Basic Research On Hot Extrusion Forming Of The Low Cost And Ultra-High Strength Mg-Ca Based Alloy

Magensium alloys,with obvious advantages of saving weight,have great potentials to be used in various fields of automotive,aerospace,3C and so on.The presently developed Mg-Rare-Earth(RE)alloys exhibit excellent mechanical properties.However,due to the high cost and strage usage of RE elements,Mg-RE alloys face greater challenges to be used in industrial applications in large scales.Consequently,it is urgent to develop new rare-earth-free and high-performance Mg alloy.In this work,based on the composition optimization and extrusion deformation,a low-cost Mg-Ca-based wrought alloy with excellent mechanical properties has been developed.The following main results can be obtainedMg-Ca dilute alloys can exhibit exceptional high ultimate strength,e.g.,?390 MPa in Mg-1.0 wt.%Ca alloy and?300 MPa in the Mg-1.0 wt.%Ca alloy,achieved by conventional extrusion.Multi-scale substructures,including the ultrafine subgrains and dynamically recrystallized grains,residual dislocations with segregations and the Mg2Ca nano-phases contribute to the high strength.Both increased activity of non-basal slips due to Ca addition and the dynamic partitioning of Ca atoms during extrusion are believed to play the critical role in facilitating formation of the ultra-fine grains in submicron scale.Based on the Mg-Ca binary alloy,a novel Mg-2Sn-2Ca(wt.%)wrought alloy with tunable ultra-high tensile yield strength(360-440 MPa)was also developed.Interestingly,there is little drop in mechanical properties of this alloy even when it is extruded at a speed several times higher than those used in conventional processes.Examination of asextruded microstructures of this alloy reveals that the ultra-high strength is associated with the presence of surprisingly ultra-fine grains(?0.32 ?m),mono-dispersed Mg2Ca nano-particles,and Ca enrichment in most sub-grain boundaries.We suggest that it is the combination of solute partitioning and numerous Mg2Ca nano-precipitates that contributes to formation of the ultra-fine grains during extrusion,which in turn contributes significantly to the enhancement of the yield strength of the extruded Mg2Sn-2Ca alloy.However,the alloy had a poor elongation of?1.2%while retaining the highest strength.Based on the multi-alloying with Mn element,Mg-Sn-Ca-Mn alloy can exhibit a good combination of strength and ductility:yield strength of?450 MPa,ultimate strength of?462 MPa,and elongation of?5%.High-density of dislocations exist in the as-extruded alloy sample,which leads to the dynamic precipitation of a large amount of nano-Mn particles both along the grain boundaries and within the grain interiors.Consequently,nucleation rate of the DRXed grains can be increased significantly.Moreover,the nano-particles can suppress growth of the DRXed grains.More importantly,fiber texture of the DRXed grains have been significantly tilted and weakened,and the comibiantion of ultra-high strength and good ductility can be achieved in the ultra-fine grained Mg alloys.