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

Ti-Ta-X形状记忆合金对于高温应用非常有吸引力，因为它们基于比竞争材料便宜得多的成分。此外，大多数高温形状记忆合金即使在高温下也是脆性的，因此难以形成。相比之下，Ti-Ta基合金具有相当的延展性，这使得加工更容易。子项目4侧重于改性Ti-Ta-X合金的功能和结构疲劳行为。 根据第一个资助期的结果，用Al和Sn合金化是抑制有害的ω相形成的有希望的方法。然而，同时，需要降低Ta含量以获得足够高的转变温度（例如Ti-20 Ta-5Al）。该子项目的主要目标是对热机械疲劳过程中控制退化的机制有一个基本的了解。最后，这应该允许准确地预测这些合金的安全使用条件的限制的基础上占主导地位的微观结构processes.In实验中，功能特性和它们的演变将在热机械载荷条件下进行研究。此外，还将在与使用相关的条件下进行耐久性试验和裂纹扩展试验。另一个关键方面是原位研究，以表征合金的局部变形和转变行为。这些高横向分辨率的研究应允许理解的微观结构-转化-性质-关系。此外，这些研究还将揭示结构退化，我们目前将其归因于ω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开发，以允许在相关操作条件下应用于新合金。这里的愿景是通过恢复微观结构来完全抑制热机械疲劳期间的退化，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.