Micro-plastic deformation behavior of Al-Zn-Mg-Cu alloy subjected to cryo-cycling treatment

The micro-plastic deformation behavior of Al-Zn-Mg-Cu alloy subjected to cryo-cycling treatment was investigated experimentally in the present work. Micro-plastic deformation was produced by continuous loading-unloading method with applied stress far below the yield strength of this alloy, and the deformation characteristics of grains were detected by Optical Microscope (OM) and Scanning Electron Microscope (SEM). The results showed that cryo-cycling treatment reduced the residual strain (epsilon(p)) after each cycle of loading-unloading, and the degree of grain deformation was decreased apparently, which indicated that cryo-cycling treatment enhanced the capacity of resistance to micro-plastic deformation. Transmission Electron Microscope (TEM) was employed to reveal the mechanisms of cryo-cycling treatment on Al-Zn-Mg-Cu alloy. The results showed that higher content of homogenous eta phases precipitated in the matrix and grain boundaries of Al-Zn-Mg-Cu alloy after cryo-cycling treatment. Meanwhile, no clear precipitate-free zones (PFZs) can be observed in the microstructure of cryo-treated specimen. The changes of microstructure had great contribution to improve the pinning effect of dislocations during micro-plastic deformation. Internal stress was released by the cryo-cycling treatment which was demonstrated by the in-suit cryogenic X-Ray (XRD) measurement, as a result, the stability of dislocation was improved. Cryo-cycling treatment had no obvious influence on the tensile strength and yield strength of Al-Zn-Mg-Cu alloy, while elongation and fracture toughness were improved by the extent of 7% and 10.3% respectively, compared to those of the untreated specimen.