Tensile ductility behaviour of fine-grained alumina at elevated temperature

Abstract

High-temperature tensile ductility behaviour of polycrystalline fine-grained alumina is shown to be classified into four regimes, depending on flow stress: (1) fast-crack growth regime, (2) single-crack growth regime, (3) microcracks growth regime, and (4) superplastic-crack growth regime, in the order of decreasing flow stress. The unique tensile ductility behaviour observed for each fracture regime is related to the type of damage accumulation. A fracture mechanics model is applied to interpret the tensile ductility of alumina in the superplastic-crack growth regime. The model correctly predicts the observed linear decrease in the true fracture strain with an increase in the logarithm of flow stress. In addition, the model is in quantitative agreement with the increase in the true fracture strain with decreasing grain size when compared at a given stress. The enhancement of tensile ductility in alumina by dilute MgO additions is attributed to an increase in the surface energy and/or decrease in the grain-boundary energy which resists the fracture process. On the other hand, the enhancement of tensile ductility in alumina by addition of a second phase of zirconia is attributed to an increase in the amount of alumina–zirconia grain boundaries which have a low grain-boundary energy. © 1998 Chapman & Hall