A mechanistic basis for high strain rate superplasticity of aluminum based metal matrix composites

Aluminum based metal matrix composites exhibit superplasticity at unusually fast strain rates (0.01 to 100 s−1). An analysis of these data suggests the presence of a superplastic anomaly, characterized by high values of the strain rate sensitivity, and by flow stresses that depend on the grain size, even though the nominal deformation mechanism lies in the power law creep regime. The anomaly is explained in terms of stochastic events of rapid grain boundary sliding. These events are nucleated by stress induced grain boundary migration which, by reducing the amplitude of the boundary shape, accelerate the rate of diffusion accommodated sliding. An equation for the strain rate where such supercritical sliding events are expected is derived. This equation has the same form as the classical equation for diffusional creep except that it is multiplied by an acceleration factor of magnitude 80–380. In this way relationships between stress, strain rate, temperature, and grain size that quantitatively describe the onset of the anomaly are obtained.