Tensile damage stages in cast A356-T6 aluminium alloy

Abstract

This paper reports an investigation of the damage stages during straining of A356-T6 alloy by means of quantitative metallography, tensile tests, in situ tensile tests, and in situ micro-extensometry experiments with a SEM. Quantitative analysis of the microstructure and the damage mechanisms have identified the eutectic spatial arrangement as the crucial feature in fracture; interdendrite fine eutectics (eutectic channels) and extended eutectic zones (eutectic clusters) play different roles, so that the Si particle breaking rate varies from one region to another. The current models of Si particle breaking were then checked and their predictions compared with the local cracking rates. None of them were consistent with the experimental results. Next, cavity growth was investigated and the experimental results compared with data reported in literature and with predictions obtained by the Rice and Tracey law. It was shown that the initial porosity generated by Si particle breaking was very low and that void growth calculations completely underestimate void growth rate; once more, the specific role of Si particles spatial arrangement was decisive. Lastly, the analysis of the fracture stage revealed that cracks mostly propagate along the interdendritic channels, and this indicates the localisation of the damage, thus invalidating the ductility predictions of current fracture models.