Characterization of Thermomechanically Processed High-Temperature Ni-Lean NiTi–20 at.% Hf Shape Memory Wires

The thermomechanical processability of Ti(Hf)-rich Ni_49.8Ti_30.2Hf₂₀ (at.%) high-temperature shape memory alloy (HTSMA) wires was examined. Hot-extruded rods with an initial diameter of 6.35 mm were hot-rolled and cold-drawn into a final diameter of 260 µm. For all samples, processing was performed in open air without extrusion canning. The HTSMA rods were processed to an area reduction of 99.83%. Experimental results showed that hot-rolling at 800 °C decreased the grain size and introduced a small amount of retained austenite, while breaking up the Ti₄Ni₂O_x intermetallic oxide phase, resulting in a 10 °C decrease of the austenite start temperature (A_s). While both hot-rolling and cold-drawing exhibited clear effects on the thermomechanical properties, neither the number of hot-rolling passes nor the number of cold-drawing passes affected the transformation temperatures of the material. Additionally, the NiTiHf HTSMA showed similar thermomechanical responses after the 5th and 25th hot pass when subjected to room temperature loading to 500 MPa (uniaxial tension) and thermally induced shape recovery.