A novel cobweb-like sub-grain structured Al-Cu-Mg alloy with high strength-plasticity synergy

Abstract

Al-Cu-Mg alloys, as the most widely used lightweight structural materials, have been recognized as promising candidates in the transportation field for a low-carbon economy. However, the tensile strength and plasticity of alloys cannot be simultaneously improved to satisfy the requirements of continuously upgraded transportation vehicles. In this work, inspired by high-tensile strength and high plasticity of cobweb structure, a novel cobweb-like sub-grain structure was developed in Al-Cu-Mg alloys by a successive solution, high-strain-rate rolling (4.4 s^-1), cryogenic treatment (–196 °C) and aging process (SRCA). Notably, the tensile strength and plasticity of this alloy were superior to those reported in the current study. An ultrahigh Vickers hardness and tensile strength value of 206.2 Hv and 619.6 MPa, which were 39.8 % and 31.8 % higher than those of traditional T6 heat-treated Al-Cu-Mg alloys, were obtained after SRCA. Meanwhile, an increase in the elongation of this alloy from 4.31 % to 8.23 % (increase of 90.9 %) was also achieved. More importantly, the high strength-plasticity (“double high”) Al-Cu-Mg alloy was attributed to a cobweb-like sub-grain structure, which was proposed for the first time by utilizing reverse thinking to enhance plasticity through elevating dislocations, due to the formation of high-density dislocations from high-strain-rate rolling and rearrangement effect of dislocations from cryogenic treatment. Furthermore, the strength-plasticity mechanism was verified using in-situ tensile electron back scatter diffraction (EBSD), molecular dynamics (MD) simulations, and crystal plasticity (CP) models. The results indicated that the cobweb-like sub-grain structure, resembling countless walls, formed barriers that hindered dislocation migration towards high-angle grain boundaries (HAGBs) and absorbed them, thereby reducing the occurrence of stress concentration zones, i.e., the dislocation absorption and stress-strain sharing mechanisms. Additionally, the strengthening mechanism was associated with synergistic strengthening by multiscale microstructures, including micron-sized grains, micron-sized high-density dislocation lattices, and nanosized Al₂CuMg phases, which were activated by successive deformation processes. Consequently, the concept of biomimetic structure design, which may serve as an effective method for achieving structural materials with high strength-plasticity synergy, can be extended to transportation fields, such as railway tracks and body structure design.