Microstructural and Thermomechanical Comparison of Ni-rich and Ti-rich NiTiHf20 High Temperature Shape Memory Alloy Wires

The thermomechanical processability of Ni-rich and Ti-rich NiTiHf₂₀ high-temperature shape memory alloy (HTSMA) wires was assessed. Hot-extruded Ni_50.7Ti_29.4Hf_19.9 and Ni_49.8Ti_30.7Hf_19.5 rods with an initial diameter of 6.35 mm, were hot-rolled at 700°C or 800°C to a final diameter of ~1 mm, over a series of 25 or 50 non-consecutive hot passes. Several hot-rolled rods were further hot-drawn into wires with less than 500 μm diameters. The evolution of various microstructural features (grain refinement, oxide growth/morphology, and nano-precipitation) were observed at incremental stages of the hot-rolling and drawing processes and linked to the thermal and mechanical responses of the respective HTSMA rods/wires. Synchrotron radiation x-ray diffraction (SR-XRD) was performed to evaluate the number of phases, texture, and phase/strain evolution of the NiTiHf rods/wires as a function of temperature. The thermal, mechanical, and microstructural properties were compared across both rolling temperatures (700°C & 800°C), and reduction rates (25 & 50 passes) to optimize the rolling/drawing process for NiTiHf HTSMA wires.