Structure and performance changes of Ni-Co-Al shape memory alloys in relation to Co/Al atomic ratio

Abstract

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 (R_Co/Al) 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 R_Co/Al. 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 Ll₀ structure with a $\left(1\bar11\right)$ twinning plane. The martensitic transformation temperature of samples ascends with increasing R_Co/Al owing to more Co embedded into the cell, which makes the valence electron concentration (e/a) of system rise. The saturation magnetization (M_s) of samples increases as R_Co/Al 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.