Elementary growth mechanisms of creep cavities in AZ31 alloy revealed by in situ X-ray nano-tomography

Abstract

To gain new insights into damage evolution during superplastic deformation, creep cavity growth in commercial Mg alloy (AZ31) was investigated by the novel technique of in situ X-ray nano-tomography. For this, a sample was subjected to deformation under tensile creep conditions (3.2 MPa and 673 K resulting in a strain rate of about 6.6 × 10−5 s−1) and simultaneously characterized by in situ 3D imaging at a pixel size of 100 nm thanks to a scan time of 7 s. The sample showed presence of several pre-existing cavities in its initial state (average equivalent radius of less than 1 µm). Evolution of 30 of these cavities was tracked during deformation and it was observed that the cavities followed intricate and unique growth routes, resulting in very complex cavity shapes. An original shape based classification of evolution of cavities was proposed. As a result five evolution types emerged and it was seen that one cavity generally grew by a combination of several evolution types. Individual evolution types were further linked to plausible growth mechanisms and compared with appropriate models. This indicated a combination of diffusion and grain boundary sliding to be the primary growth mechanism responsible for creep cavity growth in the tested condition. It is worth noting that in addition to the expected cavity growth, a reduction in volume of several cavities was also observed during deformation.