Research On The Development Of Microstructure,texture And Mechanical Behavior Of Magnesium Alloys During The Deformation

Magnesium alloys are one of the lightest metallic structural materials, and they are very attractive in various applications in automotive,communication,electronics,and aerospace industries. Due to the hexagonal close Packed(HCP) crystal structure, they have limited deformation system, such as basal slipping and pyramidal twining under room temperature, and have poor formability. So, in present works, the changing patterns on microstructures, textures and mechanic properties of pure magnesium and its alloy, which treated by hot extruded, equal channel angular extruded(ECAE) or textured initially, under tinseling, compressing and dynamic impacting are studied. The influences of rare earth element, texture, deformation mode, strain rate, critical resolved shearing stress(CRSS) and schmid factor(SF) on the deformation mechanics of pure magnesium and its alloy are studied in details. Furthermore, the mechanics of pure magnesium and its alloy are discussed in this work too,and the main conclusions are following:The microstructure and texture of AZ31 magnesium alloy after ECAEed under different conditions are analyzed and the results show that the refining effect enhanced with increasing the deformation amount or the extrusion temperature. The extrusion rates have little effect on the size of grains after deformed. In addition, increasing the deformation amount will make the {0002} basal texture be concentrated and reinforced as well as extrusion rate do, but the extrusion temperature has the converse effect. The refining mechanism of initial coarse grains was explained based on the interacted among the dislocations.When impacted along normal direction(ND), extrusion direction(ED) and transaction direction(TD) for AZ31 magnesium alloy annealed by 250 ℃ /2.5h, the microstructure are refined and characterized with large and small grains mixed. However, the different impact direction will results the different amounts and orientations of twinning grain, and the anisotropy of dynamic behavior appears. The texture of the samples results in the anisotropy of the dynamic strain-rate sensitivity in the impact deformation. The alloy shows positive strain-rate sensitivity in ND impact and negative in TD impact due to the different activated deformation modes and the different responses of the various modes to the strain-rate change. Orientation spreading results in differences in the absolute values of the strain-rate sensitivity. Increasing the strain rate, the anisotropy of dynamic behavior and the differences in the absolute values of strain-rate will increase more obvious.When impacted along ED and TD, though the basal slip can not be activated, the tension twins were activated to coordinate the deformation because of their low CRSS value for most grains. So theyielding stress was low. Furthermore, the orientation of new grain after twinning rotated to a favorable condition for pyramidal slips and compress twins to deeper deformation. Then the fracture mode will change from typical intra-granular brittle type to a mixture of cleavage and dimple.However, when impacted along ND, both the basal slips and tension twins were too difficult to be activated. The deformation depended only on the pyramidal slips and compress twins which having high CRSS values and the yielding stress was very high. In this condition, the fracture mode of magnesium alloys was brittle fracture mode.The Y and Er can retard the dynamic re-crystalliztion of pure magnesium during hot extrusion and annealing, and texture component {0001}//ED increased. After annealing, the equiaxed grains were obtained. Furthermore, the textures changed from {0001}//ED to RE-texture with increasing the Y or Er content.The addition of Y and Er will influence obviously on the mechanical behavior and property of magnesium alloy. Increasing the Y and Er content, the yielding stress of Mg-Y and Mg-ER alloys having similar size of equiaxed grains will reduce and the plastic property will increase. but with the different concentration present quite different tensile and compressive mechanical properties. The elongation and reduction of Mg-6Er sample can reach to 29.4% and 42.3%, respectively, much higher than previously presented elongation and reduction of pure magnesium. The mechanical properties change with Y and Er addition is resulted from the texture development and c-axis reduction which are responsible for the diversification of plastic deformation modes.The twinning mechanism was very important to the mechanical behavior and property of magnesium alloy. According to calculation on schmid factor and analyzing on CRSS for all twinning system of magnesium alloy during the tensile and compress along the axial direction, the influences of textures on the forming of twins and their variants are studied. The results show that the tension twins can be activated in a large range of orientation. However, the orientation range of compression twins can be activated was very narrow, and were verified by the results of impact tests and quasi-static compress tests. In addition, the compression and tension twins can be found in the large grains and only tension twins exists in the small grains. But the orientations of the matrix grains surrounding both compression and tension twinning grains are very approach. The calculation results also indicated that the Schmid factor was the important criteria to verdict the twins and their variants types but not the only criteria because the strain coordination of the grain around twinning grains may be very important too.