Phase-Field Study Of α-Mg Solidification Dendrite Growth In Mg-Zn-(Al) Alloy

Magnesium alloys,with low density,high specific strength and stiffness,and environmental friendliness,are extensively applied in transportation,electronic communication,aerospace,national defense,and other fields.The excellent properties are mainly determined by microstructures formed during solidification,e.g.,dendrites,eutectics,and second phases.The α-Mg dendrite,as one of the most common and important primary phases of magnesium alloys,plays a crucial role in mechanical properties.Nevertheless,due to the wide crystallization temperature range,low thermal conductivity,and large body shrinkage of magnesium alloys,resulting in coarse α-Mg dendrite,which decreases mechanical properties.Therefore,fully understanding the formation of α-Mg dendrite is of great significance for controlling the solidification microstructure and related properties of magnesium alloys.In this paper,the growth of α-Mg dendrite in Mg-Zn and Mg-Zn-Al alloys are studied based on the phase-field method,which is helpful to establish the quantitative relationship among composition,solidification structure and comprehensive properties of Mg-Zn-based alloys,and lay a foundation for the application of broadening Mg-Zn-based alloys.The main research contents are as follows:1.A strategy of parameter calculation and result storage is presented for phase-field simulation in α-Mg dendrite growth of Mg-5wt%Zn alloy under isothermal solidification.Results show that based on the phase diagram and empirical formulas,key parameters of the phase-field model,such as equilibrium partition coefficient 6),liquidus slope 8),solutal diffusion coefficient in liquid ,and solutal diffusion coefficient in solid can be obtained.Both structured grid method and structured point storage method can be used to store simulation results,but using the latter method will reduce about 60 % storage space and 37.5 % storage time compared with the former.Finally,convergent simulation results of α-Mg dendrite growth are obtained and they are in good agreement with the experimental results about optical micrograph,which verify the accuracy of parameters and stability of storage method.2.The thermal and solute diffusion in α-Mg dendrite growth of Mg-5wt.%Zn alloy under non-isothermal solidification were studied.Results show that solidification latent heat will be concentrated at the dendrite intersection,which makes thermal field distribution non-uniform,and it is similar to the profile formed by connecting primary dendrite tips.The increase in temperature will shorten primary dendrite arms,reduce side-branching development,increase dendrite tip temperature,and decrease dendrite tip concentration stability.Moreover,solidification latent heat will first diffuse to undercooled melt,then diffuse to other areas,finally,system temperature tends to be unified.The variation range of concentration difference is very small in the solidification process,and solute field distribution is uniform all the time.The local high-concentration areas will be formed after the superposition of the solute enrichment layer,and solute atoms will first diffuse to undercooled melt through the solid-liquid interface,and then concentrate between side-branching and dendrites.3.A phase-field lattice-Boltzmann model was developed to study α-Mg dendrite growth of Mg-5wt.%Zn alloy with forced convection.Results show that the existence of forced convection and overlap of thermal and solute fields makes thermal and solute fields distribution non-uniform.Thus,the symmetry of dendrite morphology is destroyed.The solid temperature and concentration of the downstream dendrite tip front with forced convection are higher than that without forced convection,while the concentration of the upstream dendrite tip front is lower.The solute transport through melt flow will be hindered by developed sidebranching.With flow velocity increase,the upstream temperature gradient and thickness of the downstream solute enrichment layer increase gradually,while the downstream temperature gradient and thickness of the upstream solute enrichment layer decrease gradually.Meanwhile,the upstream dendrite tip velocity will increase gradually,while the downstream dendrite tip velocity will decrease at first and then unchanged.This study is helpful to establish the relationship between α-Mg dendrite growth and melt flow,which is beneficial to understand the role of melt flow on dendrite morphologies.4.A 3D quantitative multicomponent phase-field model was proposed to investigate theα-Mg dendrite nucleation and growth.Results show that the refinement of α-Mg dendrite can be attributed to two aspects.On one hand,the addition of Al increases the alloy undercooling degree,decreasing the free energy barrier for nucleation,and increasing nuclei numbers.On the other hand,the solute atom of Al enriches in front of the liquid side of the solid-liquid interface,restricting further growth of α-Mg dendrite.In addition,the Mg-5wt.%Zn-2wt.%Al alloy with refined α-Mg dendrite and uniform distributed granule second phase has an excellent combination of strength and ductility(Ultimate tensile strength: 189 Mpa,Elongation: 30.4 %),and relatively high growth rate of solid-liquid interface energy and decline rate of bulk free energy.In this work,the phase-field simulation is consistent with experiment.This study is helpful to establish a relationship between the composition,solidification microstructures,and mechanical properties of Mg-Zn-Al alloy.