Study On The Dynamic Recrystallization Behavior Of Magnesium Alloys Containing Characteristic Structures

Various types of characteristic structures in magnesium(Mg)alloys,including twins,long period stacking ordered(LPSO)structure and quasicrystal,play important roles in improving the mechanical behavior of Mg alloys.Influence of the above structures on dynamic recrystallization(DRX)behavior of Mg alloys need to be studied,as controlling the DRX behavior is significant for designing high-performance Mg alloys.In the present study,Mg alloys containing twins,LPSO structure and quasicrystal were prepared by casting,compression and heat treatments.Then,the casting alloys containing the above structures were compressed,indirect-extruded and quenched immediately to freeze the hot-deformed microstructure.After that,the hot-deformed microstructure was observed and analyzed applying with the electron backscattered diffraction(EBSD)and transmission electron microscopy(TEM)techniques.In particular,the grain orientation spread(GOS)value(VGOS)in EBSD technique was calculated by the software OIM to identify the DRX grains.The grain reference orientation deviation(GROD)value(VGROD)was calculated to estimate the dislocation density or residual strain within grains.Finally,the effects of twins,LPSO structure and quasicrystal on microstructure evolution including recrystallization nucleation and grain growth were studied through theoretical calculation and analysis.The main conclusions were summarized as follows:The influence of {1012}<1011>extension twins(hereinafter referred as extension twins)and {1011}<1012>contraction twins(hereinafter referred as contraction twins)on the DRX behavior of AZ31 alloy was studied and revealed.The results showed that extension twins,contraction twins,and {1011}-{1012} secondary twins(hereinafter referred as secondary twins)could be pre-generated in the AZ31 alloy by room temperature compression.AZ31 alloy samples pre-generated twins were hot-compressed.During hot deformation,the preferred sites for the DRX nucleation are twin boundaries of the contraction or secondary twins rather than the extension twins.Orientations of the DRX grains were affected by the pre-generated twins.In the AZ31 sample pre-generated extension twins,the texture of DRX grains retain the initial texture.In the AZ31 sample pre-generated contraction twins,the DRX grains nucleated inside contraction or secondary twins and exhibited new orientations,resulting in the texture weakening.The influence of LPSO/?-Mg interfaces on the nucleation of DRX behavior in Mg96.5Gd2.5Zn1(at.%)alloy was studied and revealed.DRX grains nucleated preferentially on the LPSO/?-Mg interfaces.Compared to the basal LPSO/?-Mg interfaces("basal"means the basal plane of block LPSO structure),non-basal LPSO/?-Mg interfaces are favorable nucleation sites for the DRX grains.It is due to the higher GROD value(VGROD)and lower required stored energy Es for the DRX nucleation on the non-basal LPSO/?-Mg interfaces.During the 350? indirect-extrusion,the orientations of the as-nucleated DRX grains were scattered and their texture was randomized.Simulation was applied using DEFORM 3D software.The influence of deformation temperature and die bearing on the DRX microstructure evolution in theMg96.5Gd2.5Zn1 alloy during the indirect-extrusion was revealed.The ratios of x-axis and y-axis stress(x,y axes are perpendicular to the extrusion direction)to z-axis(extrusion direction)stress(namely,the ?xx/?zz,?yy/?zz)caused by the specific design of extrusion mold requires great attention.The results indicated that the larger ?xx/?zz,?yy/?zz,the strong texture components of DRX grains were inclined to the direction perpendicular to extrusion direction.The DRX behavior of Mg95.8Zn3.6Gd0.6(at.%)alloy containing quasicrystal was also studied.Abundant twins generated during indirect-extrusion at 250? and 300?,indicating the twinning-induced dynamic recrystallization(TDRX).The microstructure of the alloys after extrusion are partially recrystallized.Few twins were observed during the indirect-extrusion at 350? and the microstructure was fully recrystallized after extrusion.At the initial stage of the indirect-extrusion at 250?,one side of I-phase easily cracked to form some faceted interfaces which are favorable sites for DRX nucleation,while the other side remained flat which was not beneficial to the nucleation of DRX..The newly DRX grains nucleating at the initial stage of the deformation at 250? inherit the orientations of the initial grains.The texture of the DRX grains in the as-extruded alloy is typical fiber texture.The DRX nucleation and precipitation of nanoscale quasicrystal occurred concomitantly.By comprehensively comparing the recrystallization behavior of the Mg alloys containing these twins,LPSO structure and quasicrystal,we found that the interfaces between the characteristic structures and ?-Mg matrix can all provide nucleation sites for the DRX grains.Furthermore,the orientations of the DRX grain are closely related to the interface characteristic.In detail,the interfaces between the characteristic structures and ?-Mg matrix can be divided into two categories.One type of the interfaces are represented by"contraction twin boundaries and non-basal LPSO/?-Mg interfaces".Their strong ability in hindering dislocation slipping results in mass<c+a>slipping in front of interfaces.As a result,the orientations of the DRX grains nucleating on this type of interfaces are scattered and the texture is randomized.The other type of the interfaces are represented by“extension twin boundaries and I-phase/?-Mg interfaces".This type of the interfaces cannot induce the initiation of multi-slip systems.Therefore,the textures of DRX gains in the Mg alloys containing extension twins and quasicrystal inherit the initial texture.Texture weakening is not obvious.The results provide theoretical guidance for the optimal design for the boundaries,interfaces and texture of Mg alloys.