Study On Precipitation Behavior And Microhardness Of Rare Earth Magnesium Alloys Under Synergistic Effect Of Rapid Cooling And Cryocooling

As one of the lightest metallic structure material,magnesium alloy has good casting and corrosion resistance,and is widely used in aerospace,automotive communication and electronics industries.However,Mg-Al alloys under conventional as-cast conditions tend to form coarse eutecticβ-Mg₁₇Al₁₂ phase with low melting point.The existence of this brittle and hard phase at grain boundary seriously reduces the mechanical properties of the materials.As an important method of developing novelty materials,rapid solidification can not only effectively refine the grains,but also obtain supersaturated solid solutions by rapid cooling of the melt,which provides a thermodynamic driving force for precipitation.As a typical material modification and preparation technology,cryogenics causes lattice shrinkage in the alloy through volume shrinkage at low temperature.Consequently,the formation of considerable defects such as dislocations and vacancies provides sufficient nucleation sites for precipitation.Therefore,in-depth research on precipitation of magnesium alloy under the synergistic effect of rapid cooling and cryogenics is of great scientific significance and apparent engineering application value for the development of advanced metallic materials,where both the formation and transformation regularities of discontinuous precipitation at grain boundary and continuous precipitation within grain under different conditions can be revealed.In this paper,the effect of cryogenic treatment on the precipitation behavior of rapidly cooled Mg-6Al-1Y alloy was systematically studied by the combination of copper-mould spray casting and rare earth micro-alloying.According to the analysis of the morphology,size and volume fraction ofβphase at grain boundary and within the grain,the effect of cryogenic treatment on the precipitation structure and the micro-hardness of rapidly cooled during continuous heating process was discussed.The description of precipitation kinetics under different conditions was obtained by differential thermal analysis technology,and the corresponding transformation activation energy was calculated by JMA theory.On this basis,isothermal aging experiment at proper heat treatment temperature was performed,and the influence of cryogenic treatment on precipitation of rapid-cooled alloy under different holding time was compared.Consequently,the dependence of micro-hardness on microstructure features was discussed so as to provide a theoretical basis for the development of high-performance magnesium alloy.The results show that the quenched structure of Mg-6Al-1Y alloy sprayed by copper mould is composed ofα-Mg grain,Al₂Y rare earth phase and Al-rich phase at grain boundary.After aging to 250℃at 5 K/min,slight discontinuous precipitation occurs at the grain boundary of the RS（rapid solidification）alloy.As the temperature rises to 300℃,the volume fraction ofβphase further increases,and the microhardness reaches 69.26 HV0.1,which is 10.4%higher than that of the initial RS alloy.After cryogenic treatment for 32 h,there is no obvious precipitation in the original RS alloy,while the arising of micro-stress and resultant plastic deformation are beneficial to the fragmentation of dendrite arm,and the grain size is refined to 20μm.Moreover,high density dislocation structure is formed.Upon heating the RS+CT（rapid solidification and cryogenic treatment）alloy to 200℃at 5 K/min,discontinuous precipitation occurs at the grain boundary,indicating that cryogenic treatment can effectively promote the precipitation of rapid-cooled alloy.As the continuous heating to300℃,the volume fraction ofβphase increased significantly,and the precipitated region expands from grain boundary to crystal,indicating the coexistence mode of discontinuous precipitation and continuous precipitation.The microhardness of the alloy was further increased to 74.13 HV0.1.The DSC results show that the temperature range ofβprecipitation in grain boundary for RS alloy is 201.4℃～274.2℃,and the corresponding precipitation activation energy Q is calculated as 105.41 KJ/mol.In contrast,the precipitation temperature range of discontinuousβphase in grain boundary and continuous intracrystallineβphase of RS+CT alloy is 176.4℃～266.4℃and 266.4℃～288.3℃respectively.Due to the formation of high-density dislocation structure,the activation energy of discontinuousβphase decreases to 95.35 KJ/mol.After isothermal aging with 250℃/4 h,needle-plateβphase in RS alloy maintains a good interface coherent relationship with the matrix,and the corresponding orientation relationship is（101）_β//（11^—01）_α,（110）_β//（0111）_α,（01^—1）_β//（1010）_α.Cryogenic treatment effectively promotes the precipitation ofβphase and modifies its morphology into short rod-like shape.With aging of 250℃/16 h,the volume fraction ofβphase in RS+CT alloy is as high as 11.1%.However,the coarsening ofβphase is not favorable to the improvement of mechanical properties.Under the combined influence of fine grain strengthening,dislocation strengthening and precipitation strengthening,the strengthening effect due to the short rod-like shape ofβphase with 250℃/16 h is optimal.The microhardness of RS+CT alloy is as high as 82.64 HV0.1,which is 31.8%higher than that of the initial RS alloy.