Comparison of influence forging and extrusion on microstructure of heusler alloys

The results of investigation of the microstructure of two polycrystalline Ni-Mn-Ga alloys subjected to thermo-mechanical treatment by multiple isothermal forging and extrusion are presented. Alloy forging at a temperature of 680°C and 700°C leads to the formation of a bimodal structure which has large grains of several hundred micrometers surrounded by a layer of the fine-grained structure. As the result of the further treatment by extrusion at 710°C the volume fraction of the fine-grained structure is increased. At the same time, anisotropy of thermal expansion during the martensitic transformation is observed in the alloys in both states due to residual tensile stresses introduced in the last stages of treatment. The performed investigation shows high potential of the thermo-mechanical treatment for obtaining structurally modified Heusler alloys having a sufficient level of functional and service properties for practical applications.