Mechanical Behavior and Strengthening Mechanisms in Precipitation-Strengthened Aluminum Alloy with Gradient Structure Induced by Sliding Friction Treatment

The flat surfaces of a 7075 aluminum (Al) alloy plate were processed by sliding friction treatment (SFT), which is a technique for surface nanocrystallization of metals and alloys. The aim of this study was to investigate the microstructural evolution, mechanical behavior and strengthening mechanisms in this SFTed precipitation-strengthened Al alloy. The SFT resulted in a gradient structure (GS) with an effective depth of ~ 800 µm, and a nanostructured layer was formed within a depth of ~ 30 µm from the surface. Increasing boundary spacing and decreasing misorientation angle between boundaries were found with increasing depth from the surface, which was accompanied by a decrease in hardness from ~ 2436 MPa in the topmost surface layer to ~ 1568 MPa in the undeformed coarse grain (CG) matrix. Moreover, the GS revealed a prominent precipitate redistribution induced by the SFT. The GS revealed higher strength and especially exhibited a higher work-hardening rate at low strain (ε < 0.026) than the CG, attributed to a novel coupling of dislocations, boundaries and precipitates. Grain boundary strengthening, dislocation strengthening, precipitation strengthening and synergetic strengthening in the GS were quantitatively evaluated based on sufficient discussion.