Intergranular Deformation Compatibility In GW83 Alloy

Polycrystalline magnesium alloys develop internal strains/stresses characteristic of three length-scales,i.e.,macro-,intergranular and intragranular.At intergranular scale,strain mismatch between grains with different orientations due to the elastic and plastic anisotropy at the grain level will result in stress relaxation in individual grains and then the activation of various intragranular deformation modes.The current forming process design of magnesium made reference to that of aluminum alloys.The equipment and the forming process did not consider the orientation depended mechanical anisotropy and the deformation incompatibility intergranular in magnesium alloys.If these issues can be considered that establishing a microstructure related viscoplastic constitutive equation and setting the optimized stress the formability of magnesium component should be improved and it should shorten the development cycle of magnesium products.However,due to the lack of such studies on the deformation compatibility intergranular the design for the forming process of magnesium component lacks sufficient theoretical and experimental support.In this thesis,Mg-8%Gd-3%Y(wt.%)alloy was selected as the experimental sample.In-situ tensile method,EBSD(Electron Backscattered Diffraction),DIC(Digital Image Correlation)and VPSC(Visco-Plastic Self Consistent)model were used to study microstructure evolution and deformation mechanism of Mg-8%Gd-3%Y alloy at room temperature.First,intragranular deformation behavior was studied and the effect of grain orientation was discussed.Second,intergranular deformation behavior was studied and the effect of grain orientation and geometric compatibility of slip systems in adjacent grains on strain accommodation at grain-level was discussed.At last,effect of intergranular deformation behavior on mechanical property has been studied.From these investigations,the following conclusions can be drawn.Basal<a>slip,Prismatic<a>slip,pyramidal<c+a>slip and {10-12} tension twinning are the main deformation mode in the uniaxial tension process of Mg-8%Gd-3%Y alloy.The activation of basal<a>slip holds the highest proportion.Inhomogeneous strain distribution was found in the uniaxial tension process of Mg-8%Gd-3%Y alloy.Schmid factor was found not able to evaluate local strain accommodation.At regions surrounding grain boundaries,the majority of activated dislocation slip traces were found to form in pairs across grain boundary,and the basal-to-basal(B-B)slip pair is the dominant type.Higher probability of the occurrence of transferred slip was found at boundaries at[0001]0°-30°.A slip-transfer parameter m',describing the surrounding effect,was used to evaluate the deformation compatibility.It is found that a high m' value and a high Schmid factor of certain slip system in the adjacent grains would result in a good compatibility around grain boundaries and could accommodate higher local strains,which would improve the ductility of magnesium alloys.Strain accommodation around grain boundary interface is affected by grain orientation in the adjacent grains and grain boundary misorientation.To better describe intergranular deformation behavior in Mg alloys,a factor combining Schmid factor and m' value is proposed in this paper:mk=max(ka × mab' × kb)where ka and kb indicate respective Schmid factors of slip systems in two adjacent grains and mab' is the corresponding m' value.Here,the deformation in the grain interior and near the grain boundary interface are taken into account together,i.e.,ka and kb are for the grain interior,and mab' is for the grain boundary interface.A higher mk value was found related to a larger local strain.It is suggested that the overall ductility of Mg alloy could be evaluated by measuring its average mk value.As an evidence,the Mg-Gd-Y alloy tubes with higher mk value generally show higher elongations during tensile tests in this study.