In-situ High Energy Synchrotron Radiation X-ray Diffraction Measurements during Aging of NiTiHf High Temperature Shape Memory Alloy

NiTiHf high temperature shape memory alloys (HTSMAs) have begun to show considerable progress in their ability to be manufactured and are now on the cusp of becoming viable alloys for use in the aerospace industry, mainly as actuation devices. By aging the alloys at intermediate temperatures between 450 and 700 °C, Ni-rich NiTiHf alloys form the H-phase, a Ni-rich nanoscale precipitate which can used to strengthen the alloy and improve the shape memory response. However, precipitate phase evolution and growth has not yet been studied in-situ nor have the effects of previous thermo-mechanical processing on their formation. In this study, H-phase precipitate formation is observed experimentally in-situ during aging of a Ni-rich NiTiHf alloy using high energy synchrotron radiation X-ray diffraction (SR-XRD). The effects of prior thermo-mechanical treatments, i.e. hot and cold rolling, on its formation rate and volume fraction are compared. It is shown that growth of the H-phase can be observed in-situ using SR-XRD and the H-phase formation rate seems to exhibit linear growth when aging at 550 °C while increasing the temperature to 650 °C shows a logarithmic trend that plateaus after approximately 1 hour of aging.  In addition, little difference is noticed in the formation rate, crystallographic orientation, or volume fraction between the hot rolled and cold rolled conditions.