NiTi alloys are the most widely used shape memory alloys (SMAs) due to their superior mechanical and functional properties, however their use is restricted below 100 °C. In order to extend their utility in high temperature applications, higher martensitic transformation temperatures than 100 °C, lower temperature hysteresis, and better cyclic reversibility are required. The transformation temperatures of NiTi alloys can be increased by the addition of Hf and Pd, however, these additions lead to either the degradation of thermal cyclic response under stress due to the decrease in critical stress for slip or the reduction in fracture toughness due to the formation of detrimental precipitates/inclusions. In this study, severe plastic deformation via equal channel angular extrusion (ECAE) was used to increase the critical stress for slip by grain refinement down to submicron to nanometer range (100 to 300 nm) and by designing specific textures in NiTi and NiTi(Pd,Hf) alloys. A significant improvement in the thermal cyclic stability of the ECAE processed samples was shown which was attributed to the grain refinement. In addition, the severe simple shear deformation led to the dissolution and/or fragmentation of precipitates (mainly oxides or carbides) into very small sizes increasing the fracture strength of ternary high temperature SMAs. In this talk, we will summarize methods of engineering microstructure and texture using ECAE and resulting thermomechanical response including temperature hysteresis and enhanced cyclic reversibility in high temperature SMAs.

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1. Magnetic Field-Induced Phase Transformation in NiMnGa and NiMnCoIn Ferromagnetic Shape Memory Alloys