Effect Of Thermomechanical Treatment And Torsion Deformation On Microstructure And Mechanical Properties Of Magnesium Alloy

Magnesium alloys are lightweight structural materials with high specific strength, high stiffness and easy cycle use properties, etc. Mg alloys have been increasingly used in automotive, aeronautical and electronic devices as parts and structure. However, Mg alloys have the hexagonal close packed structure, limited slip systems and poor formability at room temperature. Furthermore, twinning can easy arise in Mg alloy during plastic deformation, which lead to low yield strength and tension-compression yield asymmetry. The application of Mg alloys was restricted due to their shortcomings.Recently, the main research directions of magnesium alloys are preparation of high strength magnesium alloy, improvement the tension-compression yield asymmetry and improvement the plastic deformation at room temperature, etc. The materials used in present study were the most common-used commercial AZ31 and ZK60 Mg alloys. A main research content of this paper is to study the effect of thermomechanical treatment on microstructure and mechanical property in Mg alloys. Another main research content of this paper is to study the effect of torsion deformation on microstructure and mechanical property in Mg alloys. To characterize the microstructure of the samples by optical microscopy（OM）, X-ray diffraction（XRD）, electron backscatter diffraction（EBSD）, scanning electron microscopy（SEM） and transmission electron microscopy（TEM）. The mechanisms are analyzed. The main results are as follows:（1） Low temperature annealing treatment（170℃） can increase the compressive yield strength of pre-twinned AZ31 Mg alloy. Segregation of solute atoms at twin boundary can pin the twin boundary migration which leading to enhanced activation stress for twinning growth. The compressive yield strength of the annealed pre-twinned AZ31 Mg alloy thus would increase during re-compression.（2） Aging treatment（180 ℃） can improve the tension-compression yield asymmetry of the pre-twinned ZK60 Mg alloy. Three morphologies of precipitates were observed in the {1012} tension twin, such as（0001）_twin plates, [11-20]_twin laths and [0001]_twin rods, which composed by Mg₄Zn₇ or MgZn₂. The precipitates are different between matrix and {10 1 2} tension twin of the pre-twinned ZK60 Mg alloy. Furthermore, segregation of solute atoms at twin boundary can pin the twin boundary migration. Both of the factors affect the increments of yield strength between tension and compression, which were dominant by de-twinning deformation and twinning growth respectively.（3） Free-end torsion deformation can effectively improve the tension-compression yield asymmetry of the extruded AZ31 Mg alloy. The compressive yield strength of the extruded AZ31 Mg alloy increases with the increase of shear strain, meanwhile the tensile yield strength of the extruded AZ31 Mg alloy has little change with the increase of shear strain. When the shear strain is low（γ<0.35）, the torsion deformation is dominant by dislocation slip. When the shear strain is high（γ>0.52）, the torsion deformation is in addition to the dislocation slip, a large number of {1012} tension twins can be generated.（4） The velocity of torsion deformation has an important influence on the rheological curve of extruded AZ31 magnesium alloy. The faster the velocity of torsion deformation is, the higher the rheological curve is. The torsion deformation can produce gradient microstructure on the cross section of the sample. The torsion deformation can improve the hardness of the sample. Gradient strain of torsion deformation is the main reason why torsion deformed samples contain gradient microstructure. The dislocation density, hardness and area fraction of twinning gradually increased along the radius direction on the cross section of the specimens. The increase of hardness is mainly attributed to the dislocation strengthening mechanism, while the effect of texture strengthening and grain refining strengthening is very small.（5） Hot torsion deformation can also make the extruded AZ31 Mg alloy specimen produce gradient microstructure on the cross section. The dynamic recrystallization grains produced at the edge on the cross section are much more than that of the dynamic recrystallization grains produced at the heart. With the increase of deformation temperature and shear stress, the area fraction of dynamic recrystallization grains also increased gradually. The increase of compressive yield strength of hot torsion deformed specimen is mainly caused by grain refining strengthening（dynamic recrystallization）.