Study On Solid Solution Aging Process Of Mg-5Li-4Zn-2Y Alloy

As an ultra-light structural material,magnesium alloy is widely used in aerospace science and technology,automotive industry,electronic information and other fields.The addition of rare earth elements and the development of heat treatment processes have improved the strength and plasticity of magnesium alloys to a certain extent,broadened its application scope and research prospects.In this paper,a Mg-5Li-4Zn-2Y alloy was prepared by vacuum casting.The optimal solution process parameters were determined experimentally.The samples after the optimal solution process were static aging at 220?,280?,and 350?respectively.Dynamic aging at 400?and 450?with strain rates of 0.001s^-1,0.01s^-1,0.1s^-1,and 1s^-1,respectively.During the experiment.The influence of the heat treatment process on the microstructure and properties of the alloy was analyzed by using experimental equipment such as microhardness tester,light microscope,scanning electron microscope,transmission electron microscope,and the best ageing process of the alloy was determined.The analysis results show that the microhardness of the as-cast alloy is 65.5HV,the second phase is mainly W-Mg₃Y₂Zn₃ phase and Mg₂₄Y₅ phase,and most of them are network-shaped at the grain boundaries.After the solution treatment of the sample at 520?,it was found that the grains did not grow significantly in the solution treatment sample with a duration of 4 hours,and the phenomenon of the second phase dissolution was more obvious.Therefore,the optimal solution treatment process was determined to be 520?×4h.In the static aging experiment,the peak hardness appears at 280?when the aging time is 8h,and the peak hardness is 110HV.Through research,it was found that during the aging process,a small and uniformly distributed flake-like?'phase was precipitated.This second phase is in a coherent state with the matrix and has a better strengthening effect on the alloy.At 280?×8h,the number of?'phases is the largest and most dense,so the hardness peak appears at 280?×8h.In addition,a large number of spherical W phases with a size of several tens of nanometers were precipitated in the later stage of static aging.After analysis by means of TEM and HAADF,it was found that the flake-shaped second phase(?'phases)precipitated during static aging was the intermediate phase of W phase.Finally,the optimal static aging treatment process was determined as:280?×8h,and the precipitation sequence of W phase during 280?static aging was obtained:solid solution state??'phase?W phase.In the dynamic aging experiment,the peak hardness appears at a strain rate of 1s^-1 at400?,and the peak hardness is 91.6HV.The study found that a large number of fine recrystallized grains appeared during the dynamic aging process,and a certain number of W phases were precipitated in a spherical shape,and the Mg₂₄Y₅ phase that was not completely dissolved during the solid solution process was fragmented.The number of precipitated second phases is large and the size is relatively small,and the recrystallized grains of the dynamic ageing sample at 400?×1s^-1 are the finest.Therefore,the peak hardness of dynamic ageing appears at 400?×1s^-1.During the dynamic aging process,due to the existence of a large number of defects in the crystalline material,it provided the location and energy for the uneven nucleation of the W phase.Therefore,the mesophase did not appear during the precipitation process of the W phase during dynamic aging.Finally,the optimal treatment process for dynamic aging was determined to be:400?×1s^-1,and the precipitation sequence of the W phase in the process of dynamic aging at 400?was:solid solution state?W phase.