Novel Ni48.5Ti45.5Hf3Ta3 high temperature shape memory alloy: Microstructure and phase transformation

A newly disclosed high-temperature shape memory alloy, Ni 48.5 Ti 45.5 Hf 3 Ta 3 , demonstrates excellent shape memory properties, thermal stability and good formability at room temperature. The microstructure and phase components of the specimens were examined using Optical Microscopy (OM), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM). The microstructure study of the as cast shows two different precipitates, tantalum-rich solid solution (β-Ta) and (Ti, Hf) 2 Ni, uniformly distributed along grain boundaries, which changed to dispersed and globular morphology after homogenization. The analysis also revealed the presence of Type I twins and that the β-Ta particles are coherent with the matrix. Dilatometric results revealed high transformation temperatures, and a 100% recovery was observed over 10 cycles in the bending test. The results of thermal cycle test revealed that the transformation temperatures decreased with an increasing number of cycles due to the rising dislocation density. Additionally, the effect of stress levels on actuation strain and thermal hysteresis was investigated. • Developed shape memory alloy shows high transformation temperatures and full recovery. • Alloy balances high tensile strength, ductility, and complex stress–strain behavior. • Thermal cycling lowers transformation temperature and hysteresis before stabilizing. • Actuation strain increases with applied stress but decreases with cycle number.