Microstructural and Thermomechanical Comparison of Ni-Rich and Ni-Lean NiTi-20 at.% Hf High Temperature Shape Memory Alloy Wires

The processability of Ni-rich Ni 50.8 Ti 29.2 Hf 20 and Ti–rich Ni 49.8 Ti 30.2 Hf 20 (at.%) high temperature shape memory alloy wires was investigated. Hot-extruded NiTiHf rods with an initial diameter of 6.35 mm, were hot-rolled (HR) at 750, 775, 800, or 825 °C to a square cross section with ~ 1.9 mm side lengths, over a series of 33 non-consecutive hot passes. The thermomechanical behavior of HR, solutionized, and aged NiTiHf wires was examined with thermal cycling using differential scanning calorimetry and tensile testing to assess the viability of the hot-rolling process. The ideal rolling temperature for Ni-rich NiTiHf wires lies just over the H-phase dissolution temperature (near 800 °C), to avoid wire embrittlement caused by H-phase precipitates and to limit compositional instability at the wire surface caused by HfO 2 internal oxidation. The characteristic oxide layers formed during the rolling process and subsequent heat treatments were compared across rolling temperatures and compositions to address several challenges associated with NiTiHf wire processing.