Effect of composition and heat treatment on the martensitic transformations in Co–Ni–Ga alloys

A series of Co–Ni–Ga samples was investigated with regard to the structural, thermal, magnetic and mechanical properties. Optical and transmission electron microscopies revealed two types of two-phase polycrystalline alloys: (a) martensite + γ precipitates and (b) parent phase + γ precipitates. X-ray and electron diffraction methods were used to identify the martensitic phase as body-centered tetragonal structure with about c0/a0 ≈ 1.2. The structures of the parent and the γ phase were determined as B2 and face-centered cubic, respectively. Influence of heat treatment on microstructure, martensitic transformation temperatures and the Curie temperature were reported. Annealing at 1073 K for 20 h causes dispersion of precipitation and led to grain boundary decoration, since, quenching from 1223 K results in a considerable increase of martensitic start and Curie temperatures. Additionally, compression tests were performed.