| Solidification and melting are the most common and typical phase transformation phenomena.In materials processing,solidification and melting phenomena often occur simultaneously,and exert a strong influence on the solidification microstructures and thereby the final properties of materials.In this thesis,the cellular automaton(CA)approach is adopted to simulate dendrite coarsening and liquid film migration caused by simultaneous solidification and melting in the mushy zone,and peritectic transformation and reaction of FeC alloys.The CA simulations,combined with directional solidification experiments,are performed to study the microstructural evolution during peritectic solidification of an Al-Ni alloy in a temperature gradient.These studies contribute to improving the fundamental understanding of the interplay between solidification and melting and its influence on the microstructures,which is essential for improving material properties.A two dimensional(2D)CA model incorporating the solidification and melting mechanisms is extended to three dimensions(3D)for simulating dendrite coarsening in the mushy zones of succinonitrile-acetone(SCN-ACE)transparent alloys in 2D and 3D.The evolution of 3D solid/liquid(S/L)interface morphology is analyzed by determining the distribution of principal curvatures and weighted mean curvature.It is found that the S/L interface is flattened during isothermal dendrite coarsening.The evolution of interfacial curvature,local equilibrium concentration and local actual concentration during different coarsening mechanisms are quantified by CA simulations.The effects of holding temperature and alloy composition on the microstructures and dendrite coarsening kinetics are investigated.It is found that the melting of small dendrite arms and interdendritic groove advancement are the two main mechanisms in dendrite coarsening.The mechanism of coalescence by joining arm tips is more likely to take place at a lower temperature or for a lower alloy composition,while the dendrite arm fragmentation mechanism tends to occur at a higher temperature.The coarsening rate constant is found to decrease with increasing holding temperature and alloy composition.The CA model including solidification and melting mechanisms is applied to study the isothermal liquid film migration(LFM),and microstructural evolution and concentration distribution of Al-Cu alloys with initially equiaxed grain structures holding in a temperature gradient.It is found that LFM is triggered by concentration fluctuations in the liquid film,and the liquid film migrates towards the higher solid concentration side.The CA simulation successfully reproduces the experimentally observed microstructures generated by LFM accompanied by a particle free zone behind the liquid film.The simulated grain structure becomes coarser and highly elongated after holding in the temperature gradient.The results reveal that the increase in transversal grain width is mainly controlled by LFM,while the grain elongation in longitudinal direction is attributed to both LFM and temperature gradient zone melting(TGZM).The solid concentration decreases from the initial composition to the local equilibrium solid concentration corresponding to the local temperature.A 2D/3D CA model for simulating peritectic solidification is proposed and validated by comparing with the analytical model and experimental measurements.The CA model is applied to study the effects of supersaturation and cooling rate on peritectic transformation(δ→γ,L→γ)kinetics of Fe-C alloys.It is found that the growth kinetics of γ-phase increases with increasing supersaturation.There exists a transition supersaturation for isothermal peritectic transformation,at which the growth rates of the γ/L and γ/δ interfaces are equal.The transition supersaturation is found to increase with decreasing temperature.During continuous cooling,the γ/δ interface propagates at a higher velocity than the γ/L interface at low cooling rates.At high cooling rates,however,the γ-phase grows more into the L-phase with a cellular morphology.Then,the 2D/3D CA model is applied to simulate the γ-platelet tip growth and concentration distribution evolution of Fe-C alloys during isothermal peritectic reaction(L+δ→γ).It is found that ahead of the γ-platelet tip,the δ-phase remelts due to the solute enrichment caused by γ-platelet growth.Thus,the L/γ/δ triple point shifts towards theδ-phase region.The γ-platelet tip growth velocity is found to increase with increasing undercooling,while γ-platelet thickness decreases with increasing undercooling.2D/3D CA simulations combined with the directional solidification experiments are carried out to investigate the microstructural evolution during directional solidification of an Al-25 at.% Ni peritectic alloy.It is found that the peritectic phase exhibits an asymmetric distribution along the temperature gradient: a thick peritectic layer forms on the front edge(high temperature edge)of the secondary dendrite arm of the primary phase,while there is almost no peritectic phase on the back edge(low temperature edge).The solidification/melting mechanisms involving both primary phase and peritectic phase in the presence of TGZM and peritectic reaction are quantified by utilizing CA simulation.The directional solidification process is divided into four stages according to different melted and solidified phases.The effects of holding time,pulling velocity and temperature gradient on the microstructure and phase volume fractions are studied by both CA simulation and experiment.It is found that the volume fractions of primary phase and liquid gradually decrease,while the volume fraction of peritectic phase increases with increasing holding time in the temperature gradient.The volume fraction of peritectic phase decreases and the asymmetric distribution of peritectic phase becomes unconspicuous with increasing pulling velocity.With increasing temperature gradient,the volume fraction of peritectic phase increases and the asymmetric distribution becomes more obvious. |
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