Study On The Ageing Behavior Of High-strength Mg-Gd-Y-Zn-Zr Alloy

The Mg-Gd-Y-Zn-Zr alloys with different initial microstructure were subjected toT5and T6ageing treatment. And the microstructure observation, DSC testing, textureanalysis and mechanical properties testing were conducted to investigate the ageingbehavior of Mg-Gd-Y-Zn-Zr alloy.It was observed that all the extruded alloys under different extrusion ram speedshowed obvious age-hardening effect when subjected to T5ageing treatment at200oC.The extruded alloy with the extrusion ram of0.4mm/s took the shortest time of28h toreach the peak hardness, which was attributed to the least degree of dynamicrecrystallization when extruded at the ram speed of0.4mm/s. With the increase ofageing temperature, it took shorter time to reach the peak hardness, while the peakhardness was decreased. It was found that the extruded alloy showed no obviousage-hardening effect when the ageing temperature was increased to250oC. In addition,the hardness of the extruded alloy was also improved greatly after T6ageing treatment.And it took more time to reach the peak hardness, which was mainly attributed to thelower density of dislocation in the alloy after solid solution treatment.The static recrystallization occurred in the extruded alloys during T5ageingtreatment. The degree of static recrystallization increased with the increase of ageingtime or ageing temperature. And there was no LPSO phases observed in therecrystallized grains. In addition, a large number of dense β’ phases were found in theT5and T6peak aged alloys. The β’ phase became coarsened with the increase ofageing time or temperature. After the extruded alloy was subjected to T5ageingtreatment at250oC for151h, some β1phases were observed to nucleate at the interfacebetween β’ and-Mg matrix.The texture evolution of the extruded alloy during T5ageing treatment wasanalyzed and it was found that both the extruded alloy and the T5peak aged alloycontained the basal fiber texture and an unusual texture component with basal planesperpendicular to the extrusion direction. After T5ageing treatment, the maximumintensity of the unusual texture component was nearly unchanged, while the basal fibertexture strengthened. The T5peak aged alloy exhibited excellent tension-compressionsymmetry, which was mainly attributed to the precipitation of fine dense second phases during T5ageing treatment.The T5peak aged alloys exhibited much higher tensile strength than the extrudedalloys, while the elongation showed a sharp decrease after T5ageing treatment. Thesuperior tensile strength of the T5peak aged alloy was mainly attributed to theprecipitation of β’ phases, the remained LPSO phases after T5ageing treatment and thetexture modification. With increasing of ageing temperature, the tensile strength of thealloy was decreased and elongation to failure was increased. Especially, the extrudedalloy with the extrusion ram speed of1.5mm/s showed the excellent tensile propertieswith TYS of395MPa, UTS of470MPa and elongation to failure of7.7%under the T5peak-aged condition. The tensile properties of the T6peak-aged alloy was much poorerthan that of the T5peak-aged alloy, indicating the T5is a suitable ageing treatmentprocess for extruded Mg-Gd-Y-Zn-Zr alloy.