Effect of aging time on shape memory properties of sputtered Ni-rich Ni₂MnGa alloy films

Ni₂MnGa alloy is an intelligent material with ferromagnetic and shape memory properties. The application of the alloy films to microactuators has been proposed. The Ni-rich Ni₂MnGa alloy films with a thickness of nearly 5 μm were deposited on Al₂O₃ substrates by a radio-frequency magnetron sputtering apparatus using a Ni₅₂Mn₂₄Ga₂₄ target. They were heat-treated at 1073 K for 36 ks for homogenization and ordering. The martensitic transformation temperatures of the heat-treated films were higher than room temperature. To investigate the effect of aging time on shape memory properties, the heat-treated films were aged at 673 K for various times between 0.9 and 57.6 ks in a constrained condition. The constraint-aged films showed the two-way shape memory effect by thermal cycling. Fine precipitates with a crystal structure of L1₂ were observed in the constraint-films aged for a long period. As for their two-way shape memory properties, a range of transformation temperature narrowed and the amount of macroscopic shape change increased with increasing aging time.