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 similar to 800 mu m, and a nanostructured layer was formed within a depth of similar to 30 mu m 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 similar to 2436 MPa in the topmost surface layer to similar 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 (epsilon< 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. (C) The Minerals, Metals & Materials Society and ASM International 2020