Integrative design of novel Mo-Si-based alloys and protective coatings for high temperature structural applications

高钛硅化钼基合金是一类具有良好性能的新型高温材料。特别是，这些多相合金比目前使用的Ni基单晶更耐蠕变，并且在超过1000°C的温度下表现出良好的抗氧化性。然而，在中等温度下，它们遭受害虫和内在抗氧化性低。该提案的总体目标仍然是综合开发技术相关的Mo-Si-B-Ti合金，并结合适应的涂层系统，在600至1300°C的干燥和潮湿环境中提供氧化和腐蚀保护。在这个项目的第一部分，我们开发了共晶/共析Mo-Si-Ti合金，它在蠕变和抗氧化之间表现出良好的折衷。氧化性能的改善一方面依赖于基于CALPHAD的合金设计提高其内在抗氧化性，另一方面依赖于新型硅基涂层的外在保护。与干燥气氛相比，新合金在潮湿条件下的氧化明显加快，并伴有从近抛物线到线性速率定律的变化。这可以通过更快的O和ti扩散来解释。硅涂层通过形成缓慢生长的sio2屏障，在1200°C的空气中提供300小时的良好氧化保护。为了提高合金与涂层之间的相容性，开发了适应的结合涂层，该涂层通过界面上的减少相反应和相互扩散来识别自己。在目前的建议中，我们针对两个主要的研究领域：第一，合金的氧化和蠕变行为将进行研究。我们将讨论(i)单相对氧化的贡献，（ii）干湿条件下氧化机制的差异，以及（iii）潜在的蠕变机制。由于本质上形成的氧化物和简单的涂层都不能保证在水蒸气环境中有足够的长期保护，因此材料系统将被环境屏障涂层（ebc）额外保护。因此，第二，关于EBC的发展以及涂层合金的氧化和蠕变行为的表征，以下几个方面将是重点：(i)涂层系统在潮湿气氛中的稳定性，（ii）基合金与EBC系统各层之间的相容性。这些问题将通过利用热力学模型和实验相结合的方法来解决。在热力学和动力学耦合模拟工具的帮助下，我们将确定热生长层和沉积涂层的哪种组合能够实现对干燥和潮湿大气的最佳保护。最后，将研究这些涂层系统在叠加蠕变或温度变化载荷条件下是否与基体合金机械相容。

Mo-silicide based alloys with high Ti-concentrations are a new class of high temperature materials with promising properties. In particular, these multiphase alloys are more creep resistant than cur-rently used Ni base single crystals and exhibit good oxidation resistance at temperatures beyond 1000°C. At moderate temperatures, however, they suffer from pesting and the intrinsic oxidation resistance is low. The overarching goal of this proposal is still the integrative development of a techni-cally relevant Mo-Si-B-Ti alloy combined with an adapted coating system which provides oxidation and corrosion protection in dry and wet environments in a temperature range between 600 and 1300°C. In the first part of this project, we developed eutectic/eutectoid Mo-Si-Ti alloys which exhibit a good compromise between creep and oxidation resistance. The improvement of the oxidation behavior relies on the one hand on increasing the intrinsic oxidation resistance by CALPHAD based alloy design and, on the other hand by extrinsic protection due to novel silica based coatings. Oxidation of the new alloys in wet conditions is significantly accelerated as compared to dry atmosphere which is accom-panied by a change from nearly parabolic to linear rate law. This can be rationalized by the faster O- and Ti-diffusion. The Si-coatings provide good oxidation protection for 300h at 1200°C in air by the formation of a slow growing SiO2-barrier. For better compatibility between the alloys and the coating, adapted bond coats were developed, which identify themselves by reduced phase reactions at inter-faces and interdiffusion. In the present proposal we target two main research areas: first, the oxidation and creep behavior of the alloys will be investigated. We will address (i) the contribution of the single phases to oxidation, (ii) the differences in oxidation mechanisms under dry and wet conditions and (iii) the underlying creep mechanisms. As neither intrinsically formed oxides nor simple coatings will guarantee sufficient long-term protection in water vapor environment, the material system will be additionally protected by Environmental Barrier Coatings (EBCs). Hence, second, regarding the development of EBC as well as the characterization of the oxidation and creep behavior of coated alloys the following aspects will be in focus: (i) stability of coating systems in wet atmosphere, (ii) compatibility between base alloys and the respective layers of the EBC-systems. These issues will be resolved by a combined ap-proach utilizing thermodynamic modelling and experiments. Aided by the developed simulation tool coupling thermodynamics and kinetics, we will identify which combination of thermally grown layers and deposited coatings will enable optimal protection against dry and wet atmospheres. Finally, it will be investigated whether these coating systems are mechanically compatible to the base alloys under superimposed creep or temperature change loading conditions.