Phase-field Method Research On Microstructure Of Multicomponent Alloys Solidification

The mechanical properties and serving life of castings depend on the microstructures in solidification, it is very important and realistic to control the formation of microstructures effectively in molding, but there has many controlling parameters, it is unpractical to test them by experiment, the method of numerical prediction is therefore essential for any processing activity. It is well known that the numerical simulation can reduce consumptions, predict solidified microstructures and materials behavior by fewer experiments, obtain the quantitative relationship between mayor processing parameters and the microstructures, and it can offer a credible basis to evaluate and optimize the castings processing so as to improve their quality.The phase field method is a computing technology, the investigators can simulate the microstructures directly with it. The phase-field model includes a set of equations to depict the phase field, temperature field and solute diffusion field, the phase-field method for simulating the processing of solidification is developed by coupling phase field, temperature field, flow field, and other accessional fields, the microcosmic scales and macrocosmic scales are combined during mold filling and solidifying of the castings.Take the influence of structural fluctuation and energetic fluctuation in interface to dendrite growth, improved binary phase-field model, studied the influence of different anisotropy coefficients,undercooling on shapes of dendrites and distribution of solute for single grain. A new phase-field model for dendrite growth of multiple grains in binary and multicomponent alloys has been developed based on the Ginzberg-Landau theory and phase-field model of single grain.The time discretization and the space discretization of the phase-field equation were discussed in this paper, the initial conditions, boundary conditions, stable conditions and their solution methods were presented. In the process of simulation, the Visual C++ code was used to calculate the program and visible software Tecplot was used to achieve the vision of results, the visible system of the phase-field simulation was achieved. The complicated data base can be transacted effectively in this system, and the characteristic and the discipline of simulation results can be analyzed conveniently.Using the binary alloys dendrite growth of multiple grains phase model, taking Al-Cu alloy for example, coupled the concentration field, simulated the dendritic growth process of multiple grains during isothermal solidification. The result of simulation showed dendritic competitive growth of multiple grains, and reappears the process of dendrite growth in practical solidification. Comparing the simulated of multiple grains microstructure and single grain, we can see that the single dendrite growth and the multiple dendrite growth without other grains influences on side are agree with the classical theory (Ivantsov theory);in the region of competitive growth, multiple grains influenced interactive, they growth aren't agree with the classical theory. The regions between the secondary arms and grains have the highest concentration, while the grain itself has a low concentration. These features show good agreement with the composition distribution given by the real equiaxed dendrite growth.Using the multicomponent alloys dendrite growth of multiple grains phase model taking Al-Cu alloy and Al-Cu-Mg alloy for example, coupled the concentration field, simulate the dendritic growth process of multiple grains during isothermal solidification. The result shows, the predicted dendritic morphology is affected by density of seed, the higher seed density it is, the more refined and globular grains become. Qualitative effects like the grain refining mechanisms of inoculants and the effect on the grain size and dendritic morphology in Al-Cu-Mg alloy, the effect of the relation between alloy composition and microstructure (Al-Cu alloy vs. Al-Cu-Mg alloy) have been shown. A conclusion can be drawn after comparing the simulated result of Al-Cu alloys and Al-Cu-Mg alloys, that the composition change (adding Mg to Al-Cu alloy in order to get Al-Cu-Mg alloy) do not significantly change the shape of grains. That solute Mg changes the concentration distributing of solute Cu in equiaxed dendrite and increases concentration of Cu in grains. That means solute Mg will increase the microsegregation of solute Cu in grains.A new phase-field model of dendritic growth of non-isothermal solidification is developed based on the phase-field model of isothermal solidification of multicomponent alloy with multiple grains, in connection with the temperature field. Taking Al-Cu-Mg alloy for example, we simulated the dendritic growth process of multiple grains during multicomponent alloy solidification. The simulation showed dendritic competitive growth of multiple grains, and reproduced the process of dendrite growth in practical solidification. The release of latent heat of alloy solidification can be showed in simulation. In same grain, the temperature of the region of earlier solidification was less than the region of late solidification. The velocity of solidification increased with the augment of temperature of undercooling. Deduce and analyse phase-field model of eutectic solidification, using the model, taking CBr4-C2Cl6 alloy for example, simulated the eutectic growth process of alloy solidification. The result shows, the eutectic lamellar growth is parallel and steady with fitting parameters. In front of the solidification interface the diffuse and distribution of solute influence the concentration of solute in solid phase, induce the concentration of solute in a phase is higher andβphase is lower than value of steady state.