High-temperature Deformation and Fracture of Ceramics

采用多种方法制备了氧化铝、莫来石、莫来石、稳定氧化锆及其复合材料等微结构可控的精细致密陶瓷材料。在氧化铝中加入少量的添加剂，如MgO或ZrO_2，以控制材料的显微组织，从而显著影响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，并伴有动态晶粒生长引起的应变硬化。然而，氧化锆等氧化物在晶界处偏析，明显抑制了静态和动态晶粒生长，进而影响了屈服/流动应力，这表明离子通过晶界的扩散率或迁移率取决于晶界成分。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。在AZ材料中，与纯细晶氧化铝相比，高温变形和动态晶粒生长受到抑制，这可能是由于少量ZrO_2掺杂的氧化铝中存在锆的晶界偏析。在MZ和MA材料中，获得了延迟断裂和慢裂纹扩展的实验数据并进行了分析。采用溶胶-凝胶法制备了Forsterite多晶，并对其高温变形进行了研究。得到了决定速率的物质的有效扩散系数，显示了晶界处残余MgO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.