Quantitative XRD and microstructure characterization of Ni-Ti-Cu shape memory alloys

Low hysteresis shape memory alloy (SMA) research makes use of theoretical models like crystallographic theory of martensite and co-factor theory to develop SMAs with a high thermo-elastic cycling stability and actuation efficiency. These theories correlate low hysteresis with alloy chemistries that are close to fulfill the co-factor condition λ₂ = 1. Physically, this translates into a high microstructural compatibility between the austenitic and the martensitic phase, minimizing interfacial lattice stresses at the transformation front. Generally, Ni-Ti-Cu alloys with a transformation path from the B2 austenitic phase to an intermediate orthorhombic B19 structure before transforming into the monoclinic B19’ martensite phase are closer to satisfy the cofactor condition λ₂ = 1 than such which exhibit a B2 to B19’ transformation. On the other hand, alloys with a single B2 to B19 transformation step contain high Cu amounts, which causes difficulties in processing. However, it is not trivial to characterize the triple transformation as the martensitic B19’ and B19 peaks in DSC analysis are usually superposed by each other and microstructural differences are difficult to reveal. In order to trace the deformation in cast and worked alloys, using XRD at different temperatures in combination with Rietveld or Pawley refinement revealed to be a powerful tool. In this work, low hysteresis Ni-Ti-Cu based alloys in different processing conditions are studied by XRD at various temperatures to quantify the amount of martensitic phases and Cu-rich precipitates. The results give insight on the impact of alloy composition on the double transformation behavior in Ni-Ti-Cu alloys.