Effect Of Alloying On Critical Resolved Shear Stress And Mechanical Behavior Of Magnesium Alloys

Magnesium(Mg) alloys have received increasing attentions for their lightweight, high specific strength, good electromagnetic shielding and damping vibration attenuation properties. However, conventional wrought Mg alloys, especially Mg alloys sheets have poor formability at room temperature, for the high critical resolved shear stress(CRSS) ratio of the non-basal slip to the basal slip leads to lack of independent slip systems in Mg alloys during room temperature deformation. Therefore, the use of Mg alloys is still limited. Appropriate alloying can decrease the CRSS ratio of the non-basal slip to the basal slip, and then improve the formability of wrought Mg alloys.In this paper, the effect of alloying on theoretical critical resolved shear strength(τmax) of magnesium crystal is studied through first-principles calculations, further to prepare corresponding Mg alloys. The microstructure and mechanical properties of different magnesium alloys during different deformation process are investigated using the optical microscope(OM), scanning electron microscope(SEM), X-Ray diffraction(XRD), electron back scattered diffraction(EBSD) and tensile properties testing.The main results of this dissertation are listed as follows:① The solute atoms Li, Sn and Y prefer to locate at the plane of }0211{. The decreased magnitude per atom of the τmax ratio of the non-basal slip to the basal slip for alloying elements is as follows: Sn>Li>Y. While the decreased magnitude per weight of the non-basal to basal τmax ratio for elements is Li>Sn>Y.② For the as-extruded Mg-Li binary sheets, with the increase of Li content, the grain orientation will change, which is the basal plane orientation of Mg-(1-2)Li alloys transforming to the non-basal plane orientation of Mg-3Li alloy. The ductility, work hardening index(n-value) and strength of Mg-3Li alloy are all better than the other two alloys, which can be attributed to the finer grains of Mg-3Li alloy and the grains with non-basal orientations in Mg-3Li alloy to coordinate the deformation. Besides, the anisotropy of the as-extruded Mg-3Li sheet is further improved after cold-rolling at the rolling reduction of 30%.③ Different deformation modes are shown by different Li contents in the as-extruded AZ31 sheets at different reduction. For LAZ131 alloy, basal slip and twinning are the main deformation mode in the entire cold-rolling process at all reductions. For LAZ331 alloy, prismatic slip and }2211{ pyramidal slip occur in the 5% cold rolling, }2211{ pyramidal slip and }2101{ tensile twinning in the 10% cold rolling, }2101{ tensile twinning and }1101{ compress twinning in the 15% cold rolling, and }1101{ compress twinning with }1101{- }2101{ secondary twinning in the 20% cold rolling. For LAZ531 alloy, }0211{ prismatic slip and pyramidal slip occur in the 5% cold rolling, pyramidal slip and }2101{ tensile twinning in the 10% and 15% cold rolling, and }2101{ tensile twinning with }1101{ compress twinning in the 20% cold rolling.④ The solute of tin(Sn) can improve the room formability of magnesium. In Mg-Sn solid solution alloys, Mg-2.5 wt.%Sn shows the best room formability with the cold-rolling reduction of 22.6%. In the as-extruded Mg-Sn alloys, the tensile strength, ductility and n-value are improved by increasing the Sn contents.⑤ After adding 0.3wt.% Y into Mg-0.5Sn alloy, the grains of the extruded sheet are refined by about 77%, then the strength and ductility along three directions improved significantly. The ductility values along ED, 45 o direction and TD increase to 30.3%, 28.1%, 28%, as well as the tensile strength to 288 MPa, 286 MPa, 302 MPa, respectively. The excellent comprehensive mechanical properties of the extruded Mg-0.5Sn-0.3Y sheet can be attributed to its finer grains(about 4μm) and the decreased CRSS ratio of the non-basal slip to the basal slip due to the solution of Sn and Y.