Surface gradient nanostructures in high speed machined 7055 aluminum alloy

Abstract The high speed machining induced surface deformation layer in a 7055 aluminum alloy was investigated by means of transmission electron microscopy (TEM) and precession electron diffraction (PED) assisted nanoscale orientation mapping. The gradient nanostructures were composed of equiaxed and lamellar nanograins and ultrafine grains decorated by coarse grain boundary precipitates (GBPs). The presence of low angle dislocation boundaries, the recrystallized nanograins and ultrafine grains showed direct evidence that dislocation activities and dynamic recrystallization are two dominant grain refinement approaches, while the large size and density differences between GBPs and grain interior precipitates (GIPs) unraveled a prominent precipitate redistribution, which can be accomplished via the thermally and mechanically induced precipitate dissolution, solute diffusion and reprecipitation. The quantitative prediction of solute diffusion in the current machining condition agreed well with the TEM observation results. The crystallographic texture of the surface nanostructured layer was proved to be a mixture of brass, cube and weak rotated cube, the severe but diversified thermomechanical effect of high strain, high strain rate and high temperature shear deformation during high speed machining is responsible for texture development.