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)允许在许多工业部门实现高效的驱动和传感设备，例如移动部门，由于当前的关键限制，SMA技术在该领域仍未得到广泛应用。近年来发展了许多有前景的高温超导材料，但这些系统仍然存在缺陷。含20at.-%Hf的Ni-Ti-Hf SMA可以通过复杂的热-机械工艺路线加工，但最高应用温度和实验确定的挠度相对较低，后者可能是由不利的微结构引起的。由于这些合金的明显脆性，需要多阶段成形路线，从而导致相对较高的加工成本。对单晶材料进行的材料性能测试表明，该方法可获得明显较高的相变应变和使用温度。高高频Ni-Ti-Hf高温超导材料(Hf含量和20at.-%)不能通过常规工艺进行稳健加工，但它具有优异的材料性能，即较高的相变温度和相变应变，使其在航空航天和汽车工业中具有很大的应用潜力。在拟议的项目中，电子束熔炼(EBM)技术将被用于制造高高频Ni-Ti-Hf高温SMA，并通过调整工艺参数来调整其微观结构(晶粒形态和织构)。该项目的重点将只放在循证医学上。假设第二种粉末床层添加制造(AM)技术的工艺特性，选择性激光熔化，将导致工艺诱导裂纹的形成。将根据EBM Ni-Ti-Hf的功能特性来评估工艺-组织-性能之间的关系。预计与传统制造的低氢Ni-Ti-Hf(Hf&lt；20at.-%)合金系统相比，高氢Ni-Ti-Hf的EBM处理将产生更好的材料性能。由于这些合金的高高频Ni-Ti-Hf高温超导合金和AM(EBM Ni-Ti-X合金将被首次加工)的技术都没有得到全面的研究，因此需要开发EBM加工的基本参数，并仔细地表征所产生的材料条件。最后，AM Ni-Ti-Hf的研究将充分揭示合金系统本身和EBM技术在加工脆性高温超导材料方面的潜力。通过这种研究方法，将引入一项战略，这一战略迄今尚未在国际社会内确立。

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.