Microstructure, phase transformation and mechanical properties of Ni-Mn-Ga-Y magnetic shape memory alloys

Influence of rare earth Y addition on the microstructure and phase transitions and mechanical properties of polycrystalline Ni₅₀Mn₂₉Ga₂₁ ferromagnetic-shape memory alloy (FSMA) are investigated. It is shown that microstructure of the Ni-Mn-Ga-Y alloys consists of the matrix and the Y-rich phase. The Y-rich phase firstly disperses homogeneously in the matrix with small amounts and then tends to segregate at the grain boundaries with increasing Y substitution for Ga. The Y-rich phase is indexed to Y(Ni,Mn)₄Ga phase with a hexagonal CaCu₅ type structure. The structural transition from 5 M to 7 M, and then to non-modulated T martensite appears with the increase of Y content. The martensitic transformation temperature increases remarkably with increasing Y content, whereas the Curie temperature almost keeps unchanged. It is revealed that the appropriate addition of Y significantly enhances the yield strength and improves the ductility of the alloys. The mechanism on the influence of Y content on the improved mechanical properties and martensitic transformation temperature is also discussed.