§1. The Swiss watch industry and the quartz revolution [00:40]
So in the continuing series of what I've been reading in the newspaper — last night I was reading The Economist, I flip through it on the weekend if I get it on Saturday. They had an ad in there for this watch, and I guess it was telling me LeBron James has a watch like this, or by the same people. It's a fifty-thousand-dollar watch or whatever.
There was a real problem for the Swiss watch industry back in the days when watches were mechanical movements — they didn't keep very good time. But there was a guy who was working at Bell Labs, an MIT grad, Bob Lodes [Laudise], who's a Course Six grad but he went to Bell Labs and became head of their chemistry department. And he developed a way for hydrothermal growth of quartz. That allowed people to start making quartz watches in the 1960s.
When nature makes quartz, nature takes millions of years to slowly lay down, in an aqueous solution, the SiO2 molecules on top of the solid quartz out of some silica solution. Bob [Laudise] basically developed hydrothermal growth of quartz where you have this silicon hydroxide liquid at very high temperatures, and over about a month you can grow a crystal that could be ten or twenty feet high and about this big around out of quartz. There are these reactors, the steel walls about ten inches thick. One of them blew up near Chicago airport a few years ago and landed on the McDonald's at the highway and killed one of the guys eating at McDonald's. Very high pressure, like 10,000 psi for hydrothermal growth of quartz. He just speeded up the natural process and all of a sudden quartz watches became available.
A Timex quartz watch in the early 1970s cost you twenty bucks. Nowadays you can probably get it for five or six bucks, maybe not Timex but some Chinese knockoff. And everybody said the Swiss watch industry was going to die. Well, they didn't. And by the way, the quartz watches are much more precise than the old mechanical movements with gears and stuff. Anybody know why?
You can break a second up when you're doing things with quartz into like a billion cycles a second. You've got a piezoelectric crystal, a battery which vibrates the crystal — a piezoelectric crystal, if you put a voltage on it'll give you a force. So you've got this little vibrating thing that's vibrating at like a gigahertz, and you're breaking time up into a billion times a second. You don't have to be that precise to get very good accuracy. Even if it's only a million times a second — the old mechanical movements are breaking it up into a few hundred times a second at best — the error rate is less when you have a lot more samples in a second.
In fact the most precise measurements we have in the world are atomic clocks, where they're using single vibrating cesium atoms vibrating at like 320 gigahertz, and they get one part in 10 to the 16th in terms of time. But anyway, these cheap little quartz watches — there's a little quartz oscillator on every computer chip. When it says your computer is working at 2.7 gigahertz, that's the clock speed of some little quartz watch that's telling everything to stay in sequence.
Anyway, everybody thought the Swiss watch industry was dead but it wasn't. They came out with Swatches, because people would pay for design and they were cheap enough that people could own — like one of my daughters likes to buy shoes, she has dozens of shoes — you could have a dozen Swatches with different colors you can wear with different things. Becomes a fashion statement, not just a timepiece.
But then the other end of the Swiss industry went to very expensive watches, some of which were still the old mechanical movements. They didn't keep very good time but hey, you show your Rolex. [Tom indicates his watch.] This is not a Rolex, this is a Seiko, I bought at Costco for like 150 bucks. Some of the Seikos look just like a Rolex. But if you want people to know that you wear a four-thousand-dollar watch — now if you're LeBron James you want people to know you're wearing a fifty-thousand-dollar watch. You can stud it with diamonds and everything else. This I read in The Economist yesterday — this is a titanium case and it has a ceramic face. It tells us about the material properties, so why did I bring this in?
"Time function features white ceramic bezel" — that's this thing around the face here. "Nine times harder than steel. Ceramic is exceptionally difficult to work." Yeah, it's hard. "Yet here it is finely brushed and polished as if it were a precious metal." Well, it's actually pretty easy to polish something that's really hard — you just have to use diamond. Somewhere else it said, "the complex form of the case is milled from a solid block of titanium, the individual facets are then micro sandblasted." Sounds wonderful, wouldn't you pay fifty thousand dollars for this? Well no, you wouldn't, but LeBron would, because fifty thousand dollars is just a technical foul fine for him.
So the point is there are lots of ways to market materials. The Swiss watch industry did not die. On the high end they became even more profitable, because you sell a watch — actually maybe these things cost a thousand dollars to make, a watch that you can sell for fifty thousand, good markup. And then you have ad people who basically start telling you about how it's milled from titanium. Who cares that it's milled from titanium? Aluminum's lighter. If you want something heavier, platinum's heavier. They will sell you a platinum watch, they'll probably sell you an aluminum watch too. There's no particular advantage to titanium over these other metals for a watch other than that it's titanium. Ooh, the wonder metal.
§2. How titanium became available: vacuum metallurgy at MIT [07:47]
Does anybody know why titanium is available to the world? When it became available, why it became available? It really became available during World War II, and the reason was — I don't know if it was tied into the Manhattan Project but it probably was — here at MIT in the mechanical engineering department, people developed vacuum furnaces that could melt very high temperature metals that were previously almost unavailable. Titanium was one of them. I suspect uranium was one of them — a lot of people were working on uranium in both the materials department and other places at MIT during the Manhattan Project. Certainly tungsten, molybdenum, niobium.
There's a firm out here in Newton, built in the sticks, now in the residential suburbs, called H.C. Starck. It's a spin-off indirectly of MIT. Now it's a subsidiary of Bayer Chemical — Bayer aspirin. The Sloan School is in the National Research building, and National Research was one of the spin-offs of this vacuum metallurgy; they developed a good vacuum furnace.
My thesis advisor did his doctoral thesis on single crystal tungsten. Then Margaret McVicker [MacVicar] — they refer to MacVicar Faculty Fellows — Margaret tunneled into niobium just two years after Brian Josephson discovered superconducting tunneling and won the Nobel Prize for it. He was about 22 or 23 when he did it, won the Nobel Prize when he was 25 or 26, or 27 when I met him. When he was about 29 or 30 he was a nervous wreck. Don't win the Nobel Prize when you're 26. You just can't handle the pressure. You become like a professional sports star — well, maybe not that bad.
There's all kinds of reactive metal metallurgy that grew out of that development in the mechanical engineering department at MIT. There's another important development in mechanical engineering at MIT that occurred around World War II, had to do with the creation of companies that made tensile testing machines. But it's not the tensile testing machines — those companies came out of the mechanical engineering department at MIT, things like Instron and MTS, those were both founded by MIT people. Anybody know what the thing was? Strain gauge. MIT developed strain gauges in mechanical engineering, and that's all they did for the mechanical testing — they had a little lever or a little beam, put a strain gauge on it, and you'd measure with the strain gauge how much the beam deflected, and that was your load cell for big tensile machines. Just a simple technology led to all kinds of things, just like the vacuum metallurgy.
§3. Class logistics and toughness as a design criterion [10:58]
A couple of things on the schedule: there's no class tomorrow. There will be class Monday, Wednesday, and Thursday this week with me, and Dr. Belmar will be here on Friday.
Over the theme from last Thursday: over the past 25 to 50 years, toughness has become equal to strength in many, but still not all, structural material specifications. So if I'm building a pipeline from Alaska to Texas — whether Keystone's supposed to be there — there's a toughness specification that's equal to the strength specification. In fact they'll pay a lot more for the toughness than they will for the strength. It's easy to get strength in steels for a steel pipeline; getting good toughness and strength at the same time costs a lot more. So pipelines today cost a lot more than they used to, because we're increasing the number of specifications on the materials.
Anybody have any questions? I'm going to go through toughness a little bit more. I understand Dr. Belmar came and told me last Friday he had lots of questions on toughness. Nobody has any for me, huh?
Something else came up this morning, and so it caused me to think: what's the single most important material for any type of manufacturing you can think of? It's why Tesla Motors made a mistake when they decided to site their new manufacturing facility in Nevada. Water. You can't manufacture anything without lots of water. The reason I thought about this is I went back to my office just a few minutes ago, and a guy who's an advisor to the governor of New Mexico is saying, well, we got lots of natural gas in New Mexico, they got more natural gas than Texas in the fracking business. So what if they built a plant to turn it into diesel, a liquid fuel, because it's hard to move that gaseous fuel around, very expensive — liquid fuel is a lot easier. That's why we make cars on gasoline not hydrogen.
I looked at the email when I got back. I said, well, the answer is water. No one builds automobile plants in deserts except Tesla. I was in the MIT Sloan Senior Executives Program once. Two guys from General Motors — we broke up in little teams — they came back and they were going to site their manufacturing facility in Nogales, New Mexico [Arizona], right across the border from Nogales, Mexico, so they could take advantage of the cheap labor and maquiladoras. And I said, where are you going to get the water? Do you know how many thousands of gallons of water it takes to make a car? There's not a lot of water in the car, but all those processes and all that equipment take thousands and thousands of gallons. You know how many gallons of water it takes to grow a pound of wheat? Probably ten or fifteen gallons. That loaf of bread takes ten or fifteen gallons of water.
Some people believe water is going to be our scarcest resource in the future, and it sort of is — a bottle of water costs you more than the same amount of gasoline right now, think about it. Here's a water bottle right there, you're drinking something more expensive than gasoline. I'd rather drink that than gasoline.
Student: So what's it going to do about this?
What's Tesla going to do? They're making enough money on their cars they can afford to drain the water table in Nevada, and nobody else will be able to afford to live in Nevada or pay for the water. Except there's all kinds of political things — all of a sudden they're going to tell the federal government, spend billions of dollars and start trucking it in from the Sierra Nevadas. Except California already takes significant amounts of water and moves it all up and down California, because the whole Fresno valley is a desert. And if you read about it right now, there's a huge drought going on, we're losing thirty percent of our crops in the Central Valley. We find deserts are wonderful places to grow things if you have any water. There's lots of sun in the desert usually, and people have found that sun works to help grow things. What do you know.
I did want to pass around one thing. I told you we're going to start the presentations probably — well, if things go well, we will be done if Dr. Belmar can do most of the next week, after that I have one more class. So sometime in October. I'm going to pass this around, it should have everybody's name on it; all you have to do is put a check mark on the days you would be available. Some of you have classes on certain days. You don't check days if you don't think you're going to be available or have a presentation available. So if you're not going to have a presentation available until November, don't check any of these. Well, you might put a little note that you've actually looked at it, because not everybody can come to class every day at 9:00 a.m. Some people watch it on video. Just check the dates that you would be available and I'll start working on putting together a schedule. Hopefully we'll have some people that will have a presentation by October 15th. If you don't, don't worry about it, but you should have one by the end of October, and we'll be running into November. Remember November 2nd we go off daylight savings time and that gets harder and harder to get up when it's dark outside.
§4. The ratio analysis diagram: toughness vs. strength [17:25]
Okay, so here's Ashby and fracture toughness versus strength — we've talked about this. But here's a plot I put together thirty years ago, which is called a ratio analysis diagram. Now, I didn't do the ratio analysis diagram. This guy Pellini, who worked at the Naval Research Lab on the Liberty ships that were breaking up due to brittle fracture, was the first person to put together a ratio analysis diagram, which is just fracture toughness versus yield strength on linear scales, not Ashby's logarithmic scales.
For steel you get a curve somewhat like this; for titanium here; aluminum; composites; polymers down here; and ceramics are somewhere over here. So that little watch thing I showed you, nine times as hard as steel — yeah, look at that, nine times as hard as steel. But in this region you're what we call elastic plane strain. I told you something's brittle if it breaks below the yield point — that's what we call elastic plane strain fracture. We generally like to design structural materials in the elastic-plastic mixed mode, where the thing starts to stretch a little bit before it snaps. And if you want to be fully plastic, where the thing will just stretch and stretch like Silly Putty until eventually it just collapses on itself or stretches so much that it falls apart, you can be in plastic general yielding. That's generally the low-strength version of all these materials — low-strength steels, low-strength polymers. Polyethylene is right in here, it's one of the toughest polymers, but it's not the toughest. It's low-strength, and we can use it for lawn chairs and lots of other things.
Did I tell you the story of how polyethylene got started? There was sort of a — I can't remember which chemical company, maybe several chemical companies after World War II — they learned how to make polyethylene and they decided to build some big plants and started producing it. But their quality control wasn't that good, so the properties weren't very uniform. No one wanted to commit to start using polyethylene, plus it's only good to about 200 degrees Fahrenheit; you get very hot and it starts sagging.
The chemical companies built these big plants — this was the 1950s — they were about to take a big bath on this, until someone came up with a product that they could use all this variable-grade polyethylene. Anyone ever heard of the hula hoop? Polyethylene heated up a little bit, extruded into a tube, turn the tube into a circle, put a little thing in the middle of it, and sell it to ten million kids my age — in the 1950s I was a hula-hooper. They had like a hundred million pounds of variable-grade polyethylene, and it was the hula hoop that got rid of all that surplus material and taught them, got them far enough out on the learning curve so they could start making uniform-quality polyethylene. And then people would start buying polyethylene because they could supply it. Now polyethylene is probably the most frequently used plastic. But it is low-strength, and low temperature capability.
This is the first, only time I've ever seen anybody take the ratio analysis diagram — this is before Ashby came up with his log plots — but I was just trying to show where things were, and I really was saying ceramics are not that hot as a structural material. They're so brittle that yeah, you can use them for watch faces and stuff but they're not much good for anything else. There are these lines here, where K over sigma, which is the critical fracture toughness over the yield strength of the material, equals 0.6 square root of inches. That comes out of the fracture toughness equation: K is proportional to sigma square root of pi C. Whether you do K over C equals square root of pi C, or whether you do K squared over sigma squared — which is what Ashby does on his plots — he has K squared over pi sigma squared. So there are different slopes for different criteria of stress, and there are some slight variations.
I can get more and more — I get very good toughness in steel and I can get up to 300 ksi strength. Titanium less strength, but actually on a specific strength basis titanium's a little bit better than steel. The original ratio analysis diagram for steel that Pellini did, you can find in the metals handbook — it looks something like this. He's plotting the dynamic tear energy — only the Navy could afford specimens that size — versus the yield strength in ksi. He calls this the plane strain region, the elastic-plastic region, the plastic region. He has the 2.5 critical section thickness. A good strength steel at 140 ksi can have a critical flaw size that's ten inches. Depending on the strength level, he puts in different stress levels — the ratio of K1C to the yield stress is one, so square root of pi C would be equal to one, and then 0.8, and these other things are critical section thicknesses.
In very high strength steels like landing gear at 250 ksi, the critical flaw size is four tenths of an inch, a centimeter. If you go to 300 ksi, it's a tenth of an inch, about three millimeters. When we get into something like this, the smallest flaw you can reliably detect by non-destructive testing is about three millimeters. So we don't use steels in this region unless we're going to have a really tight inspection program. Landing gears on aircraft are in the 250, maybe 280 ksi range, and they have a really tough inspection criteria. Triple vacuum arc melted steel, get rid of all the inclusions, go in there and do magnetic particle inspection.
Anybody know why landing gear are so critical on an aircraft, aside from the fact that whatever takes off you want to have an equal number of landings to takeoffs? When you go to design an aircraft, the first thing you do is design the landing gear, because the landing gear gets the highest stresses when you land, and that gets transferred to the wings and the fuselage. So the stiffness and strength of everything else on the aircraft is a function of how you support those landing gear and how the stresses go from the landing gear into the rest of the structure. If you talk to someone from Boeing about design of an aircraft — they're going to design a 787 or whatever — the first thing you design is the steel landing gear. And why do you use steel, why don't you use composites? Because you can't get the strengths with the critical flaw sizes in anything other than steel. So even though it's exceptionally heavy, you still use steel for a landing gear on an aircraft because it's the only thing.
Even there you shave the weight down on the design, and then you do a super inspection to make sure they don't develop big cracks. Because if you do, you lose a $250 million aircraft. If the landing gear breaks, you probably won't kill everybody unless it rolls over and burns everybody up, but you probably will lose the value of the aircraft if it goes sliding down the runway. I don't care what kind of aircraft it is at that point — you can't repair it, it's going to be too expensive to make it flight worthy again. So there's lots of repairability issues. You can't afford to have a landing gear fail for economic reasons or safety reasons.
Here's the ratio analysis diagram for aluminum alloys. Again we have a plastic region, but in steels we went out to 300 ksi in yield strength and the toughnesses went up to around 10,000. Aluminum is five times less tough, but it's less strong as well — it only goes up to about one fifth of the strength typically that we're going to be interested in. Proportionately it's pretty much the same. If we go to titanium, same story — rather than 1500 for aluminum or 10,000 for steels, you go up to 3500 as your dynamic tear energy. But you only go up to about 150 or 160 ksi for titanium.
Now it turns out the Navy developed a 100 ksi submarine steel at one time, when they wanted to build their own titanium submarine. You can see submarines they build in the fully plastic region. As Professor Pelloux, who's a fracture person here, used to say: they want a submarine so that when you go — if the submarine starts losing power and goes sinking to the bottom of the ocean — you slowly squeeze everyone, rather than fracture, so you don't die by drowning, you get slowly squeezed to death. Anyway, he's right.
So there's the fracture toughness of typical metals — difficult metals and everything else put on one plot. In the ratio analysis diagram on a linear scale, you can see the typical materials. If you want to go to all materials, you can go to Ashby — fracture toughness on a log scale. And what we were plotting was just this circle up in here. All these other — here's your ceramics, here's your fancy composites. If this is the criteria for toughness design, guess what — the only thing that pushes over in that region are metals. All your fancy composites, they can't get over to that line anywhere near as well as some of the better aluminums, copper, steels, nickel alloys. You can pay a fortune for these things but they just won't have the fracture resistance.
Having said all that about strength and toughness — what's the difference between strength and toughness, anybody know? In strength we're measuring how much force it takes to fracture something, pounds per square inch. Toughness is a measure of energy of fracture. All we've done is we've gone from force of fracture, which 150 years ago was the criteria for designing a structure — how strong is it? — to today, where we want to know how much energy will it absorb. [Tom searches briefly for a rubber sample.] We can have lots of toughness in rubber, not very good strength and not very good stiffness. I can get pretty good toughness and much more strength; if I try to get very much more strength in a polymer, I'm going to get something that snaps. I can have something that doesn't snap in a polymer, I just don't have much strength. If you want the best combination of strength and toughness, you're going to end up with a metal according to Ashby.
§5. The Ritchie high-entropy alloy paper [31:14]
Any questions? I told you why toughness is important — we have some very famous failures that explain why we need to have toughness as a criterion. When people didn't consider the energy of fracture, the Northridge earthquake in California in the early 90s cost 20 billion dollars to repair everything.
Here's a paper from Science magazine, I ripped it out 5th of September, first day of class. This got published, I didn't get it for a couple weeks. "Metal alloys." A lot of people think Science is the epitome of great publication to get into. Not that hard to get into Science. Here's Rob Ritchie, former faculty member in mechanical engineering at MIT, now at Berkeley, and a bunch of other people from Lawrence Livermore and Lawrence Berkeley, Germany and other places, doing some basic research on a fracture-resistant high entropy alloy for cryogenic applications. The Department of Energy is always interested in building big structures like the international fusion reactor out of very tough cryogenic materials.
I looked at the first of this paper and it told me it was a chrome-manganese-iron-cobalt-nickel alloy. I thought, well, that's okay, all they did was add iron to MP35N. But I looked for the composition and I couldn't find it. Finally the third time I looked at the paper I realized — oh, it's equiatomic. So there's five elements, each one twenty percent of the atoms. So here's a paper, a fairly long paper in Science. They love to get materials papers because they don't get very many of them; usually they come from physicists who don't know what materials are all about. They've got stress-strain curves, microstructures — this is all standard stuff. They've got stress and fracture toughness versus temperature because it's a cryogenic material. Here's crack extension, here's my little compact tension specimen I showed you in granite. This is all standard stuff. Then they take that little fracture toughness specimen, they put it in the scanning electron microscope, they show nice ductile dimples.
Here's the reason I broke it out — there's an Ashby plot in color right there in Science magazine. I'm sure the physicist was just all atwitter with the excitement of looking at what Michael Ashby developed 25 years ago — probably all new to them. Here are the high entropy alloys. If we look at this strength-toughness plot, you see the high entropy alloys are pushing out there — not necessarily as good as some of the copper alloys or some other metal alloys, but they're pretty good. They have a highest fracture toughness of virtually anything, around 300 megapascals root meter, which is almost the same as ksi square root of inch. And they've got yield strengths in megapascals of something like 300 ksi. So you actually could make aircraft landing gear or something out of this, particularly if you wanted to use it at cryogenic temperatures.
§6. MP35N: a cobalt-nickel-chrome-molybdenum alloy [35:09]
As I looked at this, I thought, what are these guys doing — they're just looking at one of my favorite alloys, MP35N. MP35N has been around since probably the 1960s. A guy named Gaylord Smith at International Nickel came up with a four-component system which was basically chromium, nickel, molybdenum, cobalt. All they did is they threw some iron in there. It's a cobalt alloy — cobalt's kind of expensive. Molybdenum is about thirty for both of these, about twenty for chromium. So it's sort of a nickel-chromium-cobalt steel with some aluminum, but it's a very interesting material.
First of all, it's non-magnetic, and I'll tell you why that's important to me. It's also — there are four bolts that are MP35N that hold the wing of the 747 onto the fuselage. Only four bolts. Now they're fair-sized bolts, but they're really high strength, like 300 ksi bolts. If you go to Standard Pressed Steel website — Standard Pressed Steel is down in Philadelphia, they make really high strength bolts — MP35N is one of the more expensive alloys, it is a pretty pricey alloy.
Here's a plot from Carpenter Technology. The patent is expired now so everybody can make this. They're comparing this high-strength cold-worked-plus-aged iron-base alloy — some people call it an iron superalloy — which has yield strength and tensile strength of 150 ksi, kind of typical high strength steel. Here's Pyromet A286 with more cold work, 200 ksi. Pyromet 718 — this is Inconel 718, a nickel-based superalloy, the most used Inconel superalloy — up to 210 ksi yield strength. MP35N: 280. See why I like it. Excellent strength and toughness.
Now, one of the things that makes it so tough — there's now a standard specification for it, an ASTM spec, and this goes back to 1978. Current edition approved 2013, originally approved in 1978. Made it into the ASTM specs: 35 cobalt, 35 nickel, 20 chrome, 10 molybdenum, for surgical implant applications. You can buy MP35N: 230 tensile, 230 yield, eight percent elongation — that's a lot of elongation — 35 reduction in area. This stuff really does stretch. Now, Rob Ritchie's high entropy alloys are a little better than this, but not a lot better. This was one of the best materials, and that's why they use it for bolts to hold the wing on.
Why don't they use it for the landing gear? Because this gets its strength from cold working. You can make a bolt that's one or one and a half inches in diameter and get lots of strength. But you can't make a landing gear that's six inches in diameter — we don't have the equipment that's heavy enough to get 80, 90 percent cold work into a structure like that. If they did, they would make landing gears out of MP35N.
Now why do I like it? I told you about my story — I used to consult with a division of Johnson and Johnson. By the way, if you go thinner in this material, in wire form less than two thousandths in diameter, you can get 290 to 330 tensile strength, 320 to 360. Really strong stuff as wire.
§7. MP35N in neurosurgery: MRI-safe brain clips [39:54]
I once got involved in a fight — I didn't get involved, there was a fight. There was a neurosurgeon, one of the top neurosurgeons in the country, this was 15 to 20 years ago. The companies that make medical instruments basically pay big consulting fees to a few top MDs to design new types of surgery instruments. People were concerned — there were two top neurosurgeons, and one, I can't remember his name, but Dr. McFadden was the one consulting for Johnson and Johnson.
They were worried about MRIs. People have an MRI, and if you actually had a blood clot in the brain, they come in, take a little steel or stainless steel or titanium clip, and stop the bleeding by putting this little clip on the blood vessel so it's not hemorrhaging in the brain anymore. The problem is, you then want to go get an MRI, and if that clip has any ferromagnetic moment to it and you put it in a five-tesla MRI machine, it just torques right off and you now got another hemorrhage in the brain.
So there were all these MDs who were worried about the Curie temperature of high-alloy steels. They were fighting over the fact that some steels — even 304 stainless, which is 18-8 chromium-nickel, 18 chrome 8 nickel stainless steel, the first original stainless steel from 1910 or so — if you cold work it, it can transform from the austenitic phase to a martensitic phase. The martensitic phase can be magnetic. I didn't bring my little pot with me, but I have a Faberware cooking pot that was formed and it was 304 stainless steel, and I take a little magnet and just go up that pot and you can see — at the top of the pot, I'll have to remember to bring it Wednesday — go up that little pot and you can see the increased magnetism where you get more plastic deformation in the steel.
Some medical doctor had learned a little bit of knowledge, which of course is a dangerous thing, and decided that this could be a problem if you make stainless steel clips for the hemorrhaging of the blood vessels in the brain. And that was correct. Other MDs decided any alloy that contained iron must be bad, because iron is ferromagnetic — they learned that in freshman chemistry, or maybe senior chemistry if they were an MD in the wrong field. Iron's ferromagnetic, and it must carry its ferromagnetism into the alloy. Well, that's not true. There's something called a Curie temperature — I could show you the iron-carbon phase diagram, and they plot the Curie temperature, and it decreases as you put more carbon in there. Or in stainless steel, you put more chrome in there and it becomes non-ferromagnetic. Ferromagnetism is a very complex materials property. It's a second-order phase transformation for all you materials scientists out there, it's not a first-order phase transformation.
So they were having a fight about whether you could tolerate any iron impurity whatsoever in these little clips they were going to embed in people's brains. Dr. McFadden came to my office to talk about all this. I said, well, why don't you just use MP35N — it doesn't have any iron in it. He thought that was wonderful because now he could go and stick it to his colleague, who was also a world-renowned neurosurgeon. They were actually fighting over ASTM specs for what type of materials you could use in the brain as implants. MP35N is a wonderful material for implanting in the body because it's got 20 chrome, it's got excellent corrosion resistance.
MP35N is something we've had for a long time. You can get tremendous strengths in wire form, you can even get it in bolt form; you can't really buy plate form or a great big forging form with these super properties, but we've had it for a while. The reason it has these properties: it doesn't get its strengthening by dislocation pile-ups, it actually deforms by a twinning mechanism. Which is exactly the same thing as this equiatomic alloy that Rob Ritchie's got. If you read that article, it's also a twin-induced transformation that causes it to get super strength. So it's not all that different than MP35N. Here's a paper — I'm sure they got it through the Basic Energy Sciences division of DOE, because oh, it's equiatomic, it's high entropy alloy. Yes?
Student: Is it weldable?
Yeah, it is weldable. I don't know that anybody — you lose your cold-work strength if you weld it. There's no reason why it's not weldable, it's basically just sort of like a nickel or stainless steel alloy. But you lose your strength because you lose your cold work. Yep?
Student: What about temperature applications?
I think it probably is limited to like five or six hundred degrees Fahrenheit, maybe centigrade. It's got limited high-temperature strength. But landing gear, I don't care; a little magnetic clip in my brain — or non-magnetic clip in my brain — it better not get that hot, right. So there's no one material that's good for everything, but there's at least something.
§8. Density and high-density alloys: penetrators and platinum eyelid weights [45:59]
I'll skip the fracture toughness density and we'll go on. There are other properties of materials — I told you we started this out a couple of lectures ago with: there are limits to materials properties. This is out of one of Ashby's books — he's got the density of different alloys. Here's your cobalt alloys around 8 or 9; copper 8.9; iron 7.9. You get down to aluminum and stuff — should be aluminum on here, I don't see it. It only went down to 5.2. Aluminum's about 2.7. Gold's fairly dense, uranium is very dense, platinum's dense, and osmium is even more dense than tungsten.
Anybody know about penetrators for armor? You want a high-density alloy so you have lots of momentum. What did they use in the first Gulf War? They used depleted uranium. The problem with those soldiers firing depleted uranium shells — they're not completely depleted of all radioactivity, and they left all these uranium shells all over Kuwait. The Kuwaitis weren't all that happy because then they had to send all these Pakistanis and Bangladeshis out there through the desert looking for all the shells, because they had a slight radioactivity to their desert now.
A good uranium penetrator will just go through — I've seen the effects of a uranium penetrator going through three feet of steel. Now, it has to be a three-foot penetrator, but roughly the length of the penetrator. If you have what they call an RPG, rocket-propelled grenade, a shape-impact weapon — people can carry these things like bazookas on your shoulder, and it'll blow up a tank, or certainly blow up a Humvee. They had to come up with something better than Humvees. They're looking for high-density alloys and trying to get around uranium.
Professor Schuh in this department's had a lot of money from the Army to do some fundamental studies of microcrystalline titanium alloys, because the Army wants to use titanium — high-density alloy, not radioactive — to replace the depleted uranium in our arsenal. They could have used platinum, they could have used gold — high density — but you wouldn't want to pay for them.
I did come across a little application in the Boston Globe. I didn't cut it out, but a few weeks ago — one Friday I had a sinus infection, off in April, and I got hospitalized for it, and I now have lost some of the hearing in my ear. So I went over to see the world's best optic nerve facial surgeon — she's over at Mass Eye and Ear. One of the nice things about living in Boston: lots of good doctors. Turns out she's a graduate of the Harvard-MIT Health Sciences and Technology program, so when she found out I was a professor, it sort of opens up with the doctors. Next thing I know, I'm looking at the paper on Monday morning, the Boston Globe, and there's Tessa Hadlock's picture, and she's treating this woman who had been cut severely by some boyfriend in Florida.
As you read the article — I read it because I knew Dr. Hadlock, she was treating my ear — this woman had a problem with her eyelid, she had been cut very severely, and so Dr. Hadlock was using platinum weights. She embedded platinum weights into the eyelid so this young woman could close her eye after some of the plastic surgery. The muscles weren't strong enough to close the eyelid, but with a little help from dense platinum. Why platinum? It's corrosion resistant, it's got high density, you don't have to use a very big one, it's not going to be a bulge on your eyelid, it's not going to be uncomfortable. And it's corrosion resistant to the body and non-toxic. So you start looking at the suite of properties you need — even though you're not going to sell a million of these, it's sort of a one-of-a-kind knockoff — there's an application. Two applications: penetrators for rifles for the military, and eyelid weights. I guess maybe I'll wait and hand that out on Monday or Wednesday. Anybody have any — yeah?
§9. Copper: when to pay for it [51:06]
Student: [Question about copper alloys / toughness.]
Copper alloys are excellent, except for the fact they cost about ten times as much as steel. They cost more than stainless steel. We love copper alloys when they have a property that we need, like electrical conductivity. The only thing with better electrical conductivity than copper is silver. And we don't use silver most of the time because it's a hundred times the price of copper.
We did use silver wiring in World War II. They built a bunch of magnets at Oak Ridge National Laboratory because they were using magnetic separation of uranium. Copper was short because they needed the brass cartridge cases for all the bullets. They actually borrowed something like 100 tons of silver from Fort Knox, Kentucky, where they store all the precious metals for the government, and after the war they had to return it. But they wound their magnets out of silver, which is a better material for electrical conductivity.
So we use copper for electrical conductivity, we use it for non-sparking tools, we use it for corrosion resistance in piping — underground piping. I spent nine thousand dollars to put about 50 square feet of copper on my roof, because hey, it'll be there a hundred years from now, it'll be there 300 years from now. If my house is there I won't be, but anyway. Copper alloys in many cases have excellent electrical properties, excellent corrosion-resistant properties, but we're only going to pay that price when we need those properties. As structural materials — other than piping for corrosion resistance when you're burying it in the ground — we usually have other things we can use that are a lot cheaper. That's the competition between materials. Other questions?
Okay, we'll finish two minutes earlier — that makes up for the two or three minutes I keep you late some other times, right?