NiTi-20Hf High Temperature Shape Memory Alloys: Production and Scale-Up Challenges

Development of melting and processing techniques for NiTiHf high temperature shape memory alloys at a small, laboratory scale has resulted in pronounced success and repeatability for actuation purposes. Even the Ni-rich NiTiHf formulations, which are more challenging from a compositional control standpoint, due to the fact that small changes in chemistry can result in large temperature variations in Ni-rich alloys, were successfully processed at the laboratory scale. However, while properties of the slightly Ni-rich NiTiHf alloys proved promising, large-scale production of such alloys required renewed attention. Transition to production scale of 250 kg ingot sizes and beyond uncovered several challenges ranging from upfront raw material cost to large ingot processing difficulties. Several melting techniques were used, ranging from vacuum induction to plasma melting. The resulting inclusion types and sizes, and the successive hot working steps used to get to a finish product are evaluated in this work. Thermomechanical properties were measured for several material lots that varied in size and processing technique. Finally, implications of process type to product form such as finished rods, wire or tubes are discussed, and a low cost replacement for Hf is proposed.