Numerical Simulation Of Liquid-Liquid Phase Separation In Immiscible Alloy

An immiscible alloy is the one which shows a metastable miscibility gap in the regime of undercooled melt in phase diagram.Once deeply undercooled to the position of bimodal,an initially homogeneous liquid will separate into two liquid phases with different concentrations.It is prone to form serious segregation under usual casting conditions due to the fact that their densities are much different under gravity.Thus the application of these kind of alloys is limited.Since the liquid-liquid phase seperation during alloy solidification plays important roles on the final microstructure,and with the limitations of the experimental technoques in quantitily analyzing the influence of parameters on liquid-liquid phase separation,the numerical study has got more and more attention.In this paper,a two-phase Lattice Boltzmann(LB)model is established to simulate the growth of a dispersed liquid droplet during the liquid-liquid phase separation of a Li-Na alloy.It is found that,1).LB model is verified with the relevant theoretical results in deducing the critical radius for a droplet to survive and the additional pressure at the interface of a single droplet as well as modeling the evolution of the liquid bridge where two droplets coagulate.2).In the growth of a single droplet,the growth rate is increased through increasing the initial concentration of the alloy,which results in the increase of final radius of the droplet in equilibrium state.However,the increase of interaction intensity parameter G can just decrease the equilibrium concentration of the matrix and thus increase the final radius of the droplet,but does not affect the growth rate.3).Coagulation of two droplets occurs when the driving of concentration gradient overcoming the dragging of surface tension.The critical surface spacing of two droplets determines whether the droplets coagulate or not.A larger value of interaction intensity parameter G helps the occurrence of coagulation.4).Ostwald ripening mechanism was examined in the study.A large value of interaction intensity parameter G results in the large concentration of melt,which prevents Ostwald ripening.Impacted by surface tension,the concentration of small-size droplet is larger than the larger one.Thus the solutes immigrate from the small-size droplet to the larger one and finally the small-size droplet merges.The larger the ratio of sizes,the faster the merging.5).The growth of multi-droplets is studied.In the early stage of growth,droplet coarsening is mainly affected by diffusive growth,collision and coagulation.However,in the final stage,it is mainly influenced by Ostwald ripening.The average radius of the droplets increases and the number of droplets is decreased with the elapse of time.6).Spindol decomposition occurs where the solute concentration approaches the critical point of bimodal curve,and with a concentration fluctuation in the immiscible melt.