CAVITY GROWTH PROCESSES DURING SUPERPLASTIC FLOW IN ALUMINA AT HIGH TEMPERATURES

The deformation behaviour of a superplastic alumina was characterised over a wide range of loads, strain rates and temperatures (1448-1553 K) under the three modes of loading (bending, compression and tension). The data were analysed in terms of the following equation: d epsilon/dt is about sigma exp(n) exp(-Q/RT), where d epsilon/dt is the strain rate, sigma is the flow stress, Q is the activation energy, R is the gas constant, T is the absolute temperature and n is the constant termed the stress exponent. The n values were in the range of 1.1-3.2, Q = 562 kJ/mol for bending. A good compatibility was not achieved between these three loading methods. Substantial cracks were observed on the tensile surfaces of the samples. Failure processes appeared immediately after beginning of the tensile tests, probably caused by internal cavitation. Cavities tend to be nucleated when local processes such as diffusion cannot relax the stress concentration sufficiently rapidly. 7 refs.