Flow behavior and fracture of Al-Mg-Si alloy at cryogenic temperatures

The tensile and fracture behaviors of AA6061 alloy were investigated in order to provide quantitative data about this alloy at cryogenic temperatures. Specimens of AA6061 alloy were solution heat treated before tensile tests at 298, 173 and 77 K and tested at strain rates in the range from 0.1 to 0.0001 s(-1). The results indicate the suppression of the Portevin-Le Chatelier (PLC) effect and dynamic strain aging (DSA) at 77 K. In contrast, at 298 K, a remarkable serrated flow, characteristic of the PLC effect, is observed. Furthermore, the tensile behavior at 77 K, compared with that observed at 173 and 298 K, shows a simultaneous increase in strength, uniform elongation, modulus of toughness, strain-hardening exponent and strain rate sensitivity, which is related to a decrease in the dynamic recovery rate at low temperature. These responses are reflected on the fracture morphology, since the dimple size decreases at 77 K, while the area covered by dimples increases. Comparisons of the Johnson-Cook model show that a good agreement can be obtained for tests at 173 and 77 K, in which DSA is suppressed.