Numerical Modeling Of Microstructural Evolution During Peritectic Solidification

The peritectic solidification is involved in the processing of various alloy systems,which have widely practical applications.Peritectic solidification plays an important role to the final microstructure and properties.In this thesis,based on the previous work of our research group,a two-dimensional(2D)quantitative cellular automaton(CA)model is developed to investigate the peritectic solidification process of Fe-C and Al-Ni alloys.The simulation results are compared with the experimental results.The CA model is applied to simulate the microstructural evolution during peritectic solidification of a Fe-C alloy with an initial grain structure under continuous cooling condition.The simulated microstructure compares well with the in-situ observation experimental results reported in the literature.The effect of cooling rate on the kinetics of peritectic ?-platelet growth is studied.It is found that under the isothermal condition,the growth velocity of the ?-platelet tip remains constant,and the concentration field in the vicinity of the triple junction reaches a steady state in a short time.Under continuous cooling conditions,however,the growth velocity of the ?-platelet tip increases with increasing time,and the concentration field in the vicinity of the triple junction varies continuously.The evolution of the microstructure and microsegregation during polycrystalline continuous phase transition of Fe-C alloys is simulated.The morphology of the primary dendrites and the solute distribution in liquid are found to be affected by the cooling rate,which further influences the following peritectic solidification process.The concentration gradient in the liquid phase increases with increasing cooling rate,leading to a faster transportation of solute atoms away from the interface,which promotes the growth of peritectic phase during peritectic reaction.The evolution of the microstructure and microsegregation during directional solidification of Fe-C alloy is simulated.It is found that the secondary dendrite arm coarsening and the peritectic solidification proceed simultaneously.The peritectic phase fraction increases with increasing temperature gradient,The CA model is applied to simulate the microstructural evolution during directional solidification of an Al-25 at.%Ni alloy.It is found that a thick perictic layer forms on the front edge(cold side)of the secondary dendrite arm of the primay phase,while the peritectic phase on the back edge(hot side)is almost remelted due to the effect of temperature gradient zone melting(TGZM).The simulated asymmetric peritectic phase growth agrees well with experimental observation.The mechanism of the separated peritectic reaction during the directional solidification is analyzed.The solidification process is divided into four stages according to different melted and solidified phases.The effects of processing variables on the microstrural evolution and phase formation of an Al-Ni alloy during peritectic solidification in a temperature gradient are investigated.It is found that the volume fractions of primary and liquid phases gradually decrease,and the volume fraction of peritectic phase increases with increasing holding time under the temperature gradient.The volume fractions of primary and liquid phases gradually increase,and the volume fraction of peritectic phase decreases with increasing pulling velocity.When increasing the temperature gradient,the volume fraction of liquid phase decreases,and the volume fraction of peritectic phase increases.However,the volume fraction of primary phase basically remains constant.