Remarkable near-surface microstructure of nanoparticles and oxide film in laser shock peened Al-Zn-Mg-Cu alloy

The near-surface microstructure in an Al-Zn-Mg-Cu (7075-T6) alloy after Laser Shock Peening without Coating (LSPwC) was characterized by advanced electron microscopy methods. A remarkable microstructure composed of a ~2 μm wide newly solidified matrix recast surface layer embedded with O-rich Al nanoparticles (NPs) with the close-packed orientation relationship (OR) as the surrounding Al matrix has been discovered, together with a nano-scale aluminum oxide layer formed on the outermost surface. The formation mechanism is associated with high-pressure surface ablation leading to melting, vaporization and shock-assisted rapid solidification during the LSPwC process. The close-packed OR between NPs and matrix is believed to be due to surface energy minimization. [Display omitted]