Achieving High Strength and Large Ductility of an Ultrafine-Grained 211zx Al Alloy Processed by Improved Thermomechanical Processing

Abstract

Achieving high strength and large ductility simultaneously has been a long-standing goal in Al alloys. In this study, a superior combination of ultimate tensile strength (690 MPa) and elongation (10.5%) was achieved in an ultrafine-grained (UFG) 211ZX Al alloy (205 nm) processed by improved thermomechanical treatment (TMT), which consisted of cryogenic rolling plus warm rolling followed by peak ageing. Microstructural evolution from TMT processing was investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Thermodynamic analysis and the microstructural characterization confirm that pre-existing precipitates and cryogenic temperature facilitate the formation of embryo structures and re-precipitation of grain interior precipitates (GIPs) in UFG. Compared to a conventional T6 heat treatment sample, the multi-step TMT sample has similar precipitation characteristics, which contain a GP zone and θ" phase in the UFG matrix. Quantitatively strengthening model calculations reveal that the high strength of the UFG alloy is due to a combination of grain-boundary strengthening, dislocation strengthening, and precipitation strengthening. The high plasticity of the UFG material is mainly attributed to the reduction of the dislocation density caused by recovery and recrystallization during the aging process as well as the massive production of nano-GIPs.

Keywords: 211ZX alloy, UFG, TMT processing, mechanical properties, Precipitation behavior