Non-equilibrium Solid Solution And Strengthening Mechanism Of Mg-RE-Zn Alloy Induced By Severe Deformation

Mg-RE-Zn alloys have attracted widespread attention due to the presence of longperiod stacking ordered(LPSO)phases and their excellent mechanical properties.Severe plastic deformation(SPD)utilizes high hydrostatic pressure and large cumulative strains to achieve grain refinement,accompanied by another significant effect: the deformationinduced formation of supersaturated solid solutions.This provides favorable preconditions for subsequent age-hardening precipitation.However,the studies of secondary phase fragmentation and dissolution,as well as the induced non-equilibrium solid solution phenomena in Mg-RE-Zn alloys under SPD conditions,are still insufficient.This study investigates the grain refinement,dislocation density evolution,secondary phase fragmentation and dissolution,and the resulting solute redistribution in Mg-12.12Gd-4.2Y-2.28Zn-0.36Zr(wt.%)alloy during rotary backward extrusion(RBE).It reveals the relationship between secondary phase transformation and non-equilibrium solid solution under different deformation parameters,elucidates the microstructural evolution and strengthening mechanisms under complex stress conditions,and provides theoretical and technical guidance for the preparation of high-performance magnesium alloy forgings.The research results are as follows:The solution-treated Mg-Gd-Y-Zn-Zr alloy exhibits a gradient microstructure after RBE deformation,with significant refinement of both the secondary phase and grain size in the fine-grained region,which is markedly finer than that of RUE and BE specimens.After RBE deformation,the gradient refinement of grain size from the outer wall to the inner wall is initially dominated by the continuous dynamic recrystallization(CDRX)mechanism,gradually transitioning to a particle-stimulated nucleation(PSN)mechanism induced by blocky LPSO phases.The coarse blocky LPSO phases are transformed into fine intragranular sandwich structures uniformly dispersed within the matrix.This transformation process is primarily related to the kinking of the LPSO phases under intense shear stress and the long-range diffusion of solute elements along the basal planes.Furthermore,this transformation increases the solute concentration in the matrix,forming a supersaturated solid solution.The mechanical properties of the RBE-deformed specimens show significant improvement compared to BE-deformed specimens,with a notable increase in yield strength(YS)and tensile strength(UTS)while maintaining elongation.The increase in strain has a significant impact on the microstructure refinement and non-equilibrium solid solution of the alloy.As the number of rotations increases,the deformation zone expands.The solute solubility and grain refinement,two key material parameters,do not reach saturation simultaneously during the RBE deformation process;the former requires a higher strain level to reach saturation.Under the effect of solid solution strengthening,the hardness of the fine-grained region of the alloy after 200 rotations is slightly higher than that after 100 rotations.By modifying the stress-strain state in the deformation affected zone through the adjustment of the punch shear structure,better grain refinement is achieved,with grain size refined to 550 nm.In this region,the sandwich-structured LPSO phase is completely fragmented and dissolved,rapidly decomposing into nanoparticle phases(β).The formation of ultrafine grains is attributed to the interaction between the fragmentation,dissolution,re-precipitation of the secondary phase,and high-density dislocations during the coordinated deformation process.The reprecipitation of high-density nanoscale particle phases significantly reduces the solute concentration in the matrix,leading to the decomposition of the supersaturated solid solution.Under the combined effects of grain refinement strengthening and precipitation strengthening,the hardness in the ultrafine-grained region reaches its maximum value(125.7 HV).By controlling the morphology of the intragranular lamellar phase in the initial alloy,the plasticity of the alloy is improved.In the low-strain region(mixed grain zone,shear zone),the dynamic recrystallization(DRX)process is suppressed after the introduction of a high-density lamellar phase,reducing the grain refinement efficiency.The intragranular lamellar phase enhances the dislocation pinning effect,maintaining a high dislocation density across the deformation zone without excessively compromising the grain refinement efficiency.The modification of the initial microstructure has little impact on the high-strain region(fine grain zone,ultrafine grain zone).The hardness in the lowstrain deformation zone is increased due to the presence of the lamellar phase and high dislocation density,while the hardness in the high-strain deformation zone remains essentially unchanged.Compared to the T6 treatment,both the fine-grained region and the ultrafine-grained region exhibit higher peak hardening under the T5 regime,with values of 132.5 HV and142.7 HV,respectively.The high hardness of the fine-grained region after T5 treatment is attributed to higher contributions from grain boundary strengthening,dislocation strengthening,and precipitation strengthening.In contrast,the ultrafine-grained region’s high hardness is due to higher grain boundary strengthening and secondary phase strengthening contributions.The two deformation zones exhibit different grain coarsening efficiencies and secondary phase transformation behaviors during the annealing process.After annealing at 480 ℃,the original phase structure of the fine-grained region remains stable,and no secondary phase precipitates at the grain boundaries.With a decrease in annealing temperature,intragranular lamellar phases precipitate,and high-density β phases precipitate at grain boundaries,with the size of the precipitated phases decreasing with lower annealing temperatures.In the ultrafine-grained region,the size of the β phase increases with higher annealing temperatures.The hardness of both regions decreases with increasing annealing temperature,with the ultrafine-grained region showing a greater degree of hardness degradation due to the singularity of its strengthening mechanism.