Phase Equilibria And Directional Crystal Growth Of Co-Ni-Ga Ferromagnetic Shape Memory Alloys

As a newly developed ferromagnetic shape memory alloy, Co-Ni-Ga alloy has good ductility and prefect two-way shape memory effect. Presently, the researchers concentrated on studying the martensitic transformation and magnetic-induced strain of the alloy. However, the phase equilibria, especially the existence range ofγphase and its overall effect on the properties of the alloy are still unknown.In this paper, the microstructures of wide composition range of Co-Ni-Ga alloys were investigated. Based on the isothermal diagrams at 1423K, the effect ofγphase on the martensitic transformation, mechanical properties and shape memory effect were studied. In addition, directionally solidified Co-Ni-Ga alloys with preferred orientation were prepared by liquid metal cooling method and the preferred orientation and microstructural evolution were also discussed.The effect of composition on the microstructure of as-cast and heat-treated Co-Ni-Ga alloys was significant. The volume fraction ofγphase decreased gradually with increasing Ga content. The orderedγ' phase precipitated in the alloys with composition far from the Co corner from phase diagram during annealing or aging. Theγ' precipitates exhibited rod-like, widmanst?tten and blocky structures, which were controlled by the solid-state diffusion transformation and the mismatch degree with matrix. The features of the isothermal diagrams of Co-Ni-Ga alloys are similar to those of Ni-Al based alloys. The optimal range of Co-Ni-Ga alloy possessing the ferromagnetic single martensite state at room temperature was depicted with the phase boundaries and electron concentration dependence.The volume fraction ofγphase increased with increasing the soaking time during...