Microstructure And Mechanical Properties Of High-strength Mg-Gd-Y Alloys Experienced By Hot-working Processes

Magnesium and its alloys are the lightest metal structure materials by now, and theirapplications are becoming more common in aerospace filed, automobile industry andelectronics filed due to their easy-recyclability which are known as “the greenenvironmentally-friendly materials”. Mg-Gd-Y alloys have recently received considerableattention due to the combination of high strength and heat resistance. But the Mg-Gd-Y alloyswill behave an extent of brittleness as a result of high RE elements content. However, hotworking is an effective method to get a refined microstructure and improved mechanicalproperties of alloys, so this paper is focused on the hot working of Mg-Gd-Y alloys and themain research works are carried out as follows:The mechanical properties of Mg-Gd-Y alloys and its microstructure evolution duringmulti-directional impact forging were investigated. After forging, the grains were refinedefficiently by twinning interaction. The EW75alloy was forged for150passes at450℃andminor recrystallization grains occurred at the twin boundaries. The microstructure of GW93and GW531alloys was refined efficiently by recrystallization induced by twinning, and thestructure could be refined more easily with decreasing RE elements. The mechanicalproperties could be enhanced by forging, super plasticity could be attained when tested at500℃, which means that a high strain rate and high strain processing could be applied in thefollow-up processing.The Mg-Gd-Y alloys were processed by hydrostatic forging and its microstructureevolution and mechanical properties were investigated. Recrystallization was restricted byprecipitates when compressed at410℃and440℃, the compressed-streamlines formed as aresult of that the grains were severe deformed along the compression axis. While compressedat470℃, the grain growth and rotation were inhibited by precipitates and a weak texture andfine structure with mean grain size of15um was obtained. When tested at ambient, anelongation of21%was obtained, with an improvement of20times when compared with the solutionized samples. When compressed at500℃, the grains became coarser because of thedissolution of the precipitates.The effect of extrusion temperature on the microstructure and mechanical properties ofMg-Gd-Y alloys during extrusion processing was discussed. Bi-modal structure was obtainedwhen extruded at400℃, which means that some original grains were stretched along theextrusion direction and recrystallization structure occurred at boundaries of the stretchedgrains. When extruded at higher temperature the original grains were refined gradually, andthe proportion of the recrystallization structure became higher, and the bi-modal structurenearly disappeared when extruded at480℃. The ultimate tensile strength of413MPa canobtained when extruded at400℃as a result of basal-texture strengthening by the stretchedgrains and residual stress strengthening, at the meantime, recrystallized grains could lubricatethe original grain boundaries led to a high ductility of10%. Ultra-high strength of508MPacould obtained after200℃peak aged for60H, and nanoscale precipitates along the grainboundaries could restrain dislocations slip efficiently.