Phase Equilibria and Phase Transformations in New B2-type Ferromagnetic Shape Memory Alloys of Co-Ni-Ga and Co-Ni-Al Systems

Abstract

The phase equilibria, the martensitic and magnetic phase transformations in the β (B2) phase region of the Co–Ni–Ga and Co–Ni–Al systems have been investigated. It is shown that some compositions in the range Co–(15–30)at%Ni–30 at%Ga and Co–(30–40)at%Ni–30 at%Al exhibit both the β to β^′ (L1₀) thermoelastic martensitic transformation and the para/ferromagnetic transition. Some of these undergo the martensitic transformation from the paramagnetic β to the ferromagnetic β^′. The introduction of a small amount of the γ phase into the β phase by heat treatment, based on the results of the β⁄γ phase equilibria study in these systems, is shown to significantly improve the ductility in both alloy systems. The new β single phase and β+γ two-phase ferromagnetic shape memory alloys (FSMAs) of the Co–Ni–Ga and Co–Ni–Al systems hold great promise as new smart materials.