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Achieving high strength and large ductility of an ultrafine‑grained 211ZX aluminium alloy processed by improved thermomechanical processing

Luo Tao, Lei Lei, Feng Ke, Yang Ming, Jiang Yun

Archives of Civil and Mechanical Engineering

2023

Vol. 23, no. 2

art. no. e127, 2023

Simultaneously achieving high strength and ductility has been a long-standing goal in aluminum alloys, while the increase in strength usually leads to ductility loss. In this study, a novel thermomechanical treatment (TMT) method, i.e., pre-existing precipitation in coarse grain (CG) and cryogenic rolling plus warm rolling followed by peak aging, is developed to achieve high strength and good ductility in 211ZX aluminum alloy. As a result, a composite nanostructure including ultrafine-grained (UFG) and nanoprecipitation is obtained. Compared to a conventional T6 sample, the multi-step TMT sample has a finer grain (205 nm), while numerous GP zones and θ" phases are dispersed inside the grain. The precipitation characteristics are similar to the T6 sample. The yield strength (635 MPa) and ultimate tensile strength (690 MPa) are about 81% and 53% higher than the T6 sample, respectively, with only a slight decrease in plasticity. Microstructural characterization and thermodynamic analysis confirmed that pre-existing precipitates and cryogenic temperatures facilitate the formation of the composite nanostructure. Quantitatively strengthening calculations demonstrate that the high strength is attributed to the ultra-fine grain strengthening and precipitation strengthening, while the high plasticity is mainly due to the reduction of dislocation density caused by recovery and recrystallization during the aging process as well as the massive production of nano-GIPs (interior grain precipitates).