Effect of moderate Fe doping and heat treatment on the microstructure of Ni-Mn-Ga melt-spun ribbons

The present work investigates the effects of Mn substitution with additions of Fe (0.5, 1, 2, and 3 at%) on the crystal structure, phase transformation temperatures, and magnetic properties of casted and heat-treated Ni49.8Mn28.5-xGa21.7Fex magnetic shape memory (MSM) alloys. The alloys doped with 0.5 and 1 at% Fe exhibited single-phase seven-layered modulated (14 M) and five-layered modulated (10 M) martensite structures at ambient temperature, whereas the alloys doped with 2 and 3 at% Fe exhibited L21 Heusler austenite structures at ambient temperature. The Ni49.8Mn27.5Ga21.7Fe1 alloy exhibiting the 10 M crystal structure at ambient temperature was selected for making ribbons using the melt-spinning process. The effects of heat treatment on grain growth, crystal structure, and other functional properties of the melt-spun ribbons were studied. The ribbons heat-treated at 1273 K exhibited a 10 M martensite structure and an average grain size of 220 mu m along the ribbon plane. The results show that Ni49.8Mn27.5Ga21.7Fe1 is a particularly promising composition for designing and manufacturing functional MSM micro-actuators.