Development of Cobalt-Nickel-Gallium Ferromagnetic Shape Memory Alloys (FSMAs) by Investigating the Effects of Solidification Processing Parameters

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 beta 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 Co46Ni27Ga27 and single phase Co48Ni 22Ga30 (at%) compositions. The effects of gamma+beta microstructure on the martensitic and austenitic transformation temperatures and magnetic properties were also investigated. The presence of gamma phase 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 gamma+beta phases. Moreover, in contrast with martensite forming from Co48Ni22Ga 30, the hyperperitectic martensite in supercooled Co46Ni 27Ga27 samples showed no grain boundary 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 Co50Ni50-x Gax (at%) with x ranging up to 50, and Co100-2yNi yGay(at%) with y ranging from 15 to 35 were studied as well. For the metastable single beta phase of supercooled Co50Ni 22.5Ga27.5 alloy in Co50Ni50-xGa x series, the martensitic transformation showed thermal stability during cyclic cooling and heating (up to 350°C) revealing its potential as a FSMA candidate. In Co100-2yNiyGay series, on the other hand, paramagnetic Co40Ni30Ga30 alloy showed martensite to austenite transformation temperature higher than 470°C, indicating its potential as High Temperature Shape Memory Alloy (HTSMA).