Thermomechanical Fatigue of Ti-Ta-X-Y High-Temperature Shape Memory Alloys: Cyclic Stress-Strain Response and Damage Evolution

Ti-Ta-X形状记忆合金在高温应用中非常有吸引力，因为它们的成分比竞争对手的材料便宜得多。此外，大多数高温形状记忆合金即使在高温下也是脆性的，因此很难形成。相比之下，Ti-Ta基合金具有相当高的延展性，这使得加工更容易。子项目4重点研究改性Ti-Ta-X合金的功能和结构疲劳行为。根据第一个注资期的结果，铝和锡的合金化是一种很有希望的抑制有害omega相形成的方法。但同时，为了获得足够高的相变温度(如Ti-20Ta-5Al)，需要降低Ta含量。这一分项目的主要目标是对控制热机械疲劳过程中退化的机制有一个基本的了解。最终，这应该能够基于主导的微观结构过程准确地预测这些合金的安全使用条件的极限。在实验中，将研究热机械加载条件下的功能特性及其演变。此外，还将在与服役相关的条件下进行耐久试验和裂纹扩展试验。另一个关键方面是现场研究合金的局部变形和相变行为。这些高横向分辨率的研究应该可以帮助理解microstructure-transformation-property-relationship.此外，这些研究还将揭示组织退化，我们目前将其归因于omega和α相的形成。最后，在第一个资助期内成功开发的恢复热处理应进一步发展，以便在相关操作条件下应用于新合金。这里的愿景是通过恢复显微组织来完全抑制热机械疲劳过程中的退化，从而完全恢复相变温度和应变。

Ti-Ta-X shape memory alloys are very attractive for elevated temperature applications, as they are based on substantially cheaper constituents than the competing materials. In addition, most high-temperature shape memory alloys are brittle even at high temperatures, and thus, are difficult to form. In contrast, the Ti-Ta-based alloys are fairly ductile, which makes processing easier.Subproject 4 focuses on the functional and structural fatigue behaviour of modified Ti-Ta-X alloys. Based on the results from the first funding period, alloying with Al and Sn is a promising approach to suppress the formation of the detrimental omega-phase. However, at the same time, the Ta-content needs to be reduced in order to obtain sufficiently high transformation temperatures (e.g. Ti-20Ta-5Al). The major objective of this subproject is to develop a fundamental understanding of the mechanisms that control degradation during thermomechanical fatigue. Eventually, this should allow for accurately predicting the limits of safe service conditions for these alloys based on the dominant microstructural processes.In the experiments, functional properties and their evolution will be studied under thermomechanical loading conditions. In addition, endurance tests and crack growth experiments will be conducted under conditions relevant for service. Another key aspect is in-situ studies to characterize the local deformation and transformation behaviour of the alloys. These high lateral resolution studies should allow for understanding the microstructure-transformation-property-relationship. In addition, these studies will also shed light on structural degradation, which we currently attribute to formation of the omega- and alpha-phase.Finally, the recovery heat-treatment successfully develop during the first funding period shall be developed further to allow for application to the new alloys under relevant operating conditions. The vision here is o fully suppress degradation during thermomechanical fatigue by rejuvenating the microstructure, and thus, fully recover transformation temperature and strains.