Martensitic transition and superelasticity of Co–Ni–Al ferromagnetic shape memory alloys with β + γ two-phase structure

Abstract

The martensitic and magnetic phase transitions of Co–Ni–Al β single and β + γ two-phase alloys have been systematically investigated. The Curie temperature T C and the martensitic transition temperature of the β single-phase alloys are sensitive to the Ni and Al compositions. The value of T C increases with decreasing both the Ni and Al contents, while the martensitic transition starting temperature M s decreases with decreasing the Ni content and with increasing the Al content. T C and M s of the β + γ two-phase alloy depend on the annealing temperature due to the variation of the chemical composition of β phase equilibrated with γ phase, increasing by the increase of annealing temperature. In addition, the superelasticity (SE) in tensile loading of the Co–Ni–Al polycrystal β + γ alloys have been investigated. It was found that the SE properties in the β + γ two-phase alloys strongly depend on the average β grain size as well as the volume fraction of γ phase. Consequently, the maximum recovery SE strain of about 6.3% was achieved in a sheet specimen with the large β grains.