Simultaneously enhancing the strength and ductility of as-extruded AlN/AZ91 composites via nano-precipitation and pyramidal slip

Abstract

Age hardening is often used to optimize the mechanical properties of as-deformed Mg-based materials in industry, whereas the improvement of strength is usually accompanied by the significant loss of ductility, which hinders the application of Mg-based materials in structural components. In the present work, high strength-ductility synergy (the yield strength of 263 ± 9 MPa, ultimate tensile strength of 398 ± 7 MPa and elongation to fracture of 34% ± 1%) was realized in as-extruded AlN/AZ91 composites after optimizing aging processes. Microstructural characterization shows that AlN particles induced a large number of geometrically necessary dislocations around the AlN/Mg interface during extrusion, which decreased the nucleation barrier and provided more heterogeneous nucleation sites for γ-Mg17Al12 continuous precipitation. Meanwhile, 95% of residual dislocations in as-extruded AlN/AZ91 composites were annihilated during peak-aging, suppressing the growth and coarsening of continuous precipitates. Therefore, high density of nano-sized γ-Mg17Al12 continuous precipitates was produced in as-extruded AlN/AZ91 composites after peak-aging. During tension, gliding dislocations bypassed spherical γ-Mg17Al12 nano-precipitates by Orowan looping rather than cutting mechanism, which induced a strong block on dislocation motion. So high yield strength was mainly attributed to the high density of non-shearable γ-Mg17Al12 nano-precipitates with spherical morphology, which was different from other Mg-Al-based systems. The results of texture evolution and slip trace analysis demonstrated that the suppression of extension twinning and less basal slip was due to the enhanced activity of pyramidal 〈c + a〉 slip in as-extruded AlN/AZ91 composites after peak-aging during the room temperature tension, meanwhile, the dislocation density of as-extruded AlN/AZ91 composites was significantly decreased during peak-aging, then higher elongation to fracture was achieved.