Correlation of fracture toughness with microstructural features for ultrafine-grained 6082 Al alloy

In the present work, cryorolling (CR) and room temperature rolling (RTR) followed by annealing (AN) at 200 degrees C were carried out to investigate the effects of grain size, precipitates (Mg-Si-phases), and AlFeMnSi-phases on the fracture toughness of 6082 Al alloy. Using the values of the conditional fracture toughness, (K-Q), in the critical fracture toughness (K-IC) validation criteria, it was found that the sample size is inappropriate, which implies that the conditional fracture toughness obtained cannot be considered as the critical fracture toughness. Therefore, to establish the relative improvement in fracture toughness, the equivalent energy fracture toughness (K-ee) and J-integral were calculated and used. The results show that the values of K-ee (89.91 MPa root m) and J (89.86 kJ /m(2)) obtained for the sample processed via CR followed by AN (CR + AN) are the highest when compared with the other samples processed through CR, RTR, and RTR followed by AN, RTR + AN. Microstructural features such as high fraction of low Taylor factor, high fraction of kernel average misorientation, Si-rich particles, small size AlFeMnSi-phases, and mixed mode of failure (transgranular shear and micro-void coalescence) also support the high fracture toughness in the CR + AN sample. It was also observed that the effect of residual stresses on the fracture toughness of CR and RTR samples is minimal. Therefore, the correlation between microstructure and residual stresses is not considered in the present work due to very small values of the residual stresses for CR and RTR samples and the absence of residual stress from the heat-treated samples.