In shape memory alloys, the right combination of crystal grains can achieve high strength and still retain memory

phys.org | 2/24/2016 | Staff
emilia (Posted by) Level 3
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A*STAR researchers have shown, through a supercomputer simulation, that high strength and shape memory can be realized at the same time by combining crystal grains of different sizes, a feat previously thought impossible. This finding demonstrates the potential of advanced simulations for tailoring materials to achieve previously unattainable physical properties.

Shape memory alloys (SMAs) are materials that can revert to an original shape by heating after being deformed at low temperature—a property used in applications such as nanoscale switches and medical devices like stents and braces.

Memory - Alloys - Functionality - Size - Crystal

However, shape memory alloys lose their functionality when the size of the constituent crystal grains goes below a certain limit—typically a few tens of nanometers.

"A couple of theories have been proposed for why this is happening," says Jerry Quek from A*STAR's Institute of High Performance Computing. "We think that it is more difficult for the memory transformation to take place at the grain boundaries than within the grains themselves. This creates an additional surface between the transformed memory phase within the grain and the untransformed phase at the grain boundary, which eventually leads to suppression of the transformation altogether at very small grain sizes."

SMAs - Metals - Grain - Sizes - Memory

This is important because SMAs, like most polycrystalline metals, become strong at very small grain sizes, where the memory effect is lost. While obtaining strength and memory at the same time in the same material seemed impossible, but if it was achieved, it could dramatically broaden the potential application and useful functionalities of SMAs.

"We were motivated by some earlier studies that showed that the combination of two different grain sizes could result in an amalgamation of useful properties such as strength and ductility," says Quek. "However, the role of these types of microstructures in SMAs was unknown. A simulation...
(Excerpt) Read more at: phys.org
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