The gradient structure in the surface layer of an Al-Zn-Mg-Cu alloy subjected to sliding friction treatment

Abstract

A surface gradient structure can be obtained by sliding friction treatment (SFT). This work studied the effects of SFT parameters on the microstructure evolution in the surface layer of a 7475-T7351 Al alloy. The results show that both applied load and cycle number are conducive to the surface deformation, but the former is more pronounced. Based on transmission electron microscopy (TEM) analyses, grain refinement process induced by SFT treated at a load of 300 N for 200 cycles was discussed. The grain refinement process involves the development of dense dislocation cell walls (DDWs), the formation of microbands with a high dislocation density, the formation of dislocation lines (DLs), dislocation tangles (DTs) and DDWs inside the microbands, the development of equiaxed dislocation cells in the microbands, the transformation of DDWs into ultrafine grains along the direction of the microbands and the development of ultrafine grains with random crystallographic orientations. In addition, from the substrate to the topmost surface, the matrix precipitates dissolved gradually and the grain boundary precipitates grew larger, which was mainly attributed to the severe shear plastic deformation and the formation of a large number of grain boundaries. Keywords: Aluminum alloy, Surface nanocrystallization, Gradient structure, Grain refinement, Precipitate evolution