Recyclability and Reprocessing of NiTiHf Alloy Scrap

NiTiHf shape memory alloys have emerged as promising materials not only for high-temperature actuator applications but also for tribological, thermal energy storage, and certain biomedical uses. In recent year, significant technological advancements have been made to scale-up these alloys from laboratory scale to industrial production by addressing challenges related to complex composition control, melt processing, and ingot breakdown. But now, a new challenge has emerged consisting of the rising cost and demand for Hf. Driven by its critical role in advanced aerospace alloys and semiconductor manufacturing, the price of Hf has increased by approximately 261% between 2021 and 2025 (from ~$1,404/kg to ~$5,080/kg) and continues to climb. This trend underscores the urgent need to explore recycling and recovery strategies for NiTiHf scraps to ensure a cost-effective utilization of this metal.

In this work, efforts were made to recycle scrap from several NiTiHf alloys with comparable compositions. Scraps from these NiTi-20Hf (at.%) were reused to produce alloys of similar composition and also employed as master alloys to fabricate derivative compositions with additional Zr or significantly reduced Hf. The scraps were shot blasted and chemically etched using nitric and hydrofluoric acid mixtures, followed by vacuum induction melting to produce ingots ranging from 20 to 40kg. Special focus was placed on impurity control, compositional outcomes, and the economics of NiTiHf recycling. The recycled alloys successfully retained shape memory properties despite minor compositional variations, although oxygen uptake increased by approximately 400ppm. Implications of these methods for alloy complexity, service life, and scalability are discussed.