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

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 TC and the martensitic transition temperature of the β single-phase alloys are sensitive to the Ni and Al compositions. The value of TC increases with decreasing both the Ni and Al contents, while the martensitic transition starting temperature Ms decreases with decreasing the Ni content and with increasing the Al content. TC and Ms 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.