Research On Microstructure Evolution And Mechanical Properties Of Mg-6Al-1Zn-0.9Sn Magnesium Alloy During Deformation

Magnesium and its alloys possess a good deal of merits such as high specific stiffness and strength,good vibration absorption and damping performance,strong electromagnetic shielding,desirable recyclability,and so on.Accordingly,they have immense potential application in the fields of vehicle traffic,medical instruments,aerospace industry,national defense and 3C products.However,the hexagonal close-packed crystal structure of magnesium alloys results in poor plastic deformation capability at room temperature.What's more serious,the mechanical properties can be improved only within a very limited scale by conventional plastic processing technologies.These are the main factors that hindered the extensive industrial application of magnesium alloys.Focusing on the problems mentioned above,we carried out research on microstructure evolution and mechanical properties of Mg-6Al-1Zn-0.9Sn?ATZ611?magnesium alloy during deformation.In this work,the effect of equal channel angle pressing?ECAP?process parameters on microstructure and mechanical properties was discussed and then the reason for the low strength and high ductility of the ECAPed magnesium alloy was analyzed.Moreover,we developed the process of"equal channel angle pressing combined with multi-pass rolling"and evaluated the strengthening and toughening potential of different types of grain structure.Consequently,the mechanism that governed plastic deformation in bimodal grain size structured magnesium alloys was revealed and the effect of the texture of the coarse grains on mechanical properties was explained.The main conclusions are as follows:?1?The ECAP temperature has a profound effect on the microstructure evolution of magnesium alloys.The grains were significantly refined and the precipitated particles were fully crushed and spheroidized at 250?ECAP temperature.With the formation of uniform fine-grain microstructure,the c-axis of the magnesium lattice gradually turned to the direction at an angle of 135 degrees from the horizontal channel.By contrast,ECAP at300?brought about undesirable grain refinement and the secondary phase was unable to get full precipitation and dispersion.Although the c-axis of the grains also showed a tendency of turning perpendicular to the maximum shear stress,the texture was relatively weak.?2?After four passes deformation to gain refined grains,the ECAP process was extended to eight passes at original ECAP temperatures.Eventually,uneven grain-growth occurred in the sample ECAPed at 250?,where the coarsened grains were featured with strong basal plane texture.While the mircostructure of the 300?ECAPed sample was uniformly coarsened to present a simple coarse-grain system.Further reducing the ECAP temperature to 200?,the greatest grain refinement was obtained.The average grain size and the mean diameter of precipitated particles were¹.1?m and⁴00nm,respectively.Meanwhile,the crystallographic orientation of the matrix grains aggregated to form a strong 135°texture.?3?The room temperature tensile properties of the ECAPed magnesium alloys deviate entirely from the Hall-Petch relationship.At the same ECAP temperature,the strength decreases and the plasticity increases with the decreased grain size.The reason is that texture plays a primarily crucial role in mechanical properties.The refined grains are largely turned toward the direction of high schmid factor,hence the critical stress of the base plane?a?slip system is significantly reduced,which results in the decreased yield strength;Grain refinement promotes the activitation of basal slip as well as non-basal slip,which accounts for the improved ductility.?4?With the increase of ECAP pass number,the strength at elevated temperatures of the 250?ECAPed samples decreases first and then increases,and the minimum value was obtained after the fourth ECAP pass.Due to the uniform grain size and dispersed precipitated particles,the 250?/4passes sample achieved an ultimate elongation of 500%under a condition of 250?/1.0×10^-3s^-1,which represents for a good plastic deformation capacity.The activation energy Q is calculated to be equal to 76kJ/mol,indicating that the high-temperature deformation is dominated by the grain boundary diffusion.?5?Strength and toughness of the ECAPed magnesium alloy can be enhanced effectively by means of multi-pass rolling.The sample fabricated by eight passes ECAP at250?got dramatic strength improvement by rolling and maintained its deformability to a large extent.The bimodal grain size microstructure reveals a better potential to achieve high comprehensive mechanical properties than simple fine-grain or coarse-grain microstructure.After a rolling process with a true strain of 15%,the bimodal microstructured ATZ611 magnesium alloy has gained a simultaneous high yield/ultimate strength and elongation,i.e.,³04MPa,⁴03MPa and¹6%,respectively.?6?In the rolling process of a bimodal grain size structure,the fine-grain region can effectively pin dislocation and take the advantage of work-hardening,while the coarse grains mainly produce twinning to coordinate the strain and play a role of storing dislocation in further deformation.In a bimodal microstructured magnesium alloy where coarse grains have strong basal texture in hard orientation yet fine grains locate at soft orientation,it is more conducive to play the role of coarse grains in maintaining high plasticity at the later stage of deformation,thus achieve desirable mechanical properties after rolling.