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

Phase equilibria between the β (B2 or L2₁) and γ (A1) phases in the Ni-Ga-Fe system at 900-1200 °C were determined by energy dispersion X-ray spectroscopy (EDX). The martensitic and magnetic transformation temperatures in β single- and β+γ 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 β single-phase alloy increase with decreasing annealing temperature in the region above 600 °C. While the martensitic transformation temperatures decrease with decreasing annealing temperature in the region below 600 °C, the Curie temperature increases and becomes saturated at about 400 °C. 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 β phase and the order-disorder transition from the B2 structure to the L2₁ structure.