Sustainable NiTi shape memory metallurgy

The high need for resources and the emission of still large amounts of greenhouse gases demand a transition from a linear to a circular economy. This not only holds for the most economically important alloys and metals like steel, aluminum, copper (etc.), where recycling routes have already been established during the last decades. Also for NiTi shape memory alloys (SMAs), there is a need for a sustainable metallurgy. The generation of pure Nickel and Titanium is associated with high energy requirements (Ni: 0.25 EJ yr-1; Ti: 0.07 EJ yr-1) and thus with the emission of large amounts of CO2 (Ni: 26 MT CO2 eq yr-1; Ti: 6.7 MT CO2 eq yr-1). Furthermore, the preparation of NiTi key products, such as stents, involves processes like gun drilling and laser cutting, which cause significant material losses. In the present work, we discuss sustainability aspects of NiTi SMAs. Unfortunately, NiTi SMAs are not resilient, as small amounts of impurities affect transformation temperatures and fatigue lives. In the present work, we evaluate the possibility of NiTi recycling by different remelting strategies. We use a theoretical approach which allows to assess how geometry, surface states and processing conditions affect chemical purities and phase transformation behavior of the recycled SMAs. We demonstrate first results from laboratory scale recycling experiments involving arc melting and vacuum induction melting. We also discuss “second-life” options for cases where recycling results in larger changes of functional properties.