Iron whiskers (1950s screw dislocation studies)

1940s iron whiskers with axial screw dislocations measured at 2 million PSI when pulled along the screw axis. Used to establish that theoretical strengths are achievable only in idealized geometries.

So did we ever make any material that had that strength? In the 1940s we made iron whiskers, which had a screw dislocation right up the axis of this whisker. Anybody who knows anything about dislocations — if you pull in the same direction as the screw dislocation, then there's no stress on it. A screw dislocation moves by shear through the material. They pulled these whiskers experimentally, they measured 2 million PSI. So now, forty years later, I hear about people discovering carbon nanotubes with these tremendous strengths. Except the problem is, the physicists assume a perfect crystal with no dislocations. At what temperature is that true? Absolute zero. At any temperature above absolute zero, you're going to have some vacancies in those nanotubes. And guess what's going to happen to the strength of that nanotube when you have a vacancy? I've already ripped a piece of paper for you to show you what a notch does.

Counter-example to the "defects destroy strength" rule — iron whiskers grown as single crystals with one axial screw dislocation achieved 2.2 million psi because the dislocation was not loaded.

Some physicist down at Rice University, who thought he should win the Nobel Prize — actually I think he did, for discovering buckyballs — did the calculation that had been done in the 1930s and took credit for it. "I've discovered a new material that's going to be ten times stronger than anything else we have." In the 1940s we actually measured iron whiskers with a strength of 2.2 million psi, and we got around the defects of the dislocations because the iron whisker was a single crystal with a screw dislocation right down the axis. When you pulled on it there was no stress on the dislocation — if you know about dislocations and Burgers vectors — and you got 2.2 million psi. A student about twenty years ago asked, how do you do that calculation? I'd explained how you would do it, and they challenged me. I went back to my office, in five minutes I'd scratched it out, got 2.2 million psi for the strength of iron. It's just the strength of the chemical bond between two iron atoms. The calculation was done ninety years ago. But this guy twenty years ago discovers buckyballs and wins the Nobel Prize because he goes around the press: I've discovered a new material that's going to revolutionize the world. He discovered those twenty, twenty-five years ago. Have buckyballs revolutionized your world? Don't think so. Although it has, for all of you doing research — there's a lot of research money to study this.

1950s growth of single-crystal iron whiskers containing only a screw dislocation along the axis. Pulled to 2 million PSI experimental strength — the theoretical strength of perfect crystal steel. Used to argue that physicists' claims about carbon nanotube strength (the "rope to the moon") are decades behind what materials scientists already demonstrated and refuted.

We've known that for 50 years. The physicists didn't know it because they were 50 years behind the materials scientists — actually they're still more than 50 years behind. We grew iron whiskers in the 1950s that were a single crystal, all but a screw dislocation up the axis. They pulled those and got 2 million PSI strength experimentally, because if you don't have any mobile dislocations, you pull on a screw dislocation parallel with the axis of the screw dislocation, it won't move. It was like pulling a perfect crystal, and they got 2 million PSI strength for steel. We don't have steels at 2 million PSI. But the physicists are out there telling Congress and everybody else in the world that they've discovered carbon nanotubes that will allow us to build the rope chain to the moon. You've heard about that, haven't you? Some of you saying yes.

Why are they so interested? Because the carbon-carbon bond is the second-strongest bond we know. The only stronger bond is silicon-oxygen. So very strong, that's why it's very hard. They're predicting all kinds of things for graphene sheets. They predict the strength of these carbon nanotubes is 2 million PSI. Well, when you watch some of the other videos, I'll explain that we had iron whiskers with 2 million PSI 60 years ago. The physicists don't know about it though — don't tell them.

Single-crystal iron whiskers grown in the 1950s with only a screw dislocation (no edge dislocations), measured at ~1.2 million PSI — close to the theoretical Lennard-Jones prediction of ~2 million PSI. Used to validate the dislocation theory of weakening.

So people predict tremendous strengths, and in the 1950s they actually found single crystals of iron. They grew iron whiskers and they had a screw dislocation right up the middle with no edge dislocations — a perfect crystal except for the screw dislocation. If you pull parallel to a screw dislocation, what's the force on the screw to move the dislocation? Zero. If you pulled an edge dislocation in tension, it'd be just like the cleaning lady in Hungary that showed Orowan about dislocations. But a screw dislocation — if you look at the Burgers vector — does anyone study dislocations anymore? I had to take a whole graduate course in dislocations. It was horrible. If you actually study dislocations, you find that a material might lose 90% of its strength because of the movement of dislocations.