Ni-Ti-Hf high-temperature shape memory alloys processed by additive manufacturing via selective electron beam melting – From process to properties

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High-temperature shape memory alloys (HT-SMAs) allow for realization of highly efficient actuation and sensing devices in many industry sectors, such as the mobility sector, where the SMA technology still is not widely employed due to current critical limitations. Many promising HT-SMAs have been developed in recent years, however, drawbacks still prevail for those systems.Ni-Ti-Hf SMAs containing 20 at.-% of Hf can be processed via complex thermos-mechanical processing routes, however, maximum application temperatures and experimentally determined deflections are relatively low, the latter supposedly induced by unfavorable microstructures. Due to the pronounced brittleness of these alloys, multi stage forming routes are needed resulting in relatively high processing costs. Material properties deduced from tests on single crystalline material showed that distinctly higher values for transformation strains and application temperatures can be obtained. High-Hf Ni-Ti-Hf HT-SMAs (Hf content > 20 at.-%) cannot be robustly processed via conventional processing routes, however, show superior material properties, i.e. high transformation temperatures and transformation strains, making them high potential candidates for application in aerospace and automotive industry.Within the proposed project the electron beam melting (EBM) technology will be used to manufacture high-Hf Ni-Ti-Hf HT-SMAs and tailor their microstructures (grain morphology and texture) by adjusting processing parameters. The focus within the project will be on EBM only. It is assumed that process characteristics of the second powder bed additive manufacturing (AM) technique, selective laser melting, will result in process-induced crack formation. Process-microstructure-property relationships will be evaluated in terms of functional properties for the EBM Ni-Ti-Hf. It is expected that superior material properties will result from EBM processing of high-Hf Ni-Ti-Hf in comparison to conventionally manufactured low-Hf Ni-Ti-Hf (Hf < 20 at.-%) alloy systems. Since neither the technology of high-Hf Ni-Ti-Hf HT-SMAs nor AM (EBM Ni-Ti-X alloys will be processed for the first time) of these alloys have been studied comprehensively, basic parameters for EBM processing need to be developed and resulting material conditions have to be characterized carefully. Finally, the research on AM Ni-Ti-Hf will reveal the full potential of both, the alloy system itself and the EBM technology for processing of brittle HT-SMAs. With this research approach a strategy will be introduced, which has not been established within the international community so far.