The defining role of initial microstructure and processing temperature on microstructural evolution, hardness and tensile response of Al-Mg-Sc-Zr (AA5024) alloy processed by high pressure torsion

The microstructural evolution, crystallographic texture and mechanical behaviour of an Al-Mg-Sc-Zr (AA5024) alloy subjected to severe plastic deformation through high pressure torsion (HPT) were investigated. Post HPT at ambient condition, the alloy showed coarse elongated grain structure, both in as-received and heat-treated conditions (grain size, d ~1–2 µm) near top region, while fine equiaxed ultrafine grains (d ~130–380 nm) were observed from edge to centre of the disc. Processing the alloy by HPT at elevated temperatures (473 K and 623 K) resulted in texture weakening and a more homogeneous microstructure (d ~300–800 nm); the lower the processing temperature, the finer is the grain size. The samples processed at lower temperatures exhibited higher hardness (~2.0 GPa) and significant mechanical strengthening (~800 MPa) at the expense of ductility. The mechanical response of the alloy processed by HPT at a higher temperature of 623 K was similar to that of the as-received alloy. The ductility was also restored at reduced strength. Additionally, the as-received and heat-treated alloy specimens displayed noticeable differences of in-plane tensile anisotropy and orientation-dependent mode of dynamic strain ageing with respect to the initial rolling direction. Thus, this study clearly elucidates that the ultrahigh strengths can be achieved in AA5024 alloy by grain refinement through HPT with negligible influence of prior processing history and reduced local heterogeneity in microstructure by HPT at elevated temperatures.