Crystal structures and phase transitions in ferromagnetic shape memory alloys based on Co-Ni-Al and Co-Ni-Ga

摘要

Magnetization and AC permeability measurements show that the shape memory compounds Co1.52Ni1.32Al1.16 and Co1.8Ni0.92Ga1.28 both order ferromagnetically below approximate to 280 K and show a further transition at 220 K. Neutron powder diffraction measurements have confirmed that the lower transition is martensitic and have enabled the structures of the austenite and martensite phases in both compounds to be established. The temperatures at which the transitions occur in the two alloys are similar, as are the structures of both their austenite and martensite phases. The austenite phase Of Co1.52Ni1.32Al1.16 is partially ordered in the B2 structure. The martensite phase in both compounds arises from a simple tetragonal distortion of the cubic cell. Unlike the related ferromagnetic shape memory compound Ni2MnGa, the martensitic transitions are not abrupt and there is a temperature range of at least 100 K in which both tetragonal and cubic phases coexist. The widths of the cubic Bragg peaks are resolution limited but those associated with the tetragonal structure show broadening consistent with the presence of anisotropic micro-strain.