UC Riverside

Among Ferromagnetic Shape Memory Alloys (FSMAs), Co-Ni-Ga ternary alloys have attracted great attention due to their high Curie and martensitic transformation temperatures as well as improved ductility. Single β phase undergoes the martensitic transformation in this system, which is the base of ferromagnetic shape memory effect. In this investigation, Electromagnetic Levitation (EML) technique was employed to assess the effect of bulk supercooling and rapid solidification on near Heusler-type dual phase Co ₄₆Ni₂₇ Ga₂₇and single phase Co₄₈Ni₂₂Ga₃₀ (at%) compositions. The effects of γ+β microstructure on the martensitic and austenitic transformation temperatures and magnetic properties were also investigated. The presence of γphase was found to suppress the martensitic and austenitic transformations to below room temperature. Bulk supercooling and rapid solidification led to the formation of homogeneous single martensitic phase from hyperperitectic alloy, which under normal solidification form the dual γ+β phases. Moreover, in contrast with martensite forming from Co₄₈Ni₂₂Ga₃₀, the hyperperitectic martensite in supercooled Co₄₆Ni₂₇ Ga₂₇ samples showed no grain boundaries microsegregation. The martensite also showed a high Curie temperature about 127°C and good directional magnetic properties, including different in- and out-of-plane magnetization up to saturation level, and different coercivity from around 14 Oe for in-plane direction to about 42 Oe at the 50° angle. The role of solidification variables and chemical composition in the microstructure, and phase transformation in the ternary alloys Co₅₀Ni_50-xGa_x (at%) with x ranging up to 50, and Co_100-2yNi_yGa_y (at%) with y ranging from 15 to 35 were studied as well. For the metastable single β phase of supercooled Co₅₀Ni_22.5Ga_27.5 alloy in Co₅₀Ni_50-xGa_xseries, the martensitic transformation showed thermal stability during cyclic cooling and heating (up to 350°C) revealing its potential as a FSMA candidate. In Co_100-2yNi_yGa_y40Ni₃₀Ga_{30 alloy showed martensite to austenite transformation temperature higher than 470°C, indicating its potential as High Temperature Shape Memory Alloy (HTSMA).}