Study of the precipitate phases and martensitic transformation in quaternary Heusler alloys of NiMnGaDy

The precipitate phases and martensitic transformation behavior of polycrystalline Ni50Mn29Ga21−xDyx ( x = 0 – 5 ) ferromagnetic shape memory alloys are investigated. The results show that some Dy-rich precipitates are observed in higher Dy-addition alloys. The precipitates can be indexed to Dy(Ni,Mn)4Ga phase with a hexagonal CaCu5 type structure. When the content of Dy increases from 0.5 at.% to 2 at.%, Dy(Ni,Mn)4Ga phase distributes mainly along the grain boundaries, while a eutectic structure composed of the matrix and Dy(Ni,Mn)4Ga phase is observed with the further increase of Dy content. In addition, structural transition from 5M to 7M, and then to non-modulated T martensite appears with the increase of Dy content. It is found that substitution of Dy for Ga up to 5 at.%, one-step thermoelastic martensitic transformation can still be observed in this quaternary alloy. The Dy doping markedly increases the martensitic transformation temperature from 326 K to 512 K.