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

高温形状记忆合金（HT-SMA）允许在许多工业领域（诸如移动领域）中实现高效致动和感测装置，其中SMA技术由于当前的关键限制而仍然没有被广泛采用。近年来，人们开发了许多有前途的高温自组装材料，但这些材料的缺点仍然存在。的Hf可以通过复杂的热机械加工路线进行处理，然而，最大应用温度和实验确定的偏转相对较低，后者被认为是由不利的微观结构引起的。由于这些合金的显著脆性，需要多阶段成形路线，导致相对高的加工成本。从单晶材料的试验中推导出的材料性能表明，可以获得明显更高的相变应变和应用温度值。高Hf Ni-Ti-Hf HT-SMA（Hf含量> 20 at.-%）不能通过常规的加工途径进行稳健加工，然而，显示出上级的材料性能，即高的转变温度和转变应变，在该项目中，将采用电子束熔炼（EBM）技术制备高Hf Ni-Ti-Hf HT-SMA，并对其微观结构进行定制（晶粒形态和纹理）。该项目的重点将只放在循证医学上。假设第二粉末床增材制造（AM）技术（选择性激光熔化）的工艺特性将导致工艺诱导的裂纹形成。工艺-显微组织-性能关系将根据EBM Ni-Ti-Hf的功能性能进行评估。预期与常规制造的低Hf Ni-Ti-Hf（Hf < 20 at.-%）相比，高Hf Ni-Ti-Hf的EBM处理将产生上级材料性质。合金体系由于高Hf Ni-Ti-Hf HT-SMA的技术和这些合金的AM（将首次加工EBM Ni-Ti-X合金）都没有被全面研究，因此需要开发EBM加工的基本参数，并且必须仔细表征所得材料条件。最后，对AM Ni-Ti-Hf的研究将揭示合金体系本身和EBM技术加工脆性HT-SMA的全部潜力。通过这一研究方法，将推出一项战略，国际社会迄今尚未制定这一战略。

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.