Effect of aging on phase transformation, thermoelastic and fracture behavior of Mn53Ni25Ga22 ferromagnetic shape memory alloy

The effects of aging on martensitic transformation, thermoelastic and fracture behavior were investigated in the Mn53Ni 25Ga22 ferromagnetic shape memory alloy. The results show that precipitation has obvious effects on the phase transformation behavior and fracture behavior. The transformation temperature gradually decreases and then increases when the samples are aged at increasing temperatures, reaching the minimum value at an aging temperature of 773 K for 3 h, which is mainly attributable to elemental diffusion between the matrix and the precipitate. The enthalpy and entropy changes rapidly increase and then gradually decrease with an increase in the aging temperature, reaching their minimum values at an aging temperature of 573 K. Moreover, the brittleness, low strength and poor processability of Mn–Ni–Ga alloys greatly limit their application. We attempted to improve the mechanical properties without sacrificing the magnetic and thermoelastic properties by using appropriate aging treatments. The highest compressive strength and compression strain of 1594 MPa and 20.3% were obtained in alloys aged at 673 K and 573 K for 3 h, respectively. These values are approximately 750 MPa and 8% higher than the solution-treated Mn53Ni25Ga22 alloy. As a result, the improvements in the mechanical properties of Mn53Ni25Ga22 alloys from the aging treatments are mainly due to changes in the fracture type from intergranular brittleness to a quasi-cleavage ductile fracture. By controlling the number and the size of the precipitates and aging at a temperature no higher than 773 K for no more than 3 h, a significant improvement in the compressive strength and ductility of the alloys can be expected through the fine-tuning of the small precipitates.