Effects Of Electromagnetic Stirring On Microstructure And Properties Of Al₇₀Zn₁₀Mg₁₀Cu₅Si₅ Medium Entropy Alloy

As a new lightweight structural material,Al-based medium entropy alloy has the characteristics of low density and high specific strength.Its application is of great significance to further lightweight of aerospace equipments.For Al-based medium entropy alloy with high mixed entropy and high alloying,its melt viscosity is large and the solidification temperature range is wide.In the process of unbalanced solidification,it faces the problems of serious composition segregation and easy to form coarse second phases.In this study,based on the traditional casting technology,the electromagnetic external energy fields with different frequencies were introduced to regulate the uniformity of the solidification structure in the solidification process of the alloy.The effects of electromagnetic stirring parameters on the grain size,solid solubility,morphology and distribution of phases,and macro segregation of elements of the Al-based medium entropy alloy were explored.The mapping relationship between the preparation process parameters,microstructure and mechanical properties was established,which provided theoretical support for the preparation of large-size Al-based medium entropy alloy.The main research work and conclusions are as follows:（1）The solidification path,phase evolution,phase stability and physical properties of the entropy alloy in Al₇₀Zn₁₀Mg₁₀Cu₅Si₅ were studied.The nucleation sequence of the alloy phase was found to beβ-Mg₂Si phase,α-Al solid solution,Q-Al₅Cu₂Mg₈Si₆ phase,θ-Al₂Cu phase,η-Mg Zn₂ phase and HCP-Zn phase,and the volume fractions ofα-Al solid solution and the second phases at room temperature are about 70%and30%,respectively.The physical properties of the second phases of the entropy alloy in Al₇₀Zn₁₀Mg₁₀Cu₅Si₅ were predicted by first-principles calculation.Among them,ηphase,Q phase andθphase are all ductile phases,andβphase is brittle phase.The toughness ofηphase is the best,and the compression resistance and shear resistance ofθphase are the best.（2）The mechanism of electromagnetic stirring on grain size,solid solubility,morphology and distribution of phases,and macro segregation of elements was explored.After 20 Hz electromagnetic stirring treatment,the solid solubility ofα-Al matrix was significantly improved,and the grain size was effectively refined.Especially for Mg₂Si phase,the average particle size decreased from 40.0μm to 17.7μm.（3）The effects of homogenization treatment on the evolution of alloy phase and the content and morphology of second phases were revealed.After homogenization treatment,the content of the second phases decreased significantly.Mg Zn₂ phase partially dissolved in the matrix and partially transformed into C14-Al Zn Mg Cu phase.The massive Mg₂Si phase spheroidized and the reticular Mg₂Si phase partially melted.A large number of white and dispersed HCP-Zn phases precipitated in theα-Al matrix.（4）The influence mechanism of electromagnetic stirring on the mechanical properties of Al-based medium entropy alloy was explored.After 20 Hz electromagnetic stirring treatment,the ultimate compressive strength and yield strength of the alloy at room temperature were 554 MPa and 500 MPa,respectively.The compressive strength and fracture elongation at 200°C were 319 MPa and 22%,respectively.