Twinning Behavior And Its Effect On Plastic Deformation Of Magnsium Alloys Under Different Loading Paths

In recent years,the introduction of ?twin interface? into metal structural materials has been paid much attention for its remarkable improvement in strength and toughness.{10-12} tension twinning is one of the most common activated twinning modes in magnesium and its alloys.Since its low critical shear stress,it is easy to form in the initial stage of plastic deformation,and thus affecting the subsequent mechanical behavior.On the other hands,the pole nature together with the existence of six different {10-12} twin variants results in a highly strain-path dependent twinning behavior,which will also vary with strain and strain rate.Therefore,the study of {10-12} twinning behavior under different loading paths and the effects on subsequent mechanical behavior and plastic deformation mechanism has important theoretical significance and pratical application value.In this thesis,the effects of loading paths,strains and strain rates on {10-12} twinning behavior,morphology and volume fraction in AZ31 alloy sheet were studied firstly by mechanical testing machine and EBSD technique et al.Then,the effects of preset {10-12} twins on plastic deformation behavior at ambient or elevated temperature and dynamical recrystallization mechanism under different loading paths were investigated.Meanwhile the effects of grain boundary related with grain sizes on plastic behavior and fracture mechanism of AZ31 alloy sheets at room temperature were further analyzed.Important results and conclusions are as follows:When AZ31 alloy sheet is compressed at room temperature,the {10-12} extension twin variant with the highest Schmid factor(SF)is preferentially activated and becomes the primary twin,while the twin variant with smaller SF is inhibited.With increasing of strain,the nucleation and growth of primary extension twins dominate and result in increasing of twin volume fraction.However,due to the combining of different twin lamellar at the later deformation stage,the twin boundary first increases and then decreases.On the other hand,small amount of the variant with smaller SF will be activated to intersect with primary twins.While the adjacent twin variants activated at the grain boundary for local stress concentration are related to the misorientation of the two adjacent grains.When the misorientation of adjacent grains is small,the adjacent twin variants are in parallel with the same variant.While the misorientation of the adjacent grains is large,the intersection angle between the two adjacent twin variants is less than about 20°.Increasing strain rate will promote the nucleation of {10-12} extension twins,inhibit its growth and increase stress concentration at grain boundaries,resulting in occurrence of ?chaining? extension twins through multiple grains.As AZ31 alloy sheet is compressed along RD or TD at room temperature,the strain hardening rate curves can be divided into four regions: I,II,III and IV.The corresponding dominant microscopic mechanisms of plastic deformation are {10-12} twin nucleation,twin growth,interaction of twin-dislocation and twin-twin and slip.For RD-RD recompression without changing loading path,the growth of pre-existing lamellar {10-12} twins dominates the initial plastic deformation at room temperature and significantly affects compression yielding strength.And the effect on working hardening behavior is mainly related to stage I and II.While,for RD-TD recompression with different loading path,interaction between the preset twin and the newly activated twin plays significant effects on I,III and IV stage,as well as compression yielding strength and compressive strength.The stress required for the growth of {10-12} lamellar twin is about 100 MPa per volume fraction percentage,while that required for growth of crossing-twin is about 160 MPa per volume fraction percentage.For the above two loading cases,the maximum effect of twins on the compressive strength occurs as the twin volume fraction is close to 50%.The thermoplastic deformation and dynamic recrystallization behavior of AZ31 magnesium alloy are significant influenced by preset extension twins at low compression temperature.Dynamic recrystallization mechanism of hot compression at 150 ? and 250 ? is mainly dominated by twinning induced recrystallization and continuous dynamic recrystallization.As the loading path is changed(RD-TD),the nucleation positions of recrystallized grains are mainly at the intersection of {10-12} twin with other extension twin variants and grain boundary.As the loading path is not changed(RD-RD),the nucleation and growth of recrystallization grain mainly occurs within the {10-11}-{10-12} secondary twins.Therefore,fine dynamic recrystallization grains prefer to be obtained at relatively lower temperature and slower strain rate when loading with different strain path,or can be obtained at higher temperature of 350 ? in the condition of changing loading path and increasing strain rate of predeformation.With the decrease of grain size,the dominate plastic deformation mechanism of tension along the RD is changed from {10-12} twinning into dislocation slip and grain boundary sliding.And the fracture mechanism is changed from cleavage fracture to microspore aggregation fracture at room temperature.When deformation is coordinated togheter by dislocation slip,grain boundary and twinning,the AZ31 alloy sheet with average grain size in middle presents good combining mechanical properties,which results in a mixed fracture mechanism with both micro-pore aggregation fracture and cleavage fracture.