Enhanced strength and ductility synergy in dilute aluminum alloys through cryogenic multi directional forging: synergistic effects of bimodal heterostructured grains and multi scale defects

The cryogenic multi-directional forging (Cryo-MDF) technique enables aluminum alloys to achieve a remarkable strength-ductility synergy through unique microstructural evolution. The Cryo-MDF processed T6-6061 aluminum alloy attains a high tensile strength of 412 MPa while retaining an elongation of 10.5%, overcoming the conventional strength-ductility trade-off. Post-forging microstructural characterization reveals a bimodal heterostructured grain architecture, accompanied by nano-twins, high-density stacking faults, Lomer-Cottrell (L-C) locks, and lattice distortions. These multi-scale defect configurations synergistically enhance yield and tensile strength while preserving ductility via four key mechanisms. Nano-twin boundaries act as robust barriers to dislocation glide. High-density stacking faults restrict dislocation mobility and elevate strain-hardening rates. L-C locks stabilize tetrahedral nodes through pinning effects, forcing dislocation path deviations. Lattice distortions enhance local structural stability. This hierarchical, multi-scale coordination mechanism not only elucidates the origin of exceptional mechanical properties in Cryo-MDF alloys but also establishes a microstructure design paradigm for developing next-generation high-performance aluminum alloys. (Figure presented.)