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Metallurgy: The Alloy That Remembers

3 minute read

Like many scientists before him, Metallurgist William Buehler was blessed with serendipity, the gift of finding something valuable without actually looking for it. Assigned by the Naval Ordnance Laboratory in Maryland to find a nonmagnetic and noncorroding material for tools that could be safely used in dismantling magnetic mines, he finally hit upon 55-Nitinol, a nickel-titanium alloy. During further experiments, however, he discovered that the alloy also had a strange and mysterious quality in the realm of science fiction: It had a “memory.”

During laboratory tests, Buehler and Physicist Frederick Wang reported in Ocean Engineering, they fashioned Nitinol into a complex shape at a high temperature, then cooled it and crushed it beyond recognition. When they heated the alloy again, it magically regained its original shape, “remembering” every curve and angle.

The explanation, the scientists say, is that the Nitinol alloy was heated and shaped above its “transitional” temperature range — the temperature at which there is an atomic shift, or a change in its crystalline structure — then allowed to cool. No matter how they then distorted the alloy when it was below its transitional temperature, the atoms dutifully shifted right back to their original positions as soon as the alloy was heated above that temperature again.

Why the same phenomenon does not take place in other alloys, which also undergo atomic shifts during temperature changes, is not yet clear.

Although the theory remains vague, the potential uses of Nitinol alloys seem apparent enough. Goodyear Aerospace Corp. has already demonstrated that a complex Nitinol satellite antenna crumpled up into a small ball before launching can be restored to its original form simply by heating it in space. The same procedure has been proposed for orbiting a radio telescope as large as a mile in diameter. “All we have to do,” says Buehler, “is put these large structures into suitably compact packages on the ground and then kick them into space and let them unfold from solar heating.”

The transitional temperature of these structures and of all Nitinol alloys can be “set” anywhere from — 320°F. to 330°F., Buehler explains, either by varying the percentages of nickel and titanium or by substituting cobalt for some or all of the nickel. Instead of going to the trouble of assembling structures under the sea, for example, Buehler suggests prefabricating them out of Nitinol set below seawater temperatures, cooling and compressing them and then airdropping them—still cooled —into the water. Raised above their transitional temperature by the water, they would unfold and remain rigid on the ocean bottom.

Nitinol’s habit of springing back to its original shape when heat is applied also suggests to scientists that it can be used to convert heat energy to mechanical energy. Thus, say Buehler and Wang, it could be used in fire-extinguisher activators and circuit breakers. “The beauty of Nitinol,” says Buehler, “is that it’s something you load ahead of time. Then if you put it in the correct temperature range, it pulls the trigger itself.”

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