Morphology Evolution Of Tilted Eutectic Structure Based On Phase Field Simulation

Eutectic alloy is the most widely used type of alloy in industry,and their microstructure is characterized by the simultaneous growth of two or more phases from liquid.Its eutectic structure exists in the solid-liquid phase transitions of many important structural and functional materials.The tilted eutectic structure is a eutectic formed by a non-tilted eutectic structure under a number of factors and opposite to the direction of the heat flow.Scholars have begun to explore important issues such as the growth mechanism of tilted eutectic structures and the stability of eutectic growth interfaces.It is preliminarily considered that the tilted eutectic structure is a special phenomenon of the non-tilted eutectic structure,and anisotropic surface tension has influence in the tilt angle of the eutectic structure.The phase field method continuously changed the order parameters of the solid-liquid interface,which eliminated the difficulty of tracking the solid-liquid interface during solidification.Therefore,the phase field simulation method can be used to study the formation mechanism of the tilted eutectic structure.In this paper,the phase field model is established for the eutectic growth system,and the finite difference method is used to solve the model.Under isothermal and directional solidification,the morphology evolution of the tilted eutectic and non-tilted eutectic interfaces was considered,respectively.The influence of the solid-solid interface anisotropy,solid-liquid interface anisotropy,pulling speed and temperature gradient on the morphology of the eutectic structure were obtained,and the growth mechanism of the tilted eutectic structure was revealed in essence.Through the research of this paper,the important conclusions are as follows:(1)Under isotropic and isothermal,the influence of undercooling on the eutectic structure was obtained.With increased of undercooling,the growth speed of the eutectic solid-liquid interface gradually increased.When undercooling reached a certain value,?-phase solid-liquid interface and ?-phase solid-liquid interface nucleated and grown,leading to a phase bifurcation.The tlit angle of the tilted eutectic increased under undercooling gradually decreased.(2)Under isotropic and directional solidification,the influence of pulling speed,temperature gradient,andlamellar spacing on eutectic structure were obtained.The larger the pulling speed,the smaller the eutectic lamellar spacing,which is consistent with the Jackson-Hunt eutectic theory.When the pulling speed is small,the non-tilted eutectic structure is bifurcated and mergered by the ?-phase and ?-phase to adjusted the eutectic spacing,while the tilted eutectic structure only adjusted the eutectic structure through the ?-phase microstructure bifurcation;increased the pulling speed,the eutectic spacing is adjusted by the ?-phase microstructure bifurcation.And as the pulling speed increased,the tilt angle of the tilted eutectic structure became smaller.When the initial eutectic spacing is small,the eutectic structure maintained steady state growth.When the initial eutectic spacing is large,the eutectic structure grown unstable,?-phase structure occured bifurcation,and the eutectic faster growth under small lamellar spacing.(3)Under anisotropic and isothermal,the influence of the solid-solid interface anisotropy,the solid-liquid interface anisotropy,and the lamellar spacing of the eutectic structure on the eutectic structure were obtained.When the solid-solid interface anisotropy is small,?-phase occured bifurcation in the eutectic structure with a large initial eutectic spacing;the increased in anisotropy promotes the coupled growth of the eutectic,and the larger solid-solid interface anisotropy promoted the tilted eutectic;when the solid-solid interface anisotropy is strong,the ?-phase occured bifurcation in the eutectic structure.The increased in solid-liquid interface anisotropy has the influence of stabilizing the solid-liquid interface of the eutectic structure.The solid-liquid interface of the eutectic structure has evolved into a flat interface.(4)Under anisotropic and directional solidification,the influence of solid-solid interface anisotropy,solid-liquid interface anisotropy,lamellar spacing and pulling speed on eutectic structure were obtained.The eutectic growth direction is mainly affected by the coupling of heat flow direction and solid-solid interface anisotropy.When the solid-solid interface anisotropy is small,the direction of the heat flow dominated the growth direction of the eutectic structure.Increased the solid-solid interface anisotropy to a certain value,there will be tilted eutectic structure.The solid-solid interface anisotropy influenced the spacing and the tilt angle of the eutectic structure.As the solid-solid interface anisotropy increased,the spacing of the eutectic structure decreased and the tilt angle increased.The increase in the solid-liquid interface anisotropy stabilized the growth of the eutectic structure,and the solid-liquid interface eventually evolved into a flat interface.The solid-liquid interface anisotropy influenced the eutectic structure lamellar spacing.As the solid-liquid interface anisotropy increased,the eutectic structure lamellar spacing increased.However,the solid-liquid interface anisotropy does not influenced the tilt angle of the eutectic structure.During the growth of the eutectic structure,two kinds of instability mainly occured.The first kind of instability is bifurcation or merge.The eutectic structure is adjusted for spacing.The second instability is fault line,which is mainly influenced by the large pulling speed and the solid-solid interface anisotropy.