Deformation Behavior Of Mg-Gd-Y(-Zn)-Zr Alloy Studied By In-situ Synchrotron Diffraction And Plasticity Modeling

In the present work,the Mg-8.5Gd-2.5Y-0.3Zr?wt.%??0Zn alloy?and Mg-8.5Gd-2.5Y-0.5Zn-0.3Zr?wt.%??0.5Zn alloy?were subjected to indirect extrusion at400?,450?and 500?,respectively.The microstructure and texture of the as-extruded alloys were systematically investigated to reveal the effect of trace Zn.Additionally,the deformation behaviors of as-extruded and aged 0Zn and 0.5Zn alloys under uniaxial tensile and compressive loading were studied via in-situ synchrotron diffraction measure,polycrystal plasticity modeling and microstructure analysis.Based on the experimental and modeling results,the underlying mechanisms of yield point phenomenon and tension-compression asymmetry were revealed,and the strengthening mechanisms were discussed.The as-extruded alloys have a bimodal microstructure,consisting of dynamic recrystallized?DRXed?grains and unrecrystallized?unDRXed?grains.The recrystallization fraction,mean DRXed grain size and distribution range of DRXed grain size in the as-extruded 0Zn alloys are all larger than those in the 0.5Zn alloy under same extrusion temperature.Under 400?extrusion,DRX in both 0Zn and 0.5Zn alloys mainly occurs via discontinuous dynamic recrystallization?DDRX?mechanism initially.Afterwards,the continuous dynamic recrystallization?CDRX?mechanism is also activated.However,the addition of trace Zn suppresses the CDRX,thus leading to a lower recrystallization fraction in the 0.5Zn alloy.A large amount of?phases were formed in both alloys via dynamic precipitation.These phases mainly distribute at DRXed grain boundaries.In the as-extruded 0Zn alloy,the number density of?phases varies significantly in different DRXed regions,which is mainly due to the grain boundary segregation occurred during the preheating process before extrusion.In the0.5Zn alloy,however,the grain boundary segregation is inhibited,thus leading to a homogeneous distribution of?phases in the DRXed regions.Under 450?extrusion,the DRX in both alloys mainly takes place via DDRX and CDRX mechanisms.In the0Zn alloy,the?phases are only formed in certain regions and inhibit the grain growth in these regions,while the DRXed grains in the?phases free regions are much coarser.In the 0.5Zn alloy,the?phases almost homogeneously distribute in all the DRXed regions.Therefore,all the DRXed grains in the 0.5Zn alloy have small size.At the extrusion temperature of 450?,the difference of recrystallization fraction between 0Zn and 0.5Zn alloys is primarily due to the different DRXed grain size.Under 500?extrusion,the DRX in both alloys mainly occurs via DDRX and CDRX mechanisms.No dynamic precipitation is obviously observed in both alloys.And the fine grains in both alloys are formed within the elongated unDRXed grains by CDRX mechanism.All the as-extruded alloys possess notable?101?0?fiber texture,namely,?101?0?direction parallel to the extrusion direction.With increasing the extrusion temperature,the fiber texture is weakened,while the prismatic texture,i.e.basal plane perpendicular to the extrusion direction,is gradually strengthened.The formation of prismatic texture is associated with the growth advantage of corresponding grains.At the same extrusion temperature,the as-extruded 0.5Zn alloys exhibit stronger fiber texture than the 0Zn alloy.The results of in-situ synchrotron diffraction measurement and polycrystal plasticity modeling suggest that the as-extruded alloys exhibit yield point phenomenon and stress relaxation in the{101?1}grains,which is attributed to the interaction between basal dislocations and solute atmospheres.With lowering the extrusion temperature or adding Zn element,interaction between dislocations and solute atmospheres will be strengthened,thus making the yield point phenomenon and stress relaxation more significant.Furthermore,all the as-extruded alloys exhibit good tension-compression yielding symmetry,mainly due to the strong inhibition of tensile twinning resulting from the solute Gd and Y atoms.It is noted that in the as-extruded500?-0.5Zn alloy,the yielding strength under compression is slightly higher than that under tension.This abnormal phenomenon is attributed to the notable prismatic texture.After macro yielding,the strain hardening behaviors under tension and compression show somewhat difference,due to the significant occurrence of tensile twinning under compression.Since the tensile twinning mainly takes place in the unDRXed grains,raising the extrusion temperature will narrow the difference of strain hardening behaviors between tension and compression,by reducing the fraction of unDRXed grains.Contrarily,the addition of Zn will enlarge the strain hardening difference by increasing the unDRXed fraction.Additionally,the tensile and compressive yielding strength of the as-extruded alloys will be decreased with increasing the extrusion temperature,which is due to the coarsening of DRXed grains,the reduction of dislocation density in the unDRXed grains and the decreasing of unDRXed fraction.However,the addition of trace Zn can enhance the tensile and compressive yielding strength,resulting from the refinement of DRXed grains,enhancement of solid solution strengthening for basal slip and increasing of unDRXed fraction.During aging treatment,no obvious static recrystallization or grain growth is observed,while a large amount of?'and?₁ phases are precipitated within grains.The in-situ synchrotron diffraction measurement and polycrystal plasticity modeling results suggest that the plasticity of aged alloys under tension is mainly accommodated by basal and prismatic slip.After aging treatment,the interaction between basal dislocations and solute atmospheres is weakened.As a consequence,the yield point phenomenon disappears and the elastic-plastic transition becomes more broadening.The tensile yielding strength of the alloys is effectively enhanced after aging,due to the strengthening of basal and prismatic slip by the?'phases and the larger strain hardening rate at the elastic-plastic transition stage.Finally,it is noteworthy that the present work reveals the effect of trace Zn on the microstructure,texture and mechanical properties of extruded Mg-Gd-Y-Zr alloys.Additionally,the deformation behaviors and strengthening mechanisms of as-extruded and aged Mg-Gd-Y?-Zn?-Zr alloys with complicate microstructure are also clarified.Therefore,this work is promising to provide theoretical direction for the development of high strength magnesium alloys and promote the wide application of magnesium alloys.