On reactions and related thermodynamic driving forces during recycling of NiTi shape memory alloys through vacuum induction melting

In light of the current global energy and climate challenges, research efforts on the feasibility and methodology of metal recycling are becoming increasingly important, whereas niche materials such as nickel and titanium have been underexplored in this context. This gap appears worth addressing, as their production and refinement are far more energy-intensive (Ni: up to 194 MJ/kg; Ti: 361 MJ/kg) and thus associated with higher CO₂ emissions (Ni: up to 16.1 kg CO₂-e/kg; Ti: 35.7 kg CO₂-e/kg) than low-alloy steels. In this study, we aim to assess the recyclability of NiTi shape memory alloys (SMAs) through vacuum induction melting (VIM). Firstly, a 1 kg NiTi ingot was produced by VIM from elemental nickel pellets and titanium blocks using a graphite crucible. The ingot was subsequently remelted three times and samples for microstructural and chemical characterization were taken after each melting iteration. It was investigated how the use of pure Ni and Ti compared to a pre-existing NiTi SMA as feedstock affects process temperatures and durations. The results show that using elemental feedstocks is associated with the heat of mixing acting as an internal heat source. This significantly shortens the total process duration compared to melting pre-existing NiTi. Our experiments also allowed a quantitative assessment of the specific energy input and revealed how repeated VIM remelting affects phase composition, microstructural evolution and transformation temperatures.