Simultaneous enhancement of strength and ductility by combining heterogeneous microstructure and nanoprecipitates

Abstract

Developing advanced metallic materials with high strength and ductility is an eternal theme, however, the strength and ductility are generally considered to be conflicted to each other. Here, a simultaneous enhancement of strength and ductility is achieved in a commercial Al6061 alloy by combining the heterogeneous microstructure (composed of recovered zones and partially recrystallized zones) and nanoprecipitates. This unique microstructure is produced by the cold-drawing (∼61%) and pre-torsion (∼360°) followed by low-temperature ageing treatments. When the ageing temperature decreases from ∼300 °C to ∼150 °C, the nanoprecipitates changes from the coexistence of the needle-like β′ and rod-shaped β′′ to exclusive granular β′′ with refined size and increased volume fraction, which leads to the remarkable improvement of yield strength from σy∼176.0 MPa to σy∼340.2 MPa. Concurrently, the evading of precipitate-free zones (PFZs), the suppression of the grain-boundary precipitates and the formation of extremely fine nanoprecipitates (<5 nm) contribute to the unexpected enhanced uniform elongation (from εu∼5.3–5.6%). Furthermore, the hetero-deformation induced (HDI) strengthening and work hardening by the heterogeneous microstructure are also beneficial for the synergistic enhancement of the strength and ductility.