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.