Characterization of Processing Effect on Microstructure and Hysteresis in NiTiCu Alloys

The addition of Cu to binary NiTi shape memory alloys is a common approach to decrease the thermal hysteresis between the high temperature austenitic phase and the low temperature martensitic phase. With an increasing copper level, the orthorhombic martensite becomes continuously more stable over the monoclinic morphology, which is typically observed in binary NiTi. Previous research indicates that an amount of about 7,5 at. % Cu leads to the appearance of the orthorhombic crystal structure and that it might coexist with monoclinic martensite up to a Cu content of approximately 15 at. %. While the existence of orthorhombic martensite was well established and characterized, there is still little data available about the interplay of composition and thermo-mechanical treatments which leads to alterations in the phase transformation sequence and measured hysteresis width. This work aims to demonstrate the impact of composition, heat treatments and processing on the microstructural evolution of ternary NiTiCu alloys as observed by DSC, XRD and microscopy. In addition, a Pawley refinement is applied on measured XRD data to perform a quantitative phase evaluation in as-cast and processed alloys. It is shown that by a combination of these methods, phase transformation trends during the processing of NiTiCu can be determined.