| The increasing requirement for lightweight,energy efficiency and environmental protection is driving the widespread application of Mg alloys,which are not only the lightest structural metals,but also a recyclable material.However,owing to their low strength at room and high temperature,as well as to their poor formability,two handicaps that have not been fully overcome after two decades of intense research,the widespread use of Mg alloys in engineering components is still restricted.The development of Mg-RE alloys makes it to be possible to fabricate high-performance Mg alloys and expand the application of Mg alloys.The microstructure of Mg alloy usually become more complicated with the addition of a large amount of rare earth elements.Both the solid solution treatment and aging treatment can induce a significant change in the microstructure of Mg-RE alloy.Hence,it is very important to obtain the optimum hot deformation parameters and heat treatment parameters to optimize the mechanical properties and fabricate high-performance Mg alloys.In this study,we investigated the influence of solid solution treatment on the hot deformation behavior and hot extrusion of Mg-6Gd-3Y-0.5Zn-0.5Zr alloy,the evolution of microstructure and mechanical properties of Mg-6Gd-3Y-0.5Zn-0.5Zr during hot spinning as well as the strengthening mechanism of deformed Mg-RE alloy.The aim of the research is to provide an effective guidance for the hot plastic working of Mg alloys.Firstly,the hot deformation behavior of as-cast and solid solution treated Mg-6Gd-3Y-0.5Zn-0.5Zr alloy was studied by the isothermal compression test conducted at 350?-500?with the strain rate of 0.001s-1-1s-1.The results show that with the decrease of strain rate and the increase deformation temperature until to 475°C,the precipitation of?'phase was enhanced in the solid solution treated alloy,which restricted the dynamic recrystallization process and the growth of DRXed grains.With the further increase of deformation temperature to 500°C,the?'phase could not precipitate from the matrix of solid solution treated alloy and the hinderance of?'phase on the dynamic recrystallization became negligible.Besides,the broken eutectic phase in the as-cast alloy presented a higher stability during hot deformation at high temperature.Hence,the as-cast alloy exhibited a higher flow stability during hot deformation at high temperature.Solid solution treatment improved the workability of Mg-6Gd-3Y-0.5Zn-0.5Zr at low temperature but resulted in the flow instability at high temperature.The optimum hot deformation parameters of solid solution treated alloy were 390?-440?/0.001s-1-0.01s-1,440?-465?/0.01s-1-0.1s-1and 465?-490?/0.3s-1-1s-1.And the optimum hot deformation regions of as-cast alloy were 390?-440?/0.001s-1-0.01s-1,440?-465?/0.01s-1-0.1s-1and 465?-500?/0.03s-1-1s-1.Secondly,the microstructure and mechanical properties of as-cast and solid solution treated Mg-6Gd-3Y-0.5Zn-0.5Zr alloy,as well as the age strengthening mechanism of extruded alloy was investigated by hot extrusion under different conditions(at 300?,350?,and 400?with the extrusion ratio of 9,16 and 25)and the subsequent aging treatment.The results indicate that both the as-cast alloy and the solid solution treated alloy possessed the highest yield strength after extrusion at low temperature with a small extrusion ratio,when the microstructure consisted of fine DRXed grains and coarse deformed grains.With the increase of extrusion temperature and extrusion ratio,the DRXed grains in the solid solution treated alloy exhibited a higher growth rate,and thus the yield strength decreased more quickly.An abnormal extrusion texture with<0001>//ED(extrusion direction)was formed after extrusion in the Mg-6Gd-3Y-0.5Zn-0.5Zr alloy due to the activation of non-basal slip.Higher extrusion temperature and more rare earth atoms(solid solution treatment)resulted in more activation of non-basal slip,and thus a stronger abnormal extrusion texture intensity.The combination of hot extrusion of solid solution treated alloy at low temperature and the subsequent aging treatment was an effective way of fabricating high-performance Mg alloy,in which solid solution strengthening,grain boundary strengthening and?'phase shearing strengthening were the main strengthening mechanisms.The contribution of texture strengthening was relatively smaller due to the weak extrusion texture.Besides,the higher strain energy in the deformed grains of low-temperature extruded alloy could promote the occurrence of static recrystallization during aging treatment,and thus achieve a good ductility.Thirdly,the microstructure and mechanical properties evolution during hot spinning and the strengthening mechanism of Mg-6Gd-3Y-0.5Zn-0.5Zr alloy were studied by multi-pass hot spinning and the subsequent aging treatment.The optimum spinning temperature of Mg-6Gd-3Y-0.5Zn-0.5Zr alloy was ranged from 440?to 460?.When the thinning ratio is small,the deformation was nonuniform along the thickness of the tube.With the increase of thinning ratio,the fraction of dynamic recrystallization increased,and the microstructure became more uniform along the thickness of the tube.After the 4th pass spinning,the microstructure was further refined,and the yield stress was increased with the increase of thinning ratio.The strength of the spinning tube was comparable to the alloy extruded at low temperature.The subsequent aging treatment could further improve the strength of spinning tubes.The main strengthening mechanisms in the aged spinning tube were similar to the aged extrusion alloy,including solid solution strengthening,grain boundary strengthening and precipitation strengthening.However,the repeating heating and deformation during spinning enhanced the precipitation of?'phase,and decreased the precipitation density of?'phase,and thus reduced the strength increment of the alloy at peak-aging condition.Finally,the effect of?'phase on the different deformation modes of Mg-RE alloy was revealed with the aid of single crystalline micropillar compression.The experimental results show that in grains that are favorably oriented for basal slip,the precipitation of?'phase resulted in pronounced slip localization due to the shearability of?'phase by basal dislocations,and thus leading to a very modest precipitation strengthening.In grains with the c-axis almost parallel to the compression axis,basal slip was the dominant deformation mechanism and the precipitation of?'phase enhanced the activation of pyramidal slip,but also with a minor precipitation strengthening.In grains with the c-axis almost perpendicular with the compression axis,prismatic slip dominants deformation in the solid solution state and the precipitation of?'phase favored twinning due to the concomitant lattice solute depletion.The modest hardening response of the weakly textured,polycrystalline alloy was attributed to the softening associated with basal slip localization due to particle shearing. |
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