High Pressure Torsion Effects On Shape Memory Behavior of Fe-Mn-Si-Cr Alloys

The effect of severe plastic deformation via High Pressure Torsion (HPT) on microstructure and mechanical characteristics of the Fe-Mn-Si-Cr shape memory alloys was investigated. The effects of microstructural refinement in the samples subjected to different deformation degrees consists in the formation of fine grains, ultrafine grains and also nanocristalline structures. In the experiments a modified high speed HPT technique was used, which has been ideally suited for the production of very small parts for actuators, during the processing of shape memory alloys (SMAs). Severe deformation parameters like force, pressure, number of rotation, time of torsion, were determined to obtain thin Fe based SMA discs. This type of severe plastic deformation generates large plastic deformation similar to other methods, causing changes in both mechanical and shape memory properties. An alloy subjected to this severe plastic deformation, reduces its grain size due to microstructure fragmentation by compression and rotation. The specimens exhibit different localized shearing between center and peripheral areas of the specimen which contributes to non-uniform deformation. Polycrystalline samples of Fe-Mn-Si-Cr SMA were studied before and after severe deformation. A correlation between deformation degree and the values of reversible martensitic transformation temperatures was found. Microstructural analysis was performed on the cross-sectional diameter of the specimens. The transformation temperatures in Fe-Mn-Si-Cr SMAs sujected to HPT was studied using modulated differential scanning calorimetry. The analyses of surface and hardness tests also were done.

Keywords: Severe Plastic Deformation, High Pressure Torsion, Shape Memory Alloys, Structure, Transfromation Temperature, Martensite