In situ evaluation of the transformation behaviour of NiTi-based high temperature shape memory alloys

Fine grained polycrystalline NiTi shape memory alloys containing 15 at.% Hf and Zr and zero or 3 at.% Cu fabricated by ingot metallurgy were investigated using in situ synchrotron X-ray diffraction in order to examine the viability of producing stable and affordable high temperature shape memory alloys. The alloys produced had a high thermal hysteresis, in excess of 70 °C but Af temperatures of over 250 °C were obtained for Ni50Ti35Hf15. 3 at.% Cu additions did not significantly reduce the per-cycle degradation of transformation temperatures but did reduce the transformation temperatures. The evolution of the lattice parameters during the first five thermal cycles was observed. Negative thermal expansion was found in the bB19′ cell direction in all the alloys examined and significant deviations in the lattice parameters in the region of transformation were found. A per-cycle evolution in the end-point B19′ lattice parameters was observed, but no such evolution was found for the B2 phase, which is rationalised by appealing to the increase in population of interface dislocations.