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

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' (L1 0 ) 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 y 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 + y two-phase ferromagnetic shape memory alloys (FSMAs) of the Co-Ni-Ga and Co-Ni-Al systems hold great promise as new smart materials.