Numerical Modeling Of Microstructural Evolution During Solidification Of Multycomponent And Multyphase Alloys

In this thesis,the dendritic and eutectic solidification of Al-Si-Mg ternary alloys and the peritectic transformation of Fe-C binary alloys are studied using numerical modeling.The simulation results are compared with the theoretical predictions and experimental results.The cellular automaton（CA）model for the simulation of dendritic and eutectic growth of ternary alloys is developed based on the previous work of our research group.The model validation is performed through the comparisons of the simulation results with the predictions of the lever rule and the Scheil model with respect to the relationship between solid fraction and temperature.The CA model is applied to simulate the microstructural evolution during solidification of hypoeutectic and eutectic Al-Si-Mg alloys.The effects of cooling rate and concentration on the microstructures and the microsegregation are investigated.It is found that with the increase of the cooling rate,the microstructures become finer.The eutectic microstructures can be refined by reducing the eutectic transformation temperature.The microstructures obtained from the CA simulations agree well with the experimental results.When using the zero solid diffusivity,increasing the cooling rate leads to a relatively higher composition in the early solidified region and a relatively lower composition in the late solidified region.When the realistic solid diffusivity is adopted,the effect of back diffusion in the solid phase becomes more evident as the cooling rate decreases.A CA model for the peritectic transformation of binary alloys is developed to investigate the isothermal peritectic transformation process of Fe-C alloys（δ→γ,L→γ）.The CA model is applied to simulate the kinetics of theγ-phase during the peritectic transformation with zero supersaturation in the parent phases(Ω_γL=Ω_γδ=0).It is found that the thickness of theγ-phase increases non-linearly with time.Theγ-phase thickness byδ→γtransformation is larger than the one by L→γtransformation.The carbon concentration in theγ-phase region presents a non-linear distribution.With the decrease of holding temperature,the parabolic rate constant at theγ/δinterface increases non-linearly,while it remains nearly unchanged at theγ/L interface.The kinetics ofδ→γtransformation is always larger than the kinetics of L→γtransformation.Moreover,the growth velocity of theγ-phase decreases non-linearly with the increasing thickness of theγ-phase.The simulation results are in good agreement with the analytical predictions and the experimental data.The non-zero supersaturation in the parent phases(Ω_γL>0,Ω_γδ>0)increases the diffusion flux jump significantly at theγ/L interface but only slightly at theγ/δinterface.As a result,the growth velocity increases noticeably at theγ/L interface,but it is nearly unchanged at theγ/δ interface.