Dynamic precipitation and deformation behaviors of a bimodal-grained WE43 alloy with enhanced mechanical properties

Abstract

Achieving a high strength-ductility synergy in rolled Mg alloys remains a major challenge, especially for Mg alloys with high contents of rare earth (RE) elements, which are hard-to-deform and exhibit poor formability. In the present work, a bimodal grain structure consisting of fine grains (FGs) and coarse grains (CGs) has been achieved in a novel WE43 alloy sheet prepared by the hard-plate rolling (HPR) technique, which breaks the strength-ductility trade-off dilemma of the conventional WE43 alloy. Notably, the yield strength (YS) reaches as high as ∼312 MPa, presenting a remarkable improvement of YS (∼60 MPa) in comparison to conventionally rolled counterparts. Meanwhile, the ultimate tensile strength (UTS) and elongation-to-failure (EF) are ∼332 MPa and ∼11.8%, respectively, much higher than those of the wrought WE43 alloy reported in literature. Specifically, the disparate dislocation slips and dynamic precipitation behaviors in FGs and CGs have been studied in detail by combining EBSD and TEM analysis. The discrepancy of dynamic precipitation behavior in FGs and CGs is originated from the heterogeneous dislocation slip-dominated deformation behavior during HPR, related to initial grain orientations. It reveals that basal slips are dominant in the FG area, where equilibrium β-Mg14Nd2Y phase has precipitated along grain boundaries. In contrast, non-basal slips are prevalent and the metastable short rod-shaped β1 phase has formed within CGs. The enhanced YS of the present HPRed WE43 alloy sheet is mainly attributed to the formation of large amounts of extremely fine grains with an average size of ∼0.94 μm. The enhanced Orowan strengthening comes from the interaction between short rod-shaped β1 particles and non-basal dislocations (prismatic <a>, pyramidal <c + a> and pyramidal <a> dislocations) in CGs. In particular, the interactions between β1 particles and pyramidal <a> dislocations dominate the Orowan strengthening, leading to an increase of ∼34 MPa in the YS. Basal to basal slip transfers have been observed in FGs while non-basal slips are prevalent in CGs, which could be helpful for release the stress concentration near GBs. It is beneficial for preventing earlier fracture and thus enhancing the tensile strength combined with decent ductility.