Damage and Fracture Mechanisms During High‐Temperature Creep in Hot‐Pressed Alumina

The mechanisms responsible for creep damage accumulation and fracture have been examined in two commercial hot‐pressed aluminas. Differences between the two materials can be ascribed to minor compositional variations. Three damage regimes have been identified, depending on stress. However, in all three regimes, failure is controlled by crack propagation. At high stress, a single crack, nucleated at a processing flaw, controls failure. These cracks grow in a linear elastic stress field. At intermediate stresses, crack tip stresses relax, and many microcracks are nucleated. They grow and link under strain control. The details of this process differ under tension and bending, thus invalidating the flexure test as a means of establishing creep life, even in simple, single‐phase materials. At the lowest stress, extensive cavitation, with relatively little microcrack development, is observed. However, failure continues to be dominated by the growth of cracks. The material is damage tolerant and can be thought of as superplastic. We find that processing flaws (primarily large grains) control the creep life at all stresses. These should therefore be carefully controlled in materials aimed at high‐temperature structural applications.