Backofen superplasticity rediscovery

Tom references Backofen's foundational aluminum-zinc superplasticity work (around 30% zinc or 30% aluminum) and his 1972 book where he was still wrestling with how to maintain fine grain size. Not a developed case but a recurring methodological anchor — Backofen was Tom's teacher.

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.

Backofen, working at MIT in the 1960s on fine-grain-size effects, rediscovered superplasticity. Found a 1938 German paper that had been lost during WWII. Tom inherited Backofen's original samples when he took over his office as an assistant professor and uses them as teaching artifacts.

Backofen rediscovered superplasticity. He found a 1938 paper from Germany where they had measured superplasticity with very fine grain material. Because of World War II it got lost. It wasn't until the early '60s that Backofen rediscovered it. I will bring in some of Backofen's original samples — I was, you know, the short little door 8137 as you walk down around the corner from the infinite corridor, that was my first office. I shared it as an assistant professor with Backofen's last doctoral students, and I picked up many of his samples. When I teach deformation processing, you're going to see Al Backofen's samples, his touchy-feelies, to pass around the room. These superplasticity samples should be in the Smithsonian. We couldn't build jet engines without superplasticity.

Backofen's 1965 rediscovery of German pre-WWII superplasticity work, which enabled isothermal forging of jet engine turbine discs. Tom references graphite and molybdenum dies for hot superplastic forming of nickel superalloys at 2200°F.

The third one is called isothermal forging, and it was developed because of Backofen and superplasticity in 1965, when he rediscovered it from Germany. That's Tuesday's lecture; I don't think we'll get to that today. Isothermal forging is how we make a lot of the turbine discs for airplanes, jet engines and so on. That sample should be in the Smithsonian. I recovered it from a graduate student's desk — they were using it as an ashtray.