Embrittlement of gamma titanium aluminides by high temperature oxidation: mechanisms and abatement measures

该提案的主要目的是调整γ TiAl的表面，使得热障涂层可以应用于TiAl部件，而不会损害基材的机械性能，甚至与未涂覆状态相比，可以改善它们。为此，铝化物（=铝扩散涂层）和阴极溅射的TiAl层被施加在钛铝化物合金的顶部上，随后卤化，使得在随后的热处理期间形成保护性氧化铝顶层，其充当扩散屏障以防止进一步的氧致脆化。为了达到这一目标，在各个工艺步骤，如预氧化和热障涂层的应用，以及随后的高温老化的微观结构和性能的影响，在次表面区域的热负荷必须从根本上进行研究。在此基础上，开发了一个机制的理解退化和合适的策略，抑制亚表面脆化。根据第一个两年项目阶段的结果，涂层参数，包括阻挡层的成分和厚度，将在最后的第三年进行优化。重点是评估涂层系统在高温应力下的寿命以及完成对选定变体机械性能的研究。第一个有前途的疲劳测量表明，磁控溅射层与卤化减少氧脆化由于铝富集。迄今为止进行的研究表明，在起始结构中高比例的β（o）相可被归类为由于氧诱导转化为α 2而导致的脆化特别关键。因此，在项目的第二阶段，将对Ti-48 Al-2Cr-2Nb合金（合作伙伴提供）进行额外研究，因为该合金的β（o）相含量较低。在对脆化机理和涂层系统的影响进行深入分析之后，将对选定的试样进行机械测试，以验证具有保护系统的低β相合金的性能改善。

The primary objective of the proposal is to tune the surface of gamma TiAl in such a way that thermal barrier coatings can be applied to TiAl components without impairing the mechanical properties of the base material or even with improving them compared to the uncoated state. For this purpose, aluminide (= aluminum diffusion coating) and cathode-sputtered TiAl layers are applied on top of the titanium aluminide alloys followed by halogenation, so that a protective aluminum oxide top layer is formed during the subsequent heat treatment, which acts as a diffusion barrier to prevent further oxygen induced embrittlement. To reach this objective, the influence of the thermal loading during the individual process steps such as pre-oxidation and application of the thermal barrier coating as well as a subsequent high temperature ageing on the microstructure and properties in the subsurface zone have to be fundamentally investigated. Based on this, a mechanistic understanding of degradation and suitable strategies for suppressing subsurface embrittlement are developed. Based on the results of the first two-year project phase, the coating parameters, including composition and thickness of the barrier layers, are to be optimized in the final third year. The evaluation of the lifetime of the coating systems under high temperature stress as well as the completion of the investigations of the mechanical properties of selected variants are in the focus. First promising fatigue measurements show that magnetron sputtered layers with halogenation reduce oxygen embrittlement due to the Al enrichment. The investigations carried out so far have shown that a high proportion of the beta(o)-phase in the starting structure can be classified as particularly critical with regard to embrittlement due to the oxygen-induced conversion into alpha2. Therefore, in the second phase of the project, additional investigations are to be carried out on a Ti-48Al-2Cr-2Nb alloy (available at the partners), since this alloy has a low content of beta(o)-phase. After a profound analysis of the embrittlement mechanism and the effect of the coating systems, selected specimens will be mechanically tested to verify the improved properties of the low beta phase alloy with the protection system.