Grain-boundary Cavitation and Creep Fracture of Ceramics

研究了细密陶瓷材料的高温力学性能。细晶莫来石具有良好的抗蠕变性能，在较高的应力下，由于亚临界裂纹扩展（SCG）导致静态疲劳，在较低的应力下，由于晶界空化导致蠕变断裂。其它陶瓷（氧化铝、氧化镁、氧化锆及其复合材料）也会发生类似的随应力和温度的断裂机制转变，这从寿命评估的角度来看是很重要的，因此在研究过程中进行了实验研究。研究了晶粒尺寸<1 μ m的细晶氧化铝和氧化锆（Y-TZP）的蠕变变形和蠕变断裂。Y-TZP发生超塑性变形，在晶界附近随材料输运（扩散）呈现非均匀变形。Y-TZP的晶界在高温下是稳定的；变形过程中的晶粒生长或动态晶粒生长受到限制。另一方面，细晶氧化铝在高应变速率下通过扩散（Coble）蠕变变形，受到界面反应的限制，随后由于动态晶粒生长而发生应变硬化，导致晶间空化。因此，即使添加少量氧化镁，氧化铝的晶界也相对不稳定。研究结果表明，晶界在高温下的变形和断裂中起着重要作用

The high-temperature mechanicalproperties of fine and dense ceramic materials were investigated. In fine grained mullites, which were fabricated using a fine grained sol-gel powder and have high resistance to creep deformation, static fatigue due to subcritical crack growth (SCG) at relatively higher stresses and creep fracture due to grain-boundary cavitation at lower stresses were observed at temperatures<greater than or equal>1200ﾟC.The similar transition of fracture mechanism with stress and temperature, which is important from the viewpoint of life assessment, should take place in other ceramics (alumina, magnesia, zirconia and their composites), therefore the experimental study is carried out in the process of the research. The creep deformation and creep fracture of fine grained alumina and zironia (Y-TZP), both with grain sizes<1mum, were studied. Y-TZP deforms superplastically, showing nonniform deformation with materials transport (diffusion) near the grain boundaries. The grain boundary in Y-TZP seems to be stable at high temperatures ; the grain growth during deformation, or dynamic grain growth, is limited. On the other hand, fine-grained alumina deforms by diffusional (Coble) creep at high strain rates, limited by interface reaction, followed by strain hardening due to dynamic grain growth, causing intergranular cavitation. Thus the grain boundary in alumina is relatively unstable in spite of small addition of magnesia. These results obtained in the research suggests that the grain boundary plays an important role in deformation and fracture at high temp