Stability of alumina- and mullite-based fibers by thermal exposure: experimental study and phase-field modeling

长期以来，实现高强度、高损伤容限的氧化物基陶瓷基复合材料(Ox-CMCs)的努力主要集中在寻找新的基质体系和工艺路线，以及纤维-基质-界面的调整。在过去的十年里，一些关于氧化物纤维和复合材料的研究揭示了另一种通过优化利用氧化物纤维来提高Ox-CMCs性能的方法。这些纤维作为增强体，对复合材料的力学性能起着至关重要的作用。众所周知，氧化物纤维在接收到的条件下表现出极好的强度和刚性。然而，商业上可用的多晶纤维在因颗粒生长而暴露在高温下时会出现强度损失。因此，由于在Ox-CMC的加工和应用过程中很容易达到这样的温度，所以这个问题受到了关注。本建议的目标是了解嵌入陶瓷基质中的氧化铝和莫来石基纤维的机理，并根据基质组成、工艺条件和附加热处理连续预测微观结构的变化。这将通过必要的实验和相场建模相结合来实现，以弥合所涉及的不同效应。此外，还应评估微观结构变化对纤维在高温下的准静态和长期性能的影响。为此，需要在不同的条件下制备由一根纤维束组成的微型复合材料。在高温和机械载荷作用下，将对纤维的颗粒尺寸分布和形态随基质组成的变化进行实验研究。利用力学性能表征技术研究其强度和蠕变性能的相应变化，并将其与显微组织的演变相关联。同时，将通过相场模拟结合实验结果的反馈来研究有效的生长机制。该模型将考虑晶体生长的各向异性、晶界扩散、杂质和气孔的形成和演化。将特别关注纤维和基质之间的异常颗粒尺寸分布和可能的扩散机制。作为主要结果，预计增强的纤维性能的可预测性将显示出一种确定所使用的基质组成的方法。通过这样做，可以减少陶瓷纤维在复合材料寿命期间的强度损失。因此，调整后的基质组合物将在强度和耐久性方面提供针对目标应用的复合材料性能的调整的灵活性。

The efforts to achieve high strength and damage tolerant oxide-based ceramic matrix composites (Ox-CMCs) were, for a long time, focused on searching for new matrix systems and proceeding routes, as well as on the adjustment of the fiber-matrix-interface. Nowadays, such "conventional" methods seem to have reached the limits of their capacity.In the last decade, several studies on oxide fibers and composites revealed another way to enhance the performance of Ox-CMCs by optimal utilization of the oxide fibers. Being used as reinforcements, these fibers are responsible for the mechanical performance of the composites. As it is well-known, oxide fibers present an excellent strength and stiffness in as received conditions. However, commercially available polycrystalline fibers show strength loss when they are exposed to elevated temperatures due to grain growth. Thus, attention has been given to this subject, since such temperatures can be easily reached during processing and application of Ox-CMCs.The goal of the present proposal is to understand the mechanisms and to successively predict microstructural changes of alumina- and mullite-based fibers embedded in ceramic matrices depending on the matrix composition, the processing conditions, and additional heat treatments performed. This will be achieved by combining experiments and phase-field modeling as necessary to bridge the different effects that are involved. Furthermore, the effect of the microstructural changes on the quasi-static and long-term performance of the fibers at high temperatures should be evaluated.For that, minicomposites comprised of one fiber bundle are to be manufactured under different conditions. The grain size distribution and morphology of the fibers in dependency on the matrix composition will be experimentally investigated after exposure to elevated temperatures and mechanical load. The corresponding changes of the strength and the creep performance will be investigated by means of mechanical characterization techniques and correlated to the microstructure evolution. In parallel, the effective growth mechanisms will be studied through phase-field modeling combined with the feedback from the experimental results. The model will take into account the anisotropy of crystal growth, the grain boundary diffusion, the formation and evolution of impurities and pores. Special attention will be given to the abnormal grain size distribution and possible diffusion mechanisms between fiber and matrix.As a main result, it is expected that the enhanced predictability of the fiber properties (in composites) will show a way to determine the matrix composition used. By doing so, it will be possible to reduce the strength loss of the ceramic fibers during composites lifetime. Consequently, an adjusted matrix composition will provide flexibility in the tailoring of composite properties to the target application in terms of strength and durability.