Elevated Temperature Cyclic Fatigue Behavior In Monolithic Ceramics

该项目的重点是建立一个详细的理解和定量模型，以了解高温下单片陶瓷的循环疲劳损伤和裂纹进展的物理和基本机制。本研究的主要目的是表征和模拟陶瓷在高温下的循环疲劳行为，以研究空气和真空环境下的损伤和裂纹发展机制。该方法将利用超高温（高达1800C）蠕变和疲劳裂纹扩展测试设备来检测单片氧化铝、氮化硅和碳化硅陶瓷中一系列微观结构的亚临界裂纹，选择不同的晶粒尺寸和形貌，晶界相的结晶度等。结合详细的微观结构表征（使用原位成像和失效前和失效后的电子显微镜），分析了裂纹尖端后面的屏蔽机制的循环依赖退化，以及尖端前面的固有蠕变和疲劳机制造成的累积微观结构损伤。随着科学的理解，这项工作旨在为设计改进的显微组织和成分提供指导，使耐缺陷陶瓷具有最佳的低温韧性和耐高温亚临界疲劳裂纹扩展的组合。＊＊＊

95-22134 Ritchie The proposed project is focused on developing a detailed understanding and quantitative models of the physics and fundamental mechanisms of cyclic fatigue damage and crack advance in monolithic ceramics at elevated temperatures. The central aim of the present study is to characterize and model the cyclic fatigue behavior of ceramics at high homologous temperatures to examine the comparable damage and crack advance mechanisms in both air and vacuum environments. The approach will utilize ultrahigh temperatures (up to 1800C) creep and fatigue -crack growth testing facilities to examine subcritical cracking in a range of microstructures in monolithic alumina, silicon nitride and silicon carbide ceramics, chosen to vary grain size and morphology, the crystallinity of the grain-boundary phase, etc., together with detailed microstructural characterization (using both in situ imaging and pre- and post-failure electron microscopy) of the cycle-dependent degradation of shielding mechanisms behind the crack tip and the accumulated microstructural damage by intrinsic creep and fatigue mechanisms ahead of the tip. %%% With the scientific understanding gained, this work is intended to provide guidelines for the design of improved microstructures and compositions for flaw-tolerant ceramics with optimal combinations of low-temperature toughness and elevated-temperature resistance to subcritical fatigue-crack growth. ***