Study On The Phase Selection And Solidification Kinetics Of Sub-Rapidly Solidified Mg-Zn-Y(-Zr) Alloys

As the lightest metal structural material, magnesium alloys are used widely in automotive, electronic and aeronautical industries because of a number of desirable features, such as high specific strength, high specific rigidity, damping characteristic and recycled easily. The microstructures and precipitated phases of as-cast magnesium alloys are coarse, the room temperature strength and elevated temperature strength are undesirable, which is difficult to meet the need of high performance structural materials. The materials made by rapid solidification have higher strength because of the fine grains, large solid solubility, new metastable phases and so on, however, it is difficult to prepare and the cost is high. Sub-rapid solidification, with excellencies of both conventional and rapid solidification, is more close to practical production and easily industrializing. Therefore, it is an available method to improve the strength of magnesium alloys. Presently, Mg-Zn-Y(-Zr) alloys is studied significantly to develop magnesium alloys with high strength, The phase selection and solidification kinetics of sub-rapidly solidified Mg-Zn-Y(-Zr) alloys were studied, which will provide a scientific and theoretical basis for designing and developing high-strength magnesium alloys.Conventionally solidified Mg-Zn-Y(-Zr) alloys were prepared by semi-continuous casting method, and the sub-rapidly solidified alloys were prepared by injecting into a cylindrical water-cooled copper mould with a diameter of 2mm after being remelted by high frequency induction heating equipment in a quartz tube. Microstructures and phases of both conventionally solidified and sub-rapidly solidified alloys were investigated using OM, SEM, EDAX, XRD and DSC. Microstructure evolution and phase selection under sub-rapid solidification were analyzed. Sub-rapid solidification process, nucleation and growth kinetics of the precipitated phases were discussed using time dependent transient nucleation theory and phase growth theory, and the results correspond to the experiment greatly.The microstructures of sub-rapidly solidified Mg-Zn-Y(-Zr) alloys mainly consist ofα-Mg solid solution and reticular second-phases at the grain boundaries, which is similar to that of the conventionally solidified Mg-Zn-Y(-Zr) alloys. The laminate eutectic structures of the second-phases, formed under conventional solidification, disappeared under sub-rapid solidification and were replaced by divorced eutectic structures. The solid solubility of the elements increased and the distribution at grain boundaries decreased, therefore, the segregation decreased and the distribution of elements was uniform comparatively.The grains were refined greatly by sub-rapid solidification. With the increase of Y addition, the grains were refined step by step and the matrix branched significantly. The matrix transformed from dendritic crystals to equiaxed crystals with the addition of Zr, and the effect of Y on refining matrix was enhanced simultaneously. However, under the sub-rapid solidification, the effect of elements on refining grains was no longer dominant.Mg₇Zn₂Y alloys mainly consist ofα-Mg, Mg₇Zn₃ and Mg₁₂YZn under conventional solidification, and consist ofα-Mg, Mg₃YZn₆ and Mg₁₂YZn under sub-rapid solidification. Mg7Zn3Y, Mg7Zn3YZr and Mg6Zn4YZr alloys under both conventional and sub-rapid solidification, all mainly consist ofα-Mg, Mg₃YZn₆å’ŒMg₃Y₂Zn₃. Adding Zr has no effect on phase constitution, but improved the eutectic temperature of Mg₃YZn₆. The formation of Mg₃Y₂Zn₃ and Mg₃YZn₆ was accelerated and the transition from Mg₃Y₂Zn₃ to Mg₃YZn₆ was restrained.Due to the initial cooling rate of sub-rapid solidification, the melt could not been undercooled to the liquid temperature of Mg₃YZn₆ or lower. Therefore, there was a competitive range of phase nucleation and rapid growth between Mg₃Y₂Zn₃ and Mg₃YZn₆, which determined the final contents of the phases. The solidification path of Mg₇Zn₂Y alloy wasα-Mg→Mg₁₂ YZn→Mg₃YZn₆ and which of Mg7Zn3Y, Mg7Zn3YZr and Mg6Zn4YZr alloys wereα-Mg→Mg₃Y₂Zn₃→Mg₃YZn₆.The incubation time of transient nucleation and phase growth were affected by the elements. Increasing Y/Mg ratio, the incubation time of transient nucleation curves ofα-Mg and Mg₃YZn₆ shifted right. Increasing Y/Zn ratio, the curves ofα-Mg and Mg₃YZn₆ shifted left. The curve ofα-Mg was not affected greatly with the addition of Zr, but the curve of Mg₃Y₂Zn₃ shifted left and the curve of Mg₃YZn₆ shifted right. The growth rates of both Mg₃Y₂Zn₃ and Mg₃YZn₆ increased with the increase of Y and Zr addition.