Hot Workability And Microstructure Evolution Of Ytterbium-Containing Magnesium Alloy

Magnesium alloys are among the lightest structural metals and possess several advantageous properties including recyclability,high specific strength,and biocompatibility.These attributes render it a promising candidate for applications in fields such as aerospace,automotive engineering,biomedicine,and electronic communication.Nevertheless,its low strength and susceptibility to corrosion have hindered its widespread applications.It has been acknowledged that the rare earth elements alloying combined with appropriate hot deformation processing can effectively improve the mechanical properties.In this study,a high-performance Mg–6.0 Zn–2.0 Yb–0.5 Zr（wt.%,denoted as ZYb K620）alloy was prepared.The hot workability and related microstructure evolution at different deformation regimes were studied.The main conclusions include:The effects of deformation temperature（250–350°C）and strain rate(0.001–1 s^-1)on the flow stress were investigated based on a series of hot compression tests.The results indicated that the alloy exhibited distinct dynamic softening characteristics and that peak stress increases with a decrease in deformation temperature or an increase in strain rate.The constitutive equation was obtained by fitting the Arrhenius model.And the hot processing map indicated that the alloy generally exhibited good workability with the instability region shrinking with increasing strain.When the true strain reached 0.9,the instability region was confined to a narrow range of 250–265°C/0.1–1 s^-1,with small local voids and large cracks being the main forms of instability.The favorable processing window was predicted as 300–350°C/0.001–1 s^-1,which was further verified through microstructural characterization.At 250°C/1 s^-1 and 250°C/0.1 s^-1,a large number of twins with almost no recrystallized grains was observed;While at 350°C/0.001 s^-1,the homogenous equiaxed grains dominated the microstructure;And at other deformation regimes,compressed samples all displayed partially recrystallized microstructure.The comparative study of four deformation regimes with relatively high power dissipation values in the stable region showed that strain rate and deformation temperature played important roles in the dynamic recrystallization（DRX）mechanisms of ZYb K620hot compression.When compressed at lower Zener-Holloman parameter（Z）values,corresponding to higher deformation temperatures and lower strain rates,generally resulted in the operation of discontinuous dynamic recrystallization（DDRX）and a complete DRX process;while higher Z values often made the continuous dynamic recrystallization（CDRX）in control and thus led to a partial DRX microstructure.Due to the presence of high-density second phases,the particle-stimulated nucleation（PSN）contributed to the DRX process to a certain extent at all deformation conditions,while the twin-induced recrystallization mechanism（TDRX）was rarely observed in the investigated deformation regimes.In addition,different deformation regimes resulted in the change in the operative deformation mechanisms during hot compression.When deformed at low temperatures and strain rates,twinning and basal slip accommodated plastic strain predominantly,while at elevated deformation temperatures and strain rates,non-basal slips were greatly promoted and the activation of multiple slip systems controlled the plastic deformation.Nevertheless,it was worth mentioning that the root of the readily activated non-basal slip observed in this alloy essentially lay in the sufficient alloying of Yb.Furthermore,the activation energy of ZYb K620 was sensitive to deformation conditions and the workability improved with increasing strain rate and temperature.A lower average Q（the average activation energy）value as compared with that of ZYb K610 alloy indicated improved workability obtained at most deformation regimes,which should be derived from a higher activity of non-basal slips achieved.At last,the dislocation density change model,dynamic recrystallization nucleation model and recrystallization growth model are introduced to construct a cellular automata（CA）model that accurately reflects the dynamic recrystallization evolution law of the alloy.The DRX behavior with different deformation temperatures,strain rates,and initial grain sizes was predicted.The comparative analysis of simulation and experiment results indicated that the model can well reflect the DRX characteristics of ZYb K620 alloy at tested deformation conditions.The maximum errors of DRX volume fraction and average grain size were 7.08%and 10.09%respectively,presenting a desirable simulation accuracy.