Research On Dynamic Recrystallization Kinetics Of Rare Earth Magnesium Alloys

Magnesium alloys are widely employed in automobiles,aerospace,electronic communication and other related fields due to their low density,high specific strength,good vibration absorption and electromagnetic shielding performance.However,the limitations of poor plasticity and low strength at ambient temperature is also severely constraining its large-scale application.Extensive research has been performed to improve its plasticity and workability.Among these possibilities,rare earth element alloying has proven to be one of the most effective methods.Through the reasonable addition of rare earth elements and the effective regulation of dynamic recrystallization(DRX),the microstructure and the mechanical properties of the magnesium alloy can be optimized.In this work,the DRX kinetics of the newly developed Mg-5.8 Zn-0.5 Zr-1.0 Yb magnesium alloy was systematically investigated.A series of thermal simulation tests was conducted to study the hot compression behaviors of the alloy.The influence of deformation temperature and strain rate on the flow stress was analyzed.Based on the modified Avrami model,the critical conditions for dynamic recrystallization of the test alloy were determined,and the dynamic recrystallization kinetics model of Mg-5.8 Zn-0.5 Zr-1.0 Yb alloy was established.Furthermore,the Zener-Hollomon parameter(Z parameter)was introduced to analyze the influence of deformation conditions on the dynamic recrystallization kinetics.Finally,the effect of Yb on the dynamic recrystallization kinetics was comparatively analyzed with the counterpart without Yb addition.The main conclusions are as follows:According to the intrinsic relationship between the work hardening rate and the microstructure evolution of the alloy,the critical conditions for the dynamic recrystallization of Mg-5.8 Zn-0.5 Zr-1.0 Yb alloy were determined.It is found that the ratio of critical strain to peak strain of the test alloy is between 0.446 and 0.553,which is significantly lower than that of other alloys,indicating that the test alloy can be recrystallized with a relatively small degree of deformation.Based on the modified Avrami equation,the dynamic recrystallization kinetics model was established.It is found that with increasing deformation temperature and decreasing strain rate,the critical strain and the average DRXed grain size of the test alloy decrease,while the DRX volume fraction increases.The influence of deformation conditions on the DRX kinetics of Mg-5.8 Zn-0.5 Zr-1.0 Yb alloy was further analyzed by adopting the Z parameter:(1)When compressed at a relatively high Z value condition,the alloy exhibits a typical necklace structure due to a much higher nucleation rate and lower growth rate.Therefore,the smallest average size of DRXed grains is achieved despite the lowest DRX fraction;When compressed under a medium Z value condition,the mobility of grain boundary dislocations is improved due to the enhanced atomic difusion and the longer deformation time,resulting the growth rate of grains is comparable with the nucleation rate.Hence,the DRX fraction and the average size of DRXed grains are obviously increased;When the compression performed under a relatively low Z value condition,complete DRX is achieved and the nucleation rate is lower than the growth rate of grains.With increasing deformation temperature and decreasing strain rate,the DRXed grains obtain sufficient time and energy to grow up freely,resulting in the highest average grain size and the DRX fraction.(2)When deformed at with the same deformation degree,it is obvious that the fraction of DRX increases with decreasing Z value at a constant strain and will gradually approach the constant value of 1 with increasing strain,implying the alloy will be fully recrystallized undergoing sufficient deformation degree.When the deformation conditions are constant,the DRX rate increases slowly with the increase of the recrystallization degree to the maximum value and then decreases gradually,which presents a typical 'slow-rapid-slow' characteristic.Therefore,the Mg-5.8 Zn-0.5 Zr-1.0 Yb rare earth-magnesium alloy can obtain higher recrystallization degree and DRX rate at lower Z value.(3)In order to deeply analyze the evolution of dynamic recrystallization of Mg-5.8 Zn-0.5 Zr-1.0 Yb rare earth-magnesium alloy with deformation conditions and deformation degree,the variation of DRX fraction with strain and Z parameter can be plotted as an iso-fraction contour map of the parameter XDRX.The partial recrystallization stage can be subdivided into three stages according to the difficulty of recrystallization:initial onset sub-stage(0?10%XDRX),transformation sub-stage(10?90%XDRX)and delayed finishing sub-stage(90?100%XDRX).The alloy exhibits a high feasibility of DRX as well as an overall good hot workability under the studied deformation conditions.(4)At different deformation degrees,the DRX sensitive rates exhibit a similar varying tendency and achieve their minimum values when InZ?26.The sensitivity of the dynamic recrystallization degree of the alloy with the deformation condition with the increase of deformation,and the recrystallization degree is basically not affected by the deformation condition when the strain is greater than 0.6.Therefore,by comprehensive consideration of the robustness,the velocity,and the onset of DRX,it is recommended to perform the hot working at the deformation condition with the InZ value less than 24 for the studied alloy.(5)By comparing the key parameters of dynamic recrystallization kinetics to analyze the effect of Yb on the dynamic recrystallization behavior of magnesium alloy,it is found that the Yb addition can promote the onset of DRX but the progress should be retarded to some extent.The addition of Yb acts an important role in adjusting the DRX kinetics of the Mg-Zn-Zr alloy by decreasing SFE and increasing the density of fine precipitates.On the other hand,the suppression effect on the DRX kinetics may be related to the extensive existence of fine precipitates before and during deformation.With a high density of dispersed fine precipitates,dislocations induced by straining are pinned(Zener drag),which results in hindering dislocation rearranged to suppress the growth of DRX.The research on dynamic recrystallization behavior of rare earth magnesium alloy during hot deformation can provide theoretical basis for the development and subsequent research of rare earth magnesium alloy hot deformation process.