Structure and Performance Changes of Ni-Co-Al Shape Memory Alloys in Relation to Co/Al Atomic Ratio

As a potential ferromagnetic shape memory alloy, Ni-Co-Al has excellent mechanical properties, large magentic-field-induced strain and high martensitic transformation temperature. The relationship between microstructure and performance (mechanical and magnetic properties) of Ni-Co-Al with different Co/Al atomic ratios (RCo/A1) was investigated. Samples exhibit β and γ dual-phase structure. The γ phase grows coarse and the volume fraction of γ phase increases with the rise of RCo/A1. Besides, sample with high amount of γ phase content has smaller β grains owing to the pinning effect of γ phase. The martensite, transformed from β phase, is tetragonal L10 structure with a (11) twinning plane. The martensitic transformation temperature of samples ascends with increasing RCo/A1 owing to more Co embedded into the cell, which makes the valence electron concentration (e/a) of system rise. The saturation magnetization (Ms) of samples increases as RCo/A1 rises because Co-rich γ phase has excellent magnetic property. Meanwhile, both compressive and micro-hardness tests reveal that the samples containing more γ phase have excellent ductility due to the intrinsic good ductility nature of γ phase.