Development of martensitic transformation induced by severe plastic deformation and subsequent heat treatment in polycrystalline Ni52Mn24Ga24 alloy

The results of a study of the martensitic transformation in the polycrystalline Ni52Mn24Ga24 alloy in different structural states are presented in the article. In the initial state in the alloy magnetic and martensitic phase transformation is observed with the following characteristic points: M-S=25 degrees C; M-F=15 degrees C; A(S)=35 degrees C; A(F)=45 degrees C; T-C=127 degrees C. The alloy in the initial state, after intensive plastic deformation by high pressure torsion and after the stepped annealing of the deformed material at temperatures of 400 degrees C, 500 degrees C and 600 degrees C was investigated. The microstructure of the alloy was investigated by means of scanning electron microscope equipped by the detector sensitive to orientation contrast of material. Analysis of the microstructure of the alloy in the initial state shows that the average grain size is 270 mu m. In the alloy after plastic deformation and subsequent annealing at 400 degrees C, this value is 180 nm. Annealing at 500 degrees C and 600 degrees C leads to an increase in the average grain size up to 1.08 mu m and 2.33 mu m, respectively. The results of the study of the microhardness of the alloy in different structural states are presented. As a result of plastic deformation of the alloy the microhardness increases from 2.1 GPa in the initial state to 5.1 GPa in the deformed state. After annealing, a gradual decreasing of the microhardness down to 2.8 GPa is found. The temperature dependence of magnetization of the alloy in different structural states shows that as a result of severe plastic deformation the ferromagnetic order is destroyed and the martensitic transformation is suppressed. After stepped annealing at 400 degrees C, 500 degrees C and 600 degrees C, there is a gradual recovery of ferromagnetic order. The martensitic transformation is observed in the alloy only after annealing at 500 degrees C. In this case the average grain size is increased to 1 mu m.