Plane stress fracture toughness of cryorolled 6082 Al alloy

In this paper, the plane stress fracture toughness of 6082 Al alloy produced by cryorolling is reported for the first time. The rolling was performed to achieve a 75% reduction in the sample thickness at two different temperatures, namely, liquid nitrogen temperature (−196 °C), and room temperature (25 °C). The plane stress fracture toughness was evaluated for cryorolled (CR), room temperature rolled (RTR), cryorolled and annealed (CR + AN), and room temperature and annealed (RTR + AN) samples using ASTM E1820-13 standard and compact tension specimens. Two methods, i.e., the Load point displacement (LPD)-load and the J-Integral-resistance (J-R) curves, were used to calculate the plane stress fracture toughness. The results showed that the fracture toughness calculated using the J-R curve is more accurate since it accounts for all the constraints for evaluating the critical plane stress fracture toughness of the alloy for all the processing routes. The values of the critical plane stress fracture toughness (JIC) obtained via the J-R curve were 49 kJ/m2 for CR, 31 kJ/m2 for RTR, 69 kJ/m2 for CR + AN and 39 kJ/m2 for RTR + AN samples. Microstructural features obtained through scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD) were also correlated with the fracture toughness. It is found from the SEM results that Mg-Si phases evolved at the crack tip/path during crack propagation in all the investigated samples. The EBSD results showed that CR + AN sample exhibits high kernel average missorentation and crack branches due to the evolution of small sized Mg-Si (Si-rich) precipitates during the fatigue testing. The presence of mixed grains (coarse and fine), fractographic features such as shallow ridges, transgranular facets, flat circular and elongated dimples, small sized Si-rich precipitates and high kernel average missorentation support the high plane stress fracture toughness observed for the CR + AN sample.