Microstructures And Mechanical Behaviors Of Mg-Y(-Ce)-Zn-Mn Alloys

Mg alloys,the lightest,environment-friendly and rich-resources structural materials,have important values in scientific research and future engineering application.However,there are still problems such as low strength and inferior ductility of Mg alloys.Alloying and extruded processing are the simple and effective methods to resolve the above problems.The Mg-RE-Zn alloys have attracted much attention due to their variety of precipitated phases and excellent mechanical properties.It has been reported that the beneficial strengthening roles can be achieved from compositive additions of two or more kinds of rare earths,while the precipitated rules and mechanical behaviors of Mg-Y?-Ce?-Zn-Mn alloys with two different RE elements addition is still unclear.Besides,the relationship of strain rate with the mechanical behaviors of Mg-Y?-Ce?-Zn-Mn alloys is rarely reported.Therefore,study the microstructure and mechanical behavior of Mg-Y?-Ce?-Zn-Mn alloys and the effect of strain rate on the mechanical behavior of Mg-Y?-Ce?-Zn-Mn system alloys is of great significance for the comprehensive understand of the plastic deformation behaviors and related mechanisms of Mg-Y?-Ce?-Zn-Mn alloys,and also provides theoretical basis for further design of high performance Mg-Y?-Ce?-Zn-Mn alloys.In this paper,the as-cast and extruded Mg-2.5Y-1Ce-0.5Mn-xZn alloys and the extruded Mg-9Y-3Zn-1Mn alloy were prepared by the semi-continuous casting technology and extrusion technology.Microstructures of the samples before and after tensile deformation were characterized by the optical microscope,scanning electron microscope and transmission electron microscope.The mechanical properties of the alloys were tested by tensile testing system and nanoindentation.The microstructure and mechanical properties of Mg-Y?-Ce?-Mn-Zn alloys and the effect of strain rate on the mechanical behaviors of Mg-Y?-Ce?-Zn-Mn alloys have been studied in order to explore the relative deformation mechanisms.The main contents and results of this paper are as follows:1.The effect of Zn addition on the microstructure and mechanical properties of as-cast Mg-2.5Y-1Ce-0.5Mn alloy has been studied.The result demonstrated that when the Zn addition is 1 wt.%,3 wt.%and 5 wt.%,the ternary phases are LPSO phase,LPSO phase and W phase,W phase and T phase,respectively.The precipitated rule is mainly related to the Zn/Y ratios and the eutectic reaction temperatures.The yield strength of the as-cast alloy increases linearly with increasing Zn content,while the tensile strength and elongation show nonlinear variations.The variations of the mechanical properties are related to the volume fraction,the type and the distribution of different ternary phases.2.The effect of Zn addition on the microstructure and mechanical properties of extruded Mg-2.5Y-1Ce-0.5Mn alloy has been studied.The result demonstrated that the extruded alloys almost compose of equiaxed grains and uniformly dispersed second phases.With increasing Zn content,the amount of second phases gradually increases.After adding 5wt.%Zn,the grains are significantly refined and the alloy shows obvious basal texture.Tensile strengths increase nonlinearly with increasing Zn content while the elongation linearly decreases.The variation of tensile properties is mainly associated with the amounts of second phases,intensity of basal texture,amounts of solute atoms and grain refinement.3.The effect of strain rate on the tensile properties of extruded Mg-9Y-3Zn-1Mn alloy has been studied at room temperature.The result demonstrated that with decreasing strain rate from1?10^-1s^-1to1?10^-6s^-1,the yield and tensile strengths of the alloy show slightly decrease of only 13%and 7%,respectively,while the elongation of the alloy shows an obvious increase of 176%.The dependence of tensile strengths and elongation with the strain rate is mainly associated with the special deformation behaviors of LPSO phase and the participation of grain boundaries?GBs?behaviors.4.The effect of strain rate and temperature on the tensile properties of extruded Mg-5Zn-2.5Y-1Ce-0.5Mn alloy has been studied.The result demonstrated that the yield and ultimate tensile strengths are significantly decreased with increasing temperature and decreasing strain rate.The elongation at 250?increases from 55%to 108%,and at 350?, the elongation increases from 132%to 256%.Based on analysis of strain rate sensitivity, activation energy and morphologies of deformation surfaces,the pronounced increase of elongation at 250?and 350?with decreasing strain rate should be resulted from the dynamic recrystallization behavior and the formation of intragranular fibers that are assisted by the dislocation climb controlled by lattice diffusion.5.The effect of strain rate on the tensile properties of extruded Mg-2.5Y-1Ce-0.5Mn alloy annealing at different temperatures has been studied.The result demonstrated that after annealing at 500?,the yield strength decreased obviously,the tensile strengths show unobvious variety with the strain rate and the discontinuous yielding behavior is disappeared compared with unannealed alloy.Based on the analysis of strain rate sensitivity and the morphologies of deformation surfaces,the decrease of yield strength and the insensitivity of tensile properties with strain rate arise from the growth of grains and the solution of Mg₁₂Ce phase.The disappearance of yielding behavior should be resulted from the decrease of dislocations density after annealing.