High‐Temperature Fatigue Behavior of Polycrystalline Alumina

The strength and fatigue behavior of a 99.5% polycrystalline alumina were measured as a function of temperature. Both the strength and fatigue behavior remained essentially constant up to 500°C; from 800° to 1100°C the strength and fatigue resistance decreased markedly and at >1100°C macroscopic creep was observed. It is believed that the decrease in strength and fatigue resistance is caused by a grain‐boundary glassy phase enhancing subcritical crack growth. Proof‐testing at room temperature was effective in improving the strength distributions at both room temperature and 1000°C; however, at 1000°C it was not effective, due to crack growth during the proof test. The good agreement between proof‐test results and fracture‐mechanics theory indicates that the same flaws control the strength at room temperature and at high temperatures.