The Structure and Martensitic Transformation of Deformed Ni-Mn-Ga Alloys

The results of the study of the influence of multi-axial isothermal forging on the microstructure and martensitic transformation in a Ni58Mn18Ga24 alloy have been presented. Forging is performed in two stages: the first stage is forging at 700 degrees C by four passes with true strain e approximate to 1.64; the second stage is forging at 500 degrees C by one pass with e approximate to 0.24. The forging deformation results in the transformation of the original equiaxed-grain structure. No new grains are formed after the first stage of treatment. New recrystallized grains in a very negligible extent begin to be observed only after the second stage of deformation. Probably, at the first stage, the mechanism of fragmentation of the grain structure is not started due to an insufficient density of defects upon the deformation at 700 degrees C. The characteristic temperatures of martensitic transformation are shifted after forging to the low temperature region. The anharmonic change in the sample length is observed in the region of martensitic transformation for the both treated states. In general, this indicates a low level of defect density and internal stresses in the sample.