Abstract: In this work, we investigated the mechanical properties and corresponding deformation mechanisms of an Al1Mg0.4Si alloy, which exhibited significantly higher strength and outstanding strain hardening capacity at 77 K compared to its counterparts at 298 K. The deformation mechanisms responsible for the excellent strength-ductility synergy and extraordinary strain hardening capacity at cryogenic temperature were elucidated through a combined experimental and simulation study. The results reveal the presence of numerous slip traces and microbands throughout grain surfaces during deformation at 298 K, whereas at 77 K, vague grain surfaces dominate, indicating the simultaneous operation of multiple slip systems. Transmission electron microscopy (TEM) analysis using the two-beam diffraction technique demonstrates the presence of dislocations with several different Burgers vectors inside a grain at cryogenic temperature, confirming the activation of multiple slip systems. The accumulation of dislocations facilitated by these multiple slip systems, combined with the high dislocation density, contributes to strain hardening and remarkable uniform elongation at 77 K. A modified dislocation density-based crystal plasticity model, incorporating the effect of grain boundary hardening (GBH) and temperature, was developed to gain a better understanding of the underlying mechanisms governing alloy's strength and plasticity. The GBH effect significantly enhances statistically stored dislocation (SSD) density and screw dislocation proportion, which promote homogeneous deformation and enhance strain hardening capacity at cryogenic temperature. These findings deepen the understanding of plastic deformation at cryogenic temperatures and pave the way for the development of ultrahigh-performance metallic materials for cryogenic applications.

Key words: Aluminum alloy, Cryogenic temperature, Grain boundary hardening effect, Deformation mechanism, Crystal plasticity modeling