Mechanical property enhancement in gradient structured aluminum alloy by ultrasonic nanocrystalline surface modification

The large strength difference between hard and soft components in heterogeneous structured materials leads to the evolution of high back-stress hardening, which increases the strength and ductility of materials simultaneously. Moreover, the combination of high shear strain and elevated temperature allows an increase in the strength of low-melting temperature metallic alloys by grain refinement, solute migration, and clustering. In this study, to design a new heterogeneous microstructure in the aluminum alloy, both room temperature (RT) and high-temperature (HT) ultrasonic nanocrystalline surface modification (UNSM) were conducted, and their mechanical properties and microstructural evolutions were investigated. The large shear strain from the UNSM treatment reduces the grain size at the sample surface and creates a gradient structure. The combination of shear strain and elevated temperature during UNSM treatment induces solute migration at a certain depth of the specimens, resulting in the nano-sized Mg-rich particles at the surface region. Both grain refinement and precipitation at the surface region of the HT sample provide strong back-stress hardening in the early stages of deformation that enhances the strength and ductility of materials. Therefore, a high shear strain and control of processing temperature allow the design of a unique heterogeneous microstructure in low-melting temperature metallic alloys, which is a good strategy for enhancing the mechanical properties of sheet or thin metallic products.