Effect Of Initial Microstructure On Microstructure And Mechanical Properties Of Hot Extruded Mg-6Nd-2Al Alloy

The Mg-RE alloys have excellent comprehensive properties such as high strength,high toughness,heat resistance,creep resistance and corrosion resistance.Hot extrusion deformation or heat treatment is one of the important methods to improve the properties of magnesium alloys.However,the content,distribution and morphology of the second phase in the alloy have an important influence on the microstructure and mechanical properties of the hot extruded magnesium alloy.Hence,the effect of heat treatment on the microstructure and mechanical properties of the Mg-6Nd-2Al alloy is studied.Furthermore,the Mg-6Nd-2Al alloys with different initial microstructures were obtained by solution treatment and aging treatment,and then the Mg-6Nd-2Al alloy with different initial microstructure was subjected to hot extrusion deformation,and the effect of the initial microstructure on the microstructure and mechanical properties of the hot extruded Mg-6Nd-2Al alloy was studied.The effect of heat treatment on microstructure and mechanical properties of hot extruded Mg-6Nd-2Al alloy was studied.The results show that:The microstructure of as-cast Mg-6Nd-2Al alloy is composed ofα-Mg matrix,irregular bar Mg₁₂Nd phase,needle-like Al₁₁Nd₃ phase and granular Al₂Nd phase.After the alloy is solution treatment,the Mg₁₂Nd phase at the grain boundary is dissolved,the needle-like Al₁₁Nd₃ phase is broken into the granular Al₂Nd phase,and a large number of fine needle-like Al₂Nd phases uniformly distributed inside the grains which are precipitated along the basal plane of theα-Mg matrix.With increasing the solution time,the number and size of the needle-like Al₂Nd phase separated from the inside of the grains are first increased and then decreased.After aging treatment,the alloy has a large number of nano-scale flake-like precipitates and graphite-like precipitates.The tensile strength of the as-cast Mg-6Nd-2Al alloy is 141.5MPa,the yield strength is 104.4MPa,and the elongation is 5.2%.The tensile strength,yield strength and elongation of the as-cast Mg-6Nd-2Al alloy after solution treatment at 535℃for 14h were 155.7MPa,93.6MPa,and 7.6%,respectively,and the tensile strength,the yield strength and the elongation of the alloy were significantly increased after the treatment at 535℃for 14h and 200℃for 14h,respectively,and the tensile strength,yield strength and elongation of the alloy were significantly increased,which were 189.5MPa,121.9MPa and 6.3%,respectively.The alloy in the as-cast,535℃×14h treatment and 535℃×14h+200℃×14h treatment were subjected to hot extrusion deformation.After thermal extrusion deformation,a large number of fine equiaxed grains are produced in the alloy,and the second phase is broken obviously.After extrusion deformation of the as-cast Mg-6Nd-2Al alloy,the alloy consists of fine equiaxed dynamic recrystallized grains,broken grains during extrusion and deformed grains elongated along the extrusion direction.The average grain size is 4.5±1.5μm.After the thermal extrusion deformation of the alloy treated at 535℃×14h,the grain size distribution of the alloy is relatively uniform,and the average grain size is 6.5±1.0μm.After the thermal extrusion deformation of the alloy treated with 535℃×14h+200℃×14h,the grain size distribution of the alloy is more uniform,and the average grain size is 8.0±1.5μm.The tensile strength and yield strength of the as-cast Mg-6Nd-2Al alloy after extrusion are significantly higher than those of the other two alloys,which are 271.2MPa and 247.8MPa,respectively.After hot extrusion of the alloy treated at 535℃×14h with the highest elongation is 15.3%.After solution treatment,the number of the second phase in the alloy decreases obviously,and the grain size of the alloy increases obviously due to the precipitation of lath Al₂Nd phase in the alloy.After solution treatment,the average grain size of the three extruded alloys was 38±6.0μm、20±4.0μm、27±4.5μm,respectively.The tensile strength and yield strength of the alloy are obviously decreased,but the elongation of the alloy is obviously improved.Due to the solid solution treatment of the extruded alloy,the Al atom and Nd atom released by the second phase dissolve and recombine to form a stable Al₂Nd phase,which is precipitated inside the grain,which leads to the aging strengthening of the extruded Mg-6Nd-2Al alloy notbe obvious.