Phase transformations in Ni-Ga-Fe ferromagnetic shape memory alloys

Phase equilibria between the beta (B2 or L2(1)) and gamma (A1) phases in the Ni-Ga-Fe system at 900-1200 degreesC were determined by energy dispersion X-ray spectroscopy (EDX). The martensitic and magnetic transformation temperatures in beta single- and beta + gamma two-phase Ni-Ga-Fe ferromagnetic shape memory alloys (FSMAs) were also determined by differential scanning calorimetry (DSC) and by vibrating sample magnetometer (VSM), respectively. It was found that the martensitic and magnetic transformation temperatures of the beta single-phase alloy increase with decreasing annealing temperature in the region above 600 degreesC. While the martensitic transformation temperatures decrease with decreasing annealing temperature in the region below 600 degreesC, the Curie temperature increases and becomes saturated at about 400 degreesC. Moreover, martensite phases with a 10M or 14M structure were observed by transmission electron microscopy (TEM). These characteristic features are discussed in terms of the thermal stability of the beta phase and the order-disorder transition from the B2 structure to the L2, structure. (C) 2003 Published by Elsevier B.V.