Effect Of Ytterbium Addition On The Recrystallization Mechanism And Texture Development Of Extruded ZK60 Magnesium Alloys

Rare earth alloying combined with hot processing is considered to be an effective approach to improving the mechanical properties of magnesium alloys.In this study,ZK60 magnesium alloys with Yb content of 0,1.0%,and 2.0%(mass fraction)were prepared(Mg–Zn–Zr–Yb series)and studied focusing on the evolution of microstructure and mechanical properties during hot extrusion.The effects of Yb concentration and extrusion process on recrystallization behavior,morphology and distribution of precipitated phases,texture evolution and mechanical properties of alloys were systematically investigated.The mechanisms underlying the dense precipitation of spherical Mg–Zn–Yb particles,suppression of discontinuous recrystallization,basal texture weakening and c-axis of grains tilted towards the extrusion direction(ED)of the studied alloys during hot extrusion were elucidated.This research sheds a light on the influence of Yb on the microstructure and mechanical properties of magnesium alloys and provides fundamental understanding for the development of novel Yb-containing magnesium alloys with excellent mechanical properties.The main conclusions are summarized as follows:When extruded at the same temperature,the grains of the alloys were considerably refined with increasing Yb concentration,but the volume fraction of dynamic recrystallization(DRX)decreased.The rod-shaped precipitates dominated in ZK60 alloys transformed into nano-scale spherical Mg–Zn–Yb precipitates in Yb-containing alloys,which combined with solute clusters and segregation at grain boundaries providing sufficient grain boundary drag to suppress the nucleation of discontinuous recrystallization in the form of grain-boundary bulging.Meanwhile,Yb addition reducing the stacking fault energy,hindering the dynamic recovery of dislocations and promoting the formation of shear bands and {10-12} tension twins resulted in a significant increase of dislocation density at the early stage of deformation,thus promoting the continuous recrystallization by formation and rotation of sub-grains.Therefore,as the Yb concentration increased,the dominant nucleation mechanism transited from discontinuous dynamic recrystallization to continuous dynamic recrystallization.Yb addition disrupted the tendency for developing an intense basal texture during extrusion.With the increase of Yb concentration,the texture randomization increased significantly,and the c-axis of the recrystallized grains gradually inclined to the ED.The rare earth texture(RE texture)was observed in the as-extruded ZK60–2wt% Yb(ZYb K620)alloy for the first time.It was found that the formation of the abnormal non-basal texture was attributed to the combination of effects derived from Yb solute.First,nucleation along extension twin boundaries preserved an initial “Rare-Earth” orientation at the initial deformation stage.Second,Yb addition changed the surface energy of the matrix and promoted the growth of dynamically recrystallized grains along the non-basal plane with higher surface energy.Last but not least,Yb reduces the critical resolved shear stresses of non-basal slip and thus activated a large amount of pyramidal <c + a> slip causing the crystal lattice to rotate towards the ED along the <10-10> axis.Based on the aforementioned discussion,the ZYbK620 alloy was selected to track the evolution of microstructure and mechanical properties during extrusion at 300 ℃.It was found that the temperature rise caused by the deformation-induced heat considerably affected the microstructure and mechanical properties of the extrudate.The initial bimodal-grained sample with most nanoprecipitates presented an ultra-high ultimate tensile strength of ~436 MPa and a lower elongation of ~14.1% along the extrusion direction,while the fully recrystallized ultrafine-grained sample with fewer precipitates exhibited a favorable balance in UTS and EL of ~393 MPa and ~19.1%,respectively.The Mg–Zn–Yb thermostable nanoprecipitates present in the alloy improved its strength,and the tilted weak texture that was rarely observed in the axisymmetric extrusion contributed to the ductility enhancement.This work provided the fundamental insight into the underlying mechanisms of achieving the strength-ductility synergy via precipitation and texture tailoring in a newly developed Yb-alloyed Mg alloy with conventional and readily available processing technology.