Microstructures and magnetic properties of rapidly solidified Ni54Fe27−2xGa19+2x ferromagnetic Heusler alloys

Abstract

Rapidly solidified Ni54Fe27−2xGa19+2x (x=0, 1, 2, 3, and 4) ferromagnetic shape memory alloys were made by melt-spinning with variation of Fe and Ga contents to report on the martensitic phase transformation, microstructures, and magnetic properties. Rapid solidification produced pure L21 phase by preventing the formation of γ-phase. To study the effect of heat treatment on the phase transitions, microstructures, and the magnetic properties, the melt-spun ribbons were partly heat treated at different temperatures of 800, 900, 1000, 1100, and 1200 K with holding times of 5, 10, and 15 min followed by either water quenching or air cooling. The microstructures of the as-spun ribbons as revealed by electron microscopy studies exhibited a gradual transition from cellular to dendritic structure with increasing Ga concentration and with the presence of some internal martensitic twin bands at higher Ga content. The ribbons exhibited very low coercivity with high saturation magnetization, as high as ∼87 emu/g (decreasing with Ga concentration). The above-ambient Curie temperature (TC) and the subambient martensitic transition temperature (Tm) were observed to be ∼320 and ∼195 K, respectively. At higher heat treatment temperatures formation of ductile γ phases was observed.