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Taming the temperamental metal transformation

An alloy can undergo millions of cycles of shape changes in response to stress jumps [Also see Report by Chluba et al.] One of the most delightful scientific demonstrations is the shape-memory effect in binary nickel-titanium (NiTi): Stretch out a spring of this alloy so it is apparently permanently deformed, drop it into a cup of hot coffee, and it literally jumps back to its original shape (1). However, looks can be deceiving. Repeat performances are limited—especially if the spring had to lift a weight while recovering—because failure by fracture would occur after a few thousand cycles. Also, the temperature at which the spring recovers can change by 10°C after the first few cycles. These effects are believed to be the result of progressive damage in the material as it goes through a large solid-to-solid first-order phase transformation (one requiring heating or cooling) that underlies the shape-memory effect. Thus, it comes as a breathtaking development that, on page 1004 of this issue, Chluba et al. (2) demonstrate 10 million cycles with nearly exact repeatability of a comparable phase transformation under the exceedingly demanding conditions of full stress-induced transformation.