yes, there would be one at 70. If you include the coarsening phenomena and the diffusivity in the different phases, that's probably why we get rid of the 70% peak. If you look at the structures of these alloys, you'll find they're about 30% one phase and another. For example, the aluminum-zinc alloy — this first superplastic thing that Backofen did — I can't remember if it's 70% zinc or 70% aluminum, but it's right around 30%. If you look at the phase diagram, it's right around 30% of one phase in another. That's not happenstance — it comes out of trying to introduce the most grain boundary area per unit volume and have one phase stabilize the other.
Student: Solubility also.
Solubility and diffusivity — well, they're interrelated. You have to look at the whole phase diagram. Usually it's one phase in another, and nature chooses which wants to be the minor phase and which the major phase. If solubility and everything else were all the same, you should get two peaks. Or maybe it just becomes one big thing in the middle — maybe the 50% in between isn't much of a trough. There's a lot of scatter in that data. But the point is, at 30%, from pure geometry, you get the most grain boundary area for two phases interlocking. It's hard to get superplastic alloys that are single phase. You need the stabilization of the microstructure by having the two phases intermixed.
§6. Superplastic alloy systems and the M-value plateau [21:44]
Here are some compositions of superplastic alpha-beta titanium alloys. 6-4 titanium is the workhorse titanium alloy, invented over here at Watertown Mall. Actually it was Watertown Arsenal until about 20 years ago, when it died in the first round of BRAC. It was built before the Civil War to make weapons for the Army. They made cannonballs there during the Civil War. In the 1940s they invented titanium-6-aluminum-4-vanadium, which is superplastic in the 900° Centigrade range. There are a number of different titanium alloys, but they're all in this 800 to 1000° Centigrade range, with elongations at strain rates of 10⁻⁴ per second in the 300 to 1000% range.
There are lots of different alloys that behave superplastically, not just titanium. Aluminum-copper alloys — notice 33% copper. It's actually the copper intermetallics, but if you started looking at the phases. Zinc-aluminum — here's zinc 22 aluminum, so this alloy is 70% zinc and about a third aluminum (22 by weight, but aluminum is lighter than zinc, so volume percent works out). That's superplastic at 200 to 250. It's one of the reasons Backofen did it — he liked to use low-temperature systems, it's easier. That occurs over about two orders of magnitude in strain rates.
These two are duplex stainless steels. They are both FCC and BCC, and they have a duplex structure. They're in the 30% range again, because you need that to stabilize. You can make these stainless steels superplastic. If we could make 304 stainless superplastic, believe me we would — it's the workhorse alloy. But you can't do it. You have to use a duplex stainless, because it's best to have that 30% concentration. IN-100 is a nickel-based superalloy. 738, 629, this one that actually is a copper base. IN just means International Nickel. Then nickel aluminum bronze, which is basically a copper-base alloy. So you've got copper, aluminum, iron base, and titanium base superplastic alloys. But in general they're going to be things that occur at about 30% one phase in another — plus or minus 10 or 15%, because you have to stabilize the small grain size and not have it just take off on you.
The superplastic temperature — this is the strain rate exponent M, which controls superplasticity. Typical M values for metals are down in the 0.02 range. We're down here at 0.1 and 0.2. This is T over T_M, the melting temperature — the homologous temperature. As you get up to the creep regime — creep is usually considered above six-tenths of the melting point — nickel-based alloy, titanium, 304 stainless. Even 304 stainless will get a higher M value, but it's not superplastic. 0.2 is not really superplastic. You get higher elongation, things start to stretch more, but it may only be 50 or 60 or 70%, as opposed to 30 or 40%. In general, the M value goes up rapidly once things get into the creep regime, even if they're not superplastic.
This is for a couple of aluminum alloys. If you do everything in Kelvin, you find this is the same homologous temperature — strain rate sensitivity M versus the homologous temperature. It is a creep phenomenon. You can have diffusional creep, superplasticity, or power law creep. This is the log of the stress. This is going to be a lower temperature, this an intermediate temperature, this a high temperature. Here you have low M values — 0.1 or 0.2. When you get a high M value, like a half or greater, you get superplasticity. When you get into regular creep at high temperatures, you have lower M values. So there are different types of creep.
§7. Ashby deformation maps and creep mechanisms [27:23]
There's a book in the library — I looked to see if I could buy it — that Mike Ashby wrote, called Deformation Maps for creep. They look like this. He ended up at Cambridge University and retired a couple of years ago. He's very prolific, has written many books. He has a material selector program you can buy for $50,000. It works pretty well. He's one of the top metallurgists in the world for the last 30 or 40 years — top half-dozen metallurgists in the world. He really is among the very best. He came up with these broad-scale ways of looking at things. This is the normalized tensile stress over orders of magnitude versus the homologous temperature — a creep diagram.
He had a student when he was at Harvard do a doctoral thesis at General Electric — Coble, who graduated from MIT, went to General Electric research, invented Coble creep, and came back as a faculty member when I was a student. And Nabarro creep — Nabarro was a South African metallurgist who came up with an explanation of grain boundary creep. So Nabarro creep is grain boundary creep, Coble creep is a variation on Nabarro creep with a different type of diffusional mechanism. And dislocation creep is movement of dislocations within the crystal rather than at the grain boundary. So there are different ways to look at the dislocations.
If you look at this creep phenomenon as a function of stress and strain rate, you had superplasticity in the middle, diffusional creep, and power law creep over here. Power law is Nabarro-Herring and Coble creep. If you look at the M value plotted in the same region, you see you get a peak in your M values somewhere around 10⁻³ at intermediate stresses. It's a form of creep, but it gives you a very high M value. Here's that same type of plot for what looks like aluminum-copper alloys.
This work was done by David Holt and Backofen. Holt was hired as an assistant professor to work with Backofen, and he taught me part of Backofen's course with Backofen. He didn't get tenure at MIT, went off to become an Episcopalian minister as I remember. He used to do metallurgical consulting on the side. I don't know what happened to him since then. I think there's some relationship — Carl Thompson's wife's sister is married to Dave Holt or something. Anyway, here are your M values, stress, and these are M values as a function of strain rate around 10⁻³. You start getting things above five, and when you get above five on your M value, you're getting useful superplasticity. Looking at the grain sizes here, you're seeing 1.7 to 7.7 microns. Any grain size less than 10 micron is a very fine grain material. Most commercial alloys are in the 25-micron range, just because of the heating values and the way they're formed.
Here's the correlation between activation energy for creep and self-diffusion. So it is a diffusional process — atoms are moving around at the grain boundaries and changing the shape of the grains. All you've got is atoms moving at the grain boundaries. If you started out with all equiaxed grains and you put a stress on it, the atoms move to the tensile stress direction and away from the other direction, and you get elongated grains. It's just a diffusion phenomenon.
§8. Backofen on grain growth and Schuh's nanocrystals [34:03]
There are lots of problems with trying to get fine grain size. This comes out of Backofen, sort of at the end of his book, which he published in 1972 — about ten years after he discovered superplasticity. You can see in the back of his book, this is sort of an afterthought; he was still thinking about how to maintain fine grain size. You can talk about D in millimeters, diameter of grains. D to the minus one-half is the Hall-Petch equation. If you're a physical metallurgist, you know finer grain sizes give you higher strength and higher toughness. This is millimeters, this is inches to the minus one-half, here's D in inches. Then there's the ASTM grain size number — the steel guys came up with how many grains per square millimeter at 100x on a microscope. A typical grain size in a forging or a casting might be around zero or two — the grains are big, you can see them, centimeter in size. A typical structural steel might be a grain size of eight — that might be 25 microns or so. 10 or 12 is a high-strength low-alloy steel, where we get this dynamic recrystallization to try to maintain fine grain size, same as Conform and the trolley wire. You can get 10 to 12 sometimes; if you put in some precipitates too, you might get to 13.
But if you really want superplasticity in steels, you've got to be down at ASTM 16, where you're getting down to one to five microns grain size — one or two microns. I've never seen a steel with ASTM 16. About the finest I've ever seen is ASTM 13. But it would start to act like a superplastic material. The problem is, if you heat it up to superplastic temperatures, you'll find the grains grow. That's Backofen's next plot — he's trying to think globally about this whole thing. Lifespan of succeeding generations of grains at different homologous temperatures.
If you cycle around a phase transformation — like alpha-beta in titanium, or the eutectoid transformation in aluminum-zinc — you get recrystallization as you go from one phase to the other. You have it in titanium, in steels, in the aluminum-zinc alloys. You don't have it in every metal system. If you want a 50% reduction in grain size every time you cycle through the temperature — just thermal cycling to get finer grain size — this is what happens. Backofen plotted this for a tenth of a millimeter grain size, which is ASTM 4, pretty coarse grain. Depending on whether you do 2, 4, 6, or 8 cycles, you'd have to cycle between temperatures within a second or two. That's not practical when you've got a great big thing that takes tens of seconds or minutes for the heat to penetrate.
So Backofen in the late 60s is sitting there thinking, how can I get fine grain size? Since then, people have realized that the 30% one-phase-in-another helps stabilize the grain size. You get people like Professor Schuh, who made part of his tenure basically working with ultra-fine-grain metals. His company is based on a nickel-tungsten alloy, where the tungsten — it gets into not only volume fraction but solubility and diffusivity. Nickel and tungsten have lousy solubility in each other, and tungsten has lousy diffusivity because of its high melting point. So Professor Schuh has been able to stabilize very fine grains. It took 30 years between Backofen and Professor Schuh for people to get to ultra-fine-grain materials. Some of those alloys should have excellent forming characteristics in certain temperature ranges. But you have to do your mechanical shearing before the grains get so big that they lose their elongation capability.
§9. Production quantity and the SPF niche [39:47]
I guess I bothered a few people when I asked you, for your presentation, to think about production quantity. To me, when someone comes to me about a production process to make some material, one of my first questions is: how many do you want to make? Are you going to make one? A hundred? A hundred thousand? Because what process you choose depends on how many parts you make. If I want to make some complex egg-crate construction out of titanium for some fancy aerospace application, and I'm going to make less than 10 — let's say the space shuttle — unit production cost versus production quantity for space shuttle parts, I'm going to make them by hand. I'm going to get a great big forging, get a milling machine, and hog it out, with a buy-to-fly ratio of 200 to 1. It's going to cost a fortune. The space shuttle cost about $5 billion last time we bought one, 30 years ago. Right now you can't even buy them, and they fight over them.
I was in a meeting in Houston on Thursday, and the people from Houston were very upset that New York got one of the leftover space shuttles and they didn't get one in Houston. Well, that's where the control was. Houston, if you had done a better job, we wouldn't have lost two of them anyway. The superplastic methods are competitive in a production quantity somewhere between 100 and 4 or 5,000. Now we're talking aircraft, right? How many F-22 Raptors are we supposed to make? 183. It's right in there — superplastic forming is great. It only cost us $250 billion, or whatever the number is.
If you start getting into high production volumes, you're not going to use superplastic forming. It's a slow process; it might take an hour or two in the furnace to get one part. If you're going to make 200 parts a year, no problem. One part a week, no problem. But if you're going to make a million parts a year, you're going to need an awful lot of furnaces. Superplasticity has an important niche. It just happens to have that niche in the aerospace market, which is a very important market. Any questions? That's superplasticity. We spent more time on it than maybe its inherent importance, but it illustrates lots of microstructure and principles of mechanics of forming.
§10. Farberware Millennium cookware and hydrogen cracking [42:51]
Let's talk about plain old deep drawing. [Tom produces a set of progressively drawn cups.] Here's my set of deep draws, where you go from a round circular blank and you do the first draw, the second draw, third draw, fourth draw. These have all been cut off the end. Backofen's dead, I can't find out where those came from. Farberware is the company — they came to me 15 years ago through the MIT Industrial Liaison Program. They said, "We make these very expensive pots called Millennium Cookware, and we've shipped them to Japan, and lo and behold, when they get to Japan they're cracked."
I didn't know at the time that this was in Hosford's book: stainless steel cups with stress corrosion cracking in a laboratory atmosphere within 24 hours after drawing — the cracks perpendicular to the residual hoop tension stress. The Farberware pots had cracks like this. They put them on a boat, shipped them to Japan, and when they came off the boat they had cracks running down here. [Tom passes a cup around.] If I pass it around — there are a few cracks here, more than that. Two here, two here, one. Three right there. They asked me, why do we have these cracks?
[Tom produces a magnet.] I went and got my magnet and said: because you've changed the structure of your steel. It's very magnetic up here, and it's not magnetic down here. I said, you've got hydrogen cracking. They said, what's that? I said, well, there are two types of steel. There's 18-8 stainless, which is 304 annealed, and if you strain 304 stainless, you get work hardening. This is actually a 301 stainless, this is out of Backofen. The stress, 50 ksi yield stress, and then look at this work hardening rate — it goes up above 100, 200 ksi. This percent M is the percent martensite. 301 stainless is a TRIP steel — transformation-induced plasticity. It goes from face-centered cubic non-magnetic to body-centered cubic ferromagnetic. Martensite is ferromagnetic. Right in front of them in my office — this is probably pre-1995 — I got my magnet out and showed them the different magnetism going up the wall, different amounts of strain.
When they were drawing that, the material at the bottom doesn't get strained at all, the material going up the side is hardly strained, and the material up at the top got drawn in and had lots of deformation, so you have a variation of mechanical properties. Hosford calls it stress corrosion cracking; it's actually hydrogen embrittlement. 70% of the time metallurgists don't make a distinction between stress corrosion cracking and hydrogen embrittlement. There is a difference — one occurs at the anode, one occurs at the cathode. I learned this the hard way. For two weeks I was trying to crack a piece of steel in a little test, applying a battery to try to reproduce the cracking. I couldn't get it to crack, and the literature said it was stress corrosion cracking. Corrosion occurs at the anode — so I had the piece as the positive electrode and I couldn't crack it. Finally I thought, I wonder if this paper's wrong and it's really hydrogen cracking. So I reversed the polarity, made it negative, and within one hour I had a crack. I wasted two weeks of effort because someone just called it stress corrosion cracking and it was actually hydrogen cracking. There is a difference in polarity.
70% of the metallurgical literature makes no distinction, but there is one. One is a corrosion phenomenon at the anode; the other is hydrogen embrittlement at the cathode. Here's an example: Hosford doesn't know what he's talking about — he calls it stress corrosion cracking, it's actually hydrogen embrittlement. What's the corrosion for stainless steel just sitting on a laboratory bench? So I said to the guys from Farberware: I don't know where you're getting your hydrogen. How are you getting hydrogen into your steel to cause this cracking? They said, well, would it have anything to do with the fact that to put the Teflon coating on, we spray stainless steel powders on the inside surface to make a porous surface to impregnate the Teflon, and we do that in an argon 5% hydrogen atmosphere? I said, oh, that could be a source. Duh. There we had it. It's hydrogen cracking. It's delayed cracking — it didn't exist when they put it on the boat; by the time it got off the boat in Japan, the hydrogen had done its dirty work. Even Hosford said, well, it cracked in 24 hours, well, that could be corrosion — but 24 hours is pretty fast for corrosion. It's not fast at all for hydrogen. So there's a story about deep drawing changing the properties fairly dramatically.
§11. Course logistics [49:35]
Our next class will be on Friday. Jeremy, are they going to watch the rest of the videos on their own? Let's take a vote. The videos are online for the casting class, and Jeremy you've shown them too. How many people have gone ahead and watched the other videos? Okay, one of you. Good. Do you want to have Jeremy walk you through it at 11:00 tomorrow morning? I suggest you get online to eagar.mit.edu and find the casting things. You've already seen the first two, and there are about another 10 after that. That's part of the course — you're going to watch one-third of the course by video. For the next three days, frankly, you ought to be able to finish all of that this week or next.
I'm going to be down in Florida on Monday and Tuesday, and I'm tied up Wednesday, so we may have class on Thursday or Friday of next week. Right now we'll have class this Friday, then probably a class Thursday or Friday, and then it's spring break. Sometime this week or next I'll send everybody the schedule. If you haven't signed up for which week you're going to make your presentation, some of you will be making it the week we get back from spring break, the first week in April. That's all you have to do for the course: watch the other videos, make your presentation. If all goes well, we'll be done by the end of April. We're not going to videotape the presentations unless someone really wants to see themselves on video. In general, I'm not going to put you on the web.