Research On High Temperature Deformation Mechanism Of ZK60 Alloy

Wrought alloy has a broad application prospect because of its excellent mechanical properties with few defects.ZK60 is one of the most widely used deformed magnesium alloys.However,low plasticity and poor hot forming properties have limited its large–scale application.Superplastic deformation is a good method to solve this problem.Near net shape and complex shape parts can be obtained by superplastic forming.In this work,a ZK60 alloy with original average grain size of 7.88μm obtained by extrusion rolling was studied,and tensile–to–failure experiments,tensile–to–fixed–strain experiments and strain–rate–change experiments were conducted.The tensile specimens were observed and characterized by metallographic microscopy（OM）,X–ray diffraction（XRD）,and scanning electron microscopy（SEM）.The high–temperature deformation behaviors,microstructure evolution,and superplastic deformation mechanisms of the ZK60 alloy were studied.The results show that the ZK60 alloy exhibits excellent plasticity over the applied entire range of temperature and strain rate.Under deformation conditions of（?）=5×10^-2～1×10^-3 s^-1,the alloy shows superplasticity（with an elongation of over 100%）,with the highest fracture elongation rate of 330.71%.Based on the sinusoidal hyperbolic equation,the expression for the flow stress of the ZK60 alloy was obtained,which can better predict the peak stress of the alloy.Based on the dynamic material model,a thermal processing map for the ZK60 alloy was constructed.The optimal thermal processing parameters for the alloy are as follows:temperature range of 400～450°C and strain rate range of 5×10^-3 s^-1～1×10^-3 s^-1.The fracture mode of the alloy is ductile fracture with microvoid coalescence.The average grain size of the alloy increases slightly with the increase of temperature,indicating that the microstructure of the alloy is relatively stable.The average grain sizes of the alloy at all temperatures are less than 10μm,and the stable fine–grained micstructure provides good plasticity for the alloy.The Mg Zn phase distributed at the grain boundaries and the solute Zn gathered at the grain boundaries together hinder the movement of the grain boundaries,thus making the grains of the alloy more stable.The superplastic deformation process of the alloy is controlled by two deformation mechanisms:dislocation climb creep controlled by lattice diffusion（D_L–DCC）and grain boundary sliding controlled by grain boundary diffusion(D_gb–GBS).At lower temperatures,the deformation of the alloy is primarily governed by the D_L–DCC mechanism.As the temperature increases,the reduction in dislocation density and enhanced diffusion lead to a gradual decrease in the strain rate range controlled by the D_L–DCC mechanism.At higher temperatures,deformation is dominated by the D_gb–GBS mechanism.Based on these two mechanisms,a superplastic flow equation has been developed for ZK60 alloy,which effectively describes its superplastic behavior.