Detwinning behavior of an extruded AZ31 magnesium alloy during uniaxial compression Abstract:Detwinning behavior of an extruded AZ31 magnesium alloy during compression deformation was investigated by means of metallography and X- ray diffraction (XRD)，and a detwinning mechanism was suggested．The results show that the fraction of twins increases with increasing strain when the strain is low，but decreases if the strain is higher than about 4%．It indicates that detwinning occurs in the process．The grain rotation theory can be used to explain this peculiar behavior．The rotation of matrix at higher strain results in the change of stress state of {1012} tension twins activated at lower strain，and the twinned portions are capable of re- twinning，which causes the re- orientation of the twinned- portions．Key words :magnesium alloy; twinning; detwinning; compressive deformation Due to the light weight, high specific strength, high specific stiffness, good electromagnetic shielding, easy to recycle and other excellent properties, magnesium and its alloys has broad application prospects in the aerospace, machinery manufacturing, known as the "21st century green project material ", attracted more and more attention. The crystal structure of magnesium is hexagonal close-packed (hcp), it’s axial ratio c/a is 1.624, very close to the theoretical value of 1.633. At room temperature, magnesium alloy is most likely to start the basal plane slip, the cylinder and cone plane slip is difficult to start because of its high CRSS. However，at room temperature，the basal plane slip can only provide 2 independent slip systems，can’t meet with the V on Mises rule for the coordination of plastic deformation requirements（The polycrystal need 5 independent slip systems to start uniform plastic deformation ）。

And at room temperature，the activation of non basal plane slip stress is less than the stress required to induce twinning, thus ,In the magnesium alloy, twinning is an important deformation coordination mechanism ,and plays an important role in the process of plastic deformation.In magnesium and its alloys, the most common twinning is {10 12} tensile twinning and {10 11} compression twinning。

For the metal of HCP structure, Usually when the axial ratio is less than槡3（The axial ratio of magnesium c/a is 1.624），under the action of parallel to the basal stress or perpendicular to the basal surface tensile stress ,it’s propitious to activate the {10 12} extension twinning; only in the perpendicular to the base surface compressive stress or parallel to the base the tensile stress {10 11} compression twin can occur, the corresponding angle was 86 degrees and 56 degrees .In the magnesium alloy ,the critical shear stress required to start {10 12} tensile twinning is about 2 ~ 2.8 MPa, but the critical shear stress required to start the {10 11} compression twin is about 76 ~ 153 MPa, so the tensile twins are more prone to produce than compression twins.{10 12} twinning usually occurs in the initial stage of deformation, tend to c axis parallel to the compression direction of the grain, the formation of strong basal texture, lead to the anisotropy of mechanical property,but {10 11} twinning usually occurs in the end stage of deformation.In the existing research results , a large number of magnesium alloy detwinning phenomenon is observed. The literature (9) found that during the pretension,compression twinning occurred 86 degrees with respect to the rotation matrix, this orientation is propitious to produce detwinning during the follow-up tensile deformation. In addition, in the opposite direction of the load under cyclic stress, were also observed detwinning phenomenon. Huang Hongtao studied the predeformation on subsequent changes to the compressionbehavior, found along the TD direction (c axis perpendicular to thesubstrate) produced by {1012} the following ND direction tensile twinning precompression deformation ( c axis direction parallel to the substrate ) compressive deformation to detwinning behavior. However, these studies are changing the external loading direction cause the detwinning phenomena happened, for the unidirectional compression in the evolution process of twin form and will return the twin phenomenon study reported less. Unidirectional compression in the evolution process of the twin form and in the process of detwinning phenomena in this paper, based on the crystal rotation mechanism to explain the special phenomenon.1、Materials and methodsThe experimental material is extruded AZ31 magnesium alloy bar, its average grain size is 60 um. In the extrusion bar, cut 6 mm * 9 mm cylindrical compression specimen along the extrusion direction, making the compression direction consistent with the extrusion oriention。