Achieving extraordinary strength-plasticity synergy in AZ91 alloy processed by multi-degrees of freedom forming

Abstract

Strength and plasticity trade-off in magnesium alloys is a longstanding and crucial issue that significantly limits their application, which strongly relates with microstructure distribution and texture characterization. In this study, multi-degrees of freedom forming (multi-DOF forming) technology is innovatively proposed and first employed on as-extruded AZ91 alloy. Microstructure observation reveals that the bimodal-grained structure including 25.4 % of coarse grains (CGs, ∼20 μm) and 74.6 % of fine grains (FGs, ∼2 μm) in volume fraction is fabricated in the multi-DOF formed AZ91 alloy, which is mainly attributed to inhomogeneous dynamical recrystallization (DRX), i.e., un-recrystallized plays a leading role in CGs and DRX plays a leading role in FGs. Grain orientation analysis indicates that multi-DOF forming technology leads to a significant disappearance of basal texture, bringing out the absolute predominance of prismatic texture with 83.4 % in volume fraction, which is mainly owing to the introduction of large radial-tangential coupling shear deformation and thus efficient activation of non-basal slips in magnesium alloys. Moreover, the Multi-DOF formed AZ91 alloy exhibits high ultimate tensile strength (∼402 MPa) without sacrificing elongation (∼18.1 %), realizing extraordinary strength-plasticity synergy. The high work-hardening capacity resulting from high geometrically necessary dislocations at the interface of CGs/FGs is conducive to the improved strength. The higher Schmidt factor in grains with bimodal-grained structure or non-basal texture dramatically contributes to the enhanced plasticity. Thus, we propose a novel and effective multi-DOF forming technology for achieving concurrent enhancement of strength and plasticity in magnesium alloys.