Researches On Microstructure And Mechanical Properties Of Magnesium Alloys Processed By Severe Plastic Deformation

A new Mg-10.73Li-4.49Al-0.52Y magnesium alloy was prepared by melting method. Inorder to improve the plasticity and thermo stability, meanwhile to avoid the decrease ofstrength, the magnesium alloy was designed as+β two-phase with rare earth element Yadded. Equal channel angular pressing (ECAP) was conducted on the alloy at573K from1pass to4passes using route Bc, A and C.-phase and β-phase are both significantly refinedafter deformation by three routes. After1pass deformation, the microstructures of route Bcare alternated with equiaxed and lathlike, while the microstructures of routes A and C areflaky and equiaxed respectively. Route Bc obtains the best integrated mechanical properties.The texture of {110} crystal plane for β-phase reveals that different changes exist among threeroutes as ECAP passes increase, but texture of all routes become softened, therefore thetensile strength decreases correspondingly when the texture softening is more effective thangrain refining.The alloy was processed by ECAP to6passes at573K by route Bc. The microstructuresindicate that both-phase and β-phase are elongated to be streamline at an angle of about30~45degrees to extrusion direction after ECAP, and the initial large and disperse phases ofAl2Y are refined and homogenized as the extrusion pass increases. After1pass, this alloyexhibits strongest texture on main crystal {110}, the texture intensity declines compared with1pass deformation after3and6passes, and their texture moves to periphery. After3passes,tensile strength increases to the maximum of166.4MPa from137.5MPa of as-cast. Thefracture mode of the six-pass ECAPed alloy shows typical ductile fracture and more dimples,and its elongation is up to83%.Superplasticity analysis was conducted at the temperature of523~663K on ECAPedMg-Li alloy with the strain rate of2×10~-4-5×10~-2s~-1. The alloy exhibits a superplasticbehavior with a maximum elongation of306.6%at623K with an initial rate of1.5×10~-2s~-1.The maximum value of the strain rate sensitivity under623K and1.5×10-3~1.5×10~-2s~-1is0.36, and the activation energy is121.6~160.0kJ/mol. Li loss appears in β-phase, there is nosharp interface after tensile test at the best superplastic condition of623K and1.5×10-2s-1.The dominant deformation mechanism is grain boundary sliding controlled by grain boundarydiffusion at523~573K, whereas, it was grain boundary sliding controlled by lattice diffusionat573~623K. When the temperature increases to663K, dislocation needs larger activationenergy and creep may play a critical role of maintaining the superplastic deformation process.ZK60magnesium alloy was deformed by T-shape channel pressing (TCP) from1pass to 4passes at673K using route A and route Bc. The experimental results show that the biggestplastic deformation is located at the bottom of sample, while necklace structure was foundafter2passes deformation. After4passes, the average grain sizes of route A and Bc is refinedfrom88.5μm of as-cast to2.4μm and4.6μm respectively. Nevertheless, the microstructureis more homogeneous for route Bc. Moreover, at the same pass the yield strength for route Ais higher than that for route Bc, but tensile strength and ductility are lower than the latter. Inaddition, tensile strength and yield strength at the bottom are higher than those of the top.