Shape memory alloy phase transition. (Credit: Ames Laboratory)

Researchers have compared two copper-based SMAs of the same composition but fabricated differently. After annealing, the samples were cooled at different rates. Then both samples were heated inside the transmission electron microscope (TEM), so that scientists could observe the martensitic phase transition in real time.

The rapidly cooled sample transformed at a lower temperature and with better “memory” than the more slowly cooled sample. They attributed this to the formation of tiny nickel-rich dots that appeared in the slowly cooled sample, which changed the phase transition pathway and negatively affected the alloy’s performance.

Those materials Ni-rich precipitates change the matrix alloy composition and make phase transition harder to reverse, and thus the energy loop is less reliable.

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