High-temperature Deformation and Fracture of Ceramics

采用不同的方法制备了氧化铝、莫来石、镁橄榄石、稳定氧化锆及其复合材料等细小致密的可控微结构陶瓷材料。在氧化铝中加入少量的添加剂，如氧化镁或氧化锆粉，控制了材料的显微组织，显著影响了材料的高温力学性能。细晶氧化铝的高温变形受界面反应的速率限制，并伴随着动态生长的应变硬化。然而，晶界偏析的氧化物，如氧化锆的加入，明显抑制了静态和动态的晶粒长大，并进一步影响了屈服/流动应力，这表明离子在晶界的扩散性或移动性取决于晶界成分。研究了氧化铝-氧化锆(AZ)、莫来石-氧化锆(MZ)和莫来石-氧化铝(MA)陶瓷复合材料在高温下的断裂和变形。与纯细晶Al_2O_3相比，AZ材料的高温变形和动态晶粒长大受到抑制，这可能是由于在Al_2O_3中掺入少量ZrO_2的情况下，锆晶界偏析所致。在MZ和MA材料中，获得并分析了关于延迟断裂和缓慢裂纹扩展的实验数据。对溶胶-凝胶法制备的镁橄榄石多晶进行了高温变形实验研究。得到了速度决定组分的有效扩散系数，反映了晶界残留的氧化镁和玻璃相的影响。

Fine and dense ceramic materials with controlled microstructures, including alumina, mullite, forsterite, stabilized zirconia and their composites, were fabricated by using various methods. Small amounts of additives, e.g., MgO or ZrO_2 in alumina, were introduced to control the microstructures, which affected significantly high-temperature mechanical properties of the materials. The hightemperature deformation of fine grained alumina is rate-limited by interface reaction, accompanied with hardening with strain due to dynamic grain growth. However, the addition of an oxide such as zirconia, which segregates at grain boundaries, suppresses appreciably the static and dynamic grain growth, and furthemore affects the yield/flow stress, suggesting that diffusivity or mobility of ions through grain boundaries depend on grain boundary compositions. Fracture and deformation of ceramic composites such as alumina-zirconia (AZ), mullite-zirconia (MZ) and mullite-alumina (MA), were investigated at high temperatures. In AZ materials, high temperature deformation and dynamic grain growth were depressed as compared with pure fine-grained alumina, which may be due to grain boundary segregation of zirconium as in the case of the alumina doped with small amounts of ZrO_2. In MZ and MA materials, experimental data concerning delayd fracture and slow crack growth were obtained and analyzed. Forsterite polycrystals fabricated by sol-gel derived powders were tested to study the high-temperature deformation. Effective diffusion coefficients for rate-determining species were obtained, showing the effects of residual MgO and glassy phases at grain boundaries.