Microstructural evolution of Al-Zn-Mg-Cu alloy during ultrasonic surface rolling process

This paper systematically investigates the microstructural evolution of Al-Zn-Mg-Cu alloy during ultrasonic surface rolling process (USRP). Al-Zn-Mg-Cu alloys with different gradient deformation degrees were prepared by USRP conducted with 1, 2, 6, 8, 10 and, 12 processing cycles, respectively, and then the deformation structures from the surface of samples with different deformation degrees were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. The results indicated that with the increase of processing cycle, the microstructural evolution at different depths of surface deformation layer is basically the same. But the closer to the surface, the faster microstructure evolves. The basic microstructural evolution order of Al-Zn-Mg-Cu alloy during USRP is scattered dislocation, dislocation cells (DCs), elongated dense dislocation cell walls (DDWs), thick laminated structure and nanolaminated structure. According to results of crystallography and atomic level analysis, the microstructural evolution was described in detail. Scattered dislocation structure is formed by dislocation proliferation. According to the low energy dislocation structures (LEDS) principle, the scattered dislocation structure is transformed into DCs structure through dislocation movement. Under the action of large shear stress, DCs structure is elongated along the direction of shear stress to form elongated DDWs structure. The transformation of elongated DDWs into thick laminated structure and the continuous refinement of laminated structure are mainly achieved by the formation of the low angle grain boundaries (LAGBs) and the transformation from LAGBs to high angle grain boundaries (HAGBs). In some nanolaminated structure regions, the laminated structure is further refined by twin deformation. Our work provides a theoretical basis for the adjustment of the actual processing technology.