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 beta (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 beta to beta' (L1(0)) thermoelastic martensitic transformation and the para/ferromagnetic transition. Some of these undergo the martensitic transformation from the paramagnetic beta to the ferromagnetic beta'. The introduction of a small amount of the gamma phase into the beta phase by heat treatment, based on the results of the beta/gamma phase equilibria study in these systems, is shown to significantly improve the ductility in both alloy systems. The new beta single phase and beta + gamma two-phase ferromagnetic shape memory alloys (FSMAs) of the Co-Ni-Ga and Co-Ni-Al systems hold great promise as new smart materials.