Effects of thermo-mechanical processing on precipitate evolution in Ni-rich high temperature shape memory alloys

Abstract

NiTiHf-based high temperature shape memory alloys have attracted considerable attention for their high temperature (>115 °C) reversible phase transformations. Additionally, the shape memory response of the material can be tailored based on microstructure. In the case of Ni-rich NiTiHf shape memory alloys, this response can be tuned with the formation of a nano-phase that can increase the transformation temperature as well as improve mechanical properties. However, NiTiHf-based shape memory alloys are not without their drawbacks, such as difficulty in processing and controlling of nano-precipitation. In this study, the thermo-mechanical processability of a Ni-rich NiTiHf high temperature shape memory alloy, Ni50.5Ti34.5Hf15, is presented. Samples are hot-rolled to 25% and 50% reductions and then further heat treated at 550 and 750 °C to examine the formation of H-phase nano-precipitates. The size and morphology of the nano-precipitates is examined using synchrotron radiation X-ray diffraction during in situ heat treatments and compared with high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) results from ex situ heat treated samples. Changes in transformation temperatures due to the thermo-mechanical processing are determined from differential scanning calorimetry measurements. Results from this study show that SR-XRD can be used to estimate the size and observe the growth and coarsening behavior of H-phase nano-precipitates.