Study On Static And Dynamic Mechanical Behavior Of Mg-Al-Y Alloys

As one of the earliest magnesium alloys used in foundry industry,Mg-Al alloys have high specific strength,specific stiffness and good casting performance,which are widely used in near-net shape structural parts with complex shapes,such as engine housing,electronic products and other components.However,the poor processing ability at room temperature and thermal instable of Mg-Al alloy,which restrict its application as structural parts.Therefore,the room temperature processing property and thermal stability of Mg-Al alloy are improved by adding rare-earth element Y,which results in the application of Mg-Al alloy is expanded.In this paper,the Mg-Al-x Y（x=4,6,8 wt.%）alloys are prepared by the casting,and the static and dynamic mechanical behaviors at room temperature and high temperature were systematically characterized by X-ray diffractometer（XRD）,scanning electron microscope（SEM）,transmission electron microscope（TEM）and electron backscatter diffraction（EBSD）.The Mg-Al-x Y（x=4,6,8 wt.%）alloys were prepared by adding different contents of Y element,and the influence of Y content on the microstructures and mechanical properties of the as-cast and as-extruded Mg-Al-x Y alloys were studied.The results show that a large number of fine Al₂Y precipitates are observed in Mg-Al-x Y alloys,which effectively suppress the precipitation of Mg₁₇Al₁₂ and promotes the refinement of grains;thereby plasticity and strength of the alloy are improved.In addition,the residual internal stress in the alloy was eliminated and the dynamic recrystallization was more completed after the annealing heat treatment process of as-extruded Mg-1Al-6Y alloy,the plasticity was greatly improved while the strength of the Mg-1Al-6Y alloy was reduced,and the tensile and compressive strains of the alloy were up to 26.25%and 38.4%,respectively.The in-situ tensile deformation of the extruded-annealed Mg-1Al-6Y alloy at room temperature was observed via EBSD technology,and the microstructure evolution during the tensile deformation was studied.The results show that,in the initial stage after yielding,the deformation of Mg-1Al-6Y alloy was mainly dominated by prismatic slip and basal slip;with the increase of the tensile strain,the pyramidal<c+a>slip and a small amount of{101 2}extension twins was activated;with the further increase of the tensile strain,the pyramidal<c+a>dislocations continued to multiply,and the{101 2}-{101 2}double extension twin in grains gradually merged to form{101 2}single extension twin;finally,with the continuous increase of strain,the crack began to form at the grain boundary and propagated along the grain boundary until alloy fracturing.In addition,it was found that the interface bonding of Al₂Y phase and Mg matrix was not the source of cracks based on the observation of in-situ tensile SEM.The static tensile properties and deformation behaviors of the extruded-annealed Mg-1Al-6Y alloy at elevated temperatures were investigated at the deformation temperatures of 200℃to 350℃and strain rates of 8×10^-5 s^-1-1.6×10^-3 s^-1.The results showed that the peak flow stresses decreased and the elongation to fracture increased with the increase of deformation temperatures or the decrease of strain rates,and the strain rate sensitivity indexes（m）increased with the increase of deformation temperatures.When the deformation temperature was relatively lower,the true stress-strain curve of the alloy has high strain hardening;with the increase of tensile deformation temperatures,the stress-strain curve of the alloy gradually transformed to strain softening,and the plastic deformation ability of the alloy was further improved.In addition,the activation energy Q value of the thermal tensile deformation was 98.8k J/mol,it indicated that the deformation mechanism was dominated by grain boundary slip.The quasi-static compression behavior of the extruded-annealed Mg-1Al-6Y alloy was studied via Gleeble-3500 thermal simulator at the deformation temperatures of 250℃to 400℃and strain rates 0.001 s^-1-1 s^-1.The results showed that the peak flow stresses decreased gradually with the increasing of deformation temperatures or the decreasing of strain rates approximately.In addition,the stress-strain curves of the alloy had strain hardening effects at lower deformation temperatures under any strain rates and at elevated deformation temperatures under higher strain rates.On the contrary,at elevated temperatures and lower strain rates,the dynamic softening effect due to the increased dynamic recovery and recrystallization,and the strain hardening effect caused by the dislocation multiplication reached a dynamic balance,which made the stress-strain curve show the steady flow deformation.By observing the microstructures of the alloy under varied compression strains at temperature of 350℃and strain rate of 0.1 s^-1,it was found that the combined effect of the existing<21 1 1>rare earth texture,the{101 2}extension twin and the activated pyramidal<c+a>slip resulted in the hardening of the stress-strain curve.The dynamic mechanical behaviors of the extruded and annealed Mg-1Al-6Y alloy were studied by separated Hopkinson pressure bar at varied temperatures and strain rates.The Johnson-Cook constitutive equation was revised based experimental data.The microstructures of the samples after impact were observed and analyzed.The results showed that at the same strain rate,the strengths of the alloy decreased with the increasing of deformation temperatures while the plasticity of the alloy was improved;when the deformation temperature was constant,the plasticity of the alloy increased with the increasing of the strain rates,and the yield strengths and tensile strengths also increased.It was observed that the improvement of strengths and plasticity with the increasing of strain rates was attributed to texture strengthening and fine grain strengthening,and the texture strengthening caused by twins.In addition,the revised Johnson-Cook constitutive equation based on experimental data predicted the well results at lower temperatures.However,there were some differences between the predicted values and the actual results at higher temperatures.