TENSILE DUCTILITY BEHAVIOR OF FINE-GRAINED ALUMINA AT ELEVATED-TEMPERATURE

High-temperature tensile ductility behaviour of polycrystalline fine-g rained 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 grow th regime, in the order of decreasing flow stress. The unique tensile ductility behaviour observed for each fracture regime is related to th e type of damage accumulation. A fracture mechanics model is applied t o interpret the tensile ductility of alumina in the superplastic-crack growth regime. The model correctly predicts the observed linear decre ase in the true fracture strain with an increase in the logarithm of f low stress. In addition, the model is in quantitative agreement with t he increase in the true fracture strain with decreasing grain size whe n compared at a given stress. The enhancement of tensile ductility in alumina by dilute MgO additions is attributed to an increase in the su rface energy and/or decrease in the grain-boundary energy which resist s the fracture process. On the other hand, the enhancement of tensile ductility in alumina by addition of a second phase of zirconia is attr ibuted to an increase in the amount of alumina-zirconia grain boundari es which have a low grain-boundary energy. (C) 1998 Chapman & Hall.