Issue Date: June 1, 2015 | Web Date: May 28, 2015
Alloy Deforms, Springs Back Into Shape Millions Of Times
By adding a touch of cobalt to an alloy of titanium, nickel, and copper, an international team of researchers has come up with a shape-memory alloy film that can be deformed at least 10 million times and still snap back to its original shape. The finding represents a remarkable improvement on previous shape-memory alloys, which, at best, could withstand only a thousand deformations before succumbing to structural failure.
Shape-memory alloys, as their name implies, possess the ability to return to their original shape after being stretched or squished. Nickel-titanium shape-memory alloys, for example, are used as stents to open blood vessels and as orthodontic wires. Other applications, however, have been limited because this and other shape-memory alloys tend to deform permanently or break apart after a certain number of deformations. The new alloy film shows remarkable resilience, returning to its original shape even after being deformed a record-breaking number of times (Science 2015, DOI: 10.1126/science.1261164).
“The alloys were designed to have improved properties, but we were surprised to discover this superhigh fatigue life,” says the University of Maryland’s Manfred Wuttig, who spearheaded the work along with Eckhard Quandt of the University of Kiel, in Germany. “When you do something like this, you expect to improve by a factor of two or a factor of 10. This improvement was astounding,” Wuttig adds.
The alloy, which has the formula Ti54.7Ni30.7Cu12.3Co2.3, seems to achieve its fatigue-defying ability by switching easily from one crystalline form to another and back again. The films contain Ti2Cu precipitates embedded in the base alloy that act as sentinels, easing the transformation between crystalline forms.
“This alloy shows incredible reversibility under the most demanding conditions people can imagine,” comments Richard D. James, an expert on shape-memory alloys at the University of Minnesota, Twin Cities. “What’s also interesting to me is that there’s a strategy buried in there for making lots of phase transformations reversible like this,” James adds. “It’s probably the most important thing to come along in shape-memory alloys since the invention of NiTi.”
- Chemical & Engineering News
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