Production of NiTiHf High Temperature Shape Memory Alloy actuator components via hot-rolling and additive manufacturing

For decades, High-Temperature (HT) shape memory alloys (SMAs) have been studied by many researchers of different scientific fields, and several alloy systems that exhibit high Phase Transition Temperatures (PTT) have been introduced. Of these, NiTiHf is one of the most promising systems, since adding Hf increases the PTTs significantly without any serious drawbacks in the functional properties. Additionally, Hf is far less expensive than Pd or Pt, being used in prominent NiTi ternary derivatives. However, the production of NiTiHf components is still challenging due to the increased brittleness present in this alloy.

In this work, actuator components are presented that have been processed with a special hot-rolling technique being capable to handle the brittle as-cast microstructure. Thin sheets with a M_f – temperature above 150˚C were produced, which serve as actuator components in a passive security valve in nuclear powerplants currently undergoing the qualification process by the Swiss licensing authority. In another approach, we show recent results of our activities dealing with additive manufacturing of NiTiHf components. Here, other challenges such as processing of high-quality powder via gas atomization and the identification of suitable process parameters in order to obtain defect-free components via powder bed fusion processes, i.e. electron beam melting or selective laser melting, have to be overcome. The powder-metallurgic route is a promising process for near-net shape production of high temperature actuator components capable of eluding the risk of brittle failure during conventional rolling, swaging or wire drawing of NiTiHf SMAs.