Cooperative enhancements in ductility and strain hardening of a solution-treated Al–Cu–Mn alloy at cryogenic temperatures

Abstract An experimental research was conducted on the enhanced ductility and strain-hardening of a solution-treated Al–Cu–Mn alloy for understanding its deformation mechanism at the cryogenic temperature of liquid nitrogen (LN2). Both quasi-in situ tensile and rigid punch bulging tests were performed, enabling the deformation behaviors under uniaxial stress and complex stress states to be considered comprehensively. The capabilities of resisting fracture and plastic deformation were evaluated at both room temperature (RT) and −196 °C. Based on the EBSD/SEM observations, the correlations were quantitatively elucidated between the macroscopic deformation and the microstructure evolution, underlying the slip band, local misorientation, geometrically necessary dislocation, stored strain energy and relative slip distance. It is concluded that the Al–Cu–Mn alloy exhibits cooperatively enhanced ductility and strain-hardening at −196 °C. The average limiting dome height (LDH) is increased from 20.6 mm to 26.8 mm as the temperature varies from RT to −196 °C. The maximum load is 2.25 times higher than that at RT under a biaxial stress state. Moreover, the capabilities of resisting crack propagation and localized thinning are also substantially improved. In terms of the observation of local misorientations at −196 °C, the enhanced ductility is primarily a result of the diminished accumulation of movable dislocations along the grain boundaries, and the increased storage capacity in grain interiors. The increased strain hardening is attributed to the decreased relative slip distance of the activated dislocations, and the inhibited massive collapses of the tangles and cells of dislocations during the cryogenic deformation.