Dynamic Precipitation,dynamic Recrystallization Behavior And Texture Evolution Of The Mg-5Zn-X(Mn?Ca?Sr)alloys

The Mg-Zn alloy system,with excellent heat treatment strengthening ability,is one of the most widely used Mg alloys.However,the Mg-Zn alloy system with high Zn content has a wide solidification temperature ran ge,and the ingots are prone to microporosity.Therefore,it is necessary to add other alloying elements to inhibit the hot cracking tendency and refine the as-cast structure of the Mg-Zn alloys,so as to improve its formability,damping and biomedical properties.The Mg-Zn-Mn alloy system has the advantages of age hardening,excellent comprehensive mechanical properties and damping capacities,which is an important research field in the wrought Mg-Zn alloys;and the Mg-Zn-Ca alloy system is one of the important biomedical materials.In addition,the mechanical properties at high temperature,plasticity and corrosion resistance of the Mg-Zn alloys can be improved simultaneously by adding an appropriate amount of Sr element.High strain rate rolling(HSRR)can enhance the dynamic recrystallization(DRX)behavior and promote the dynamic precipitation of nano-scale phases in the Mg alloys.Combining HSRR with alloying is expected to prepare high strength and high plasticity fine-grained Mg-Zn alloy sheets.However,the dynamic precipitation mechanism of nano-scale phases,the interaction between nano-scale phases and DRX,the influence of alloying elements and the texture characteristics of the Mg-Zn alloy sheets during HSRR are still unclear.In order to solve the above problems,the hot compression simulation experiment and HSRR process were used to enhance the DRX behavior and adjust the characteristics of dynamic precipitates(type,size,morphology,volume fraction,coherent relationship and orientation relationship with matrix)in the Mg-5Zn-X(Mn,Ca,Sr)alloys.The DRX mechanism,texture evolution and mechanical properties during high strain rate deformation were also discussed.The main conclusions are as follows:(1)On the basis of hot compression experiments at 300?,the effects of strain rate(?)and strain(?)on hot deformation behavior,DRX behavior and precipitate characteristics of the Mg-5Zn-0.6Mn alloys were investigated.The results show that the peak strain(?_p)and DRX critical strain(?_c)decrease with increasing the strain rate.Grain boundary bowing is the main DRX mechanism at the strain rate of 1 s^-1,while the twin-aided DRX mechanism is dominate at the higher strain rates(??10 s^-1).The dynamic precipitation occurs before DRX.With the identical strain,(0001)?plates(?₂?)are dominant at the strain rate of 1 s^-1,while[0001]_?rods(?₁?),(0001)_?plates(?₂?)and spherical precipitates(?₂?)occur and?₁?phase is dominant at the higher strain rate(??10 s^-1)in the initial stage of deformation(?=0.11).?₂?phase becomes the main precipitate in the later deformation stage(?=1.61)at each strain rate.At the identical strain rate(?=10 s^-1),the alloy is featured with a small amount of[0001]_?rods(?₁?),(0001)_?plates(?₂?)and spherical precipitates(?₂?)at the initial stage of deformation(?=0.05).Increasing the strain can't change the types of the precipitates,but leads to the reduction of?₁?phases and the increase of?₂?phases.All the precipitates become spheroid.Compared with the traditional solution-aging process,the plastic deformation accelerates the dynamic precipitation process and promotes the transformation of?₁?phases to?₂?phases.(2)The effects of rolling temperature and strain rate on the texture chara cteristics and mechanical properties of the Mg-5Zn-0.6Mn alloys were discussed.As for rolling at 300?,the maximum intensity values of the(0002)texture in the alloy sheets decrease from 2.74 to 2.09 with the strain rate increas ing from 10 s^-1 to 25 s^-1.The reason for texture weakening is that the tensile twins and the DRX volume fraction increase,since the orientation of DRX grains is much more random than that of deformed grains and the tensile twins bring about the grain rotation.At the identical rolling strain rate(?=10 s^-1),the(0002)texture intensity in Mg-5Zn-0.6Mn alloy sheets decreases firstly and then increases with the rolling temperature increases from 250?to 350?.The weakest intensity obtained at 300?is attributed to the relative high DRX volume fraction,and the hinder effect of the fine high-density precipitates on the rotation of DRX grains.(3)The effects of alloying elements on the precipitate characteristics,DRX behavior,texture intensity and mechanical properties of the Mg-5Zn alloy with the addition of Mn,Ca and Sr elements during HSRR were studied.The results show that Ca improves the strength and Sr enhances the plasticity of the Mg-5Zn alloys.The mechanical properties of the alloy with 0.4Ca+0.2Sr are better t han those with the sole addition of Ca or Sr,but still lower than those with Mn.The reason is that the DRX grain refining effect of Mn is stronger,and the Mg-5Zn alloy with Mn has smaller precipitate size,higher precipitate density,stronger basal text ure and higher dislocation density.In this paper,the dynamic precipitation,DRX behavior,texture characteristics and mechanical properties of the Mg-5Zn-X(Mn,Ca,Sr)alloys during high strain rate deformation were clarified,which provides experimental evidences and scientific bases for the development of high strength and high plasticity Mg-5Zn alloy sheets based on controlling the microstructure and properties.