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Nanoscale Cu clusters and α′ phase affect the hardness and brittleness of Fe–Cr–based stainless steel. The separation of Cr-enriched nanoscale α′ phase with Cu shell in Fe–Cr–Al–Cu alloys was revealed by three-dimensional phase-field simulation. The influences of Cu content and temperature on the evolution kinetics and element distribution were clarified. In the phase separation process, the Al is distributed in the matrix, Cu is first distributed in the matrix, then enriched around the Cr-enriched α′ phase, forming the nanoscale composite precipitates with Cr/Cu core-shell structure. The Cu shells have a retardant effect on the diffusion of Cr atoms, and the Cu shell becomes wider with the increased Cu content. The high Cu content delays the phase separation, growth, and coarsening kinetics of the α′ phase, while high temperature accelerates the phase separation kinetics. The influence of Cu content and temperature on the evolution of Cr/Cu core-shell nanoparticles reveals the complex nanoparticles formation and kinetics evolutions in the Fe–Cr–Al–Cu alloys. [ABSTRACT FROM AUTHOR]

A two-dimensional multiphase cellular automaton (CA) model is proposed for the prediction of growth kinetics and microstructural evolution during peritectic transformation of Fe-C alloys. The proposed model is validated by comparing the simulation results with the experimental measurements and analytical predictions for the growth kinetics of the γ -phase and the concentration distributions. The simulated time evolution of the γ -phase thickness and the concentration distribution in the γ -phase agree well with the experimental data, demonstrating the quantitative capabilities of the proposed model. The influences of the holding temperature and γ -phase thickness on the γ -phase growth behavior are analyzed based on the simulation results. The γ -phase growth velocity is found to decrease with increasing the γ -phase thickness and holding temperature. Simulations are also performed for the microstructural evolution during isothermal peritectic transformation of Fe-C alloys with the primary δ -phase being an equiaxed dendrite under different holding temperatures. It is found that the driving force for γ -phase growth increases with decreasing temperature. [ABSTRACT FROM AUTHOR]

The temporal evolution of microstructures and carbon distributions in a Fe-0.323C-1.231Mn-0.849Si (mol pct) dual-phase steel during heat treatments are simulated using a two-dimensional cellular automaton model. The model involves austenite nucleation, phase transformations controlled by ferrite (α)/austenite (γ) interface mobility and the local carbon concentration, and long-range carbon diffusion. It is also coupled with a solute drag model to account for the effect of substitutional elements on the interface migration. The results show that after holding at 800 °C for 300 seconds the transformed γ-volume fraction is lower than the paraequilibrium prediction. During subsequent cooling at 6 °C s−1, the γ → α transformation takes place after a stagnant stage; the carbon concentrations in both the α- and γ-phases increase and become non-uniform. When cooled below 450 °C, the γ-volume fraction is nearly unchanged. A small amount of carbon enriched martensite, transformed from the remaining γ-phase, exists in the room temperature microstructure. The simulated microstructures and carbon concentrations in martensite compare reasonably well with the experimental micrographs and atom-probe tomographic measurements. During tempering at 400 °C, martensite decomposes and the carbon concentration in the α-matrix increases. The simulation results are used to understand the mechanisms of yield strength variations after different heat treatments. [ABSTRACT FROM AUTHOR]

In this work, a multi-phase cellular automaton (CA) model is extended for the quantitative simulation of peritectic phase transition. First, the effects of cooling rate/supersaturation and temperature on the peritectic transformation kinetics in Fe-C alloys are investigated by utilizing the present CA model. The CA simulations show that supersaturations in the parent phases (liquid and δ-ferrite) increase the L/γ interface growth velocity remarkably, but tinily for the δ/γ interface migration velocity. There exists a transition supersaturation for isothermal transformations, at which the growth rates of the two interfaces are equal. The transition supersaturation is found to increase with decreasing temperature. Microstructural evolution at different cooling rates during peritectic transformation is simulated using the experimental conditions. At low cooling rates, the δ/γ interface propagates at a higher velocity than the L/γ interface. At high cooling rates, however, the γ-phase grows more into the L-phase with a cellular morphology. Then, the proposed CA model is applied to simulate the microstructural evolution during peritectic reaction. It is observed that the γ-phase propagates along the L/δ interface and finally encircles the δ-phase. Meanwhile, the intervenient γ-phase grows in thickness through peritectic transformation. The CA simulations are compared reasonably well with the experimental data and analytical calculations. [ABSTRACT FROM AUTHOR]

A two dimensional (2D) cellular automaton (CA) - lattice Boltzmann (LB) model is presented to investigate the effects of forced melt convection on the solutal dendritic growth. In the model, the CA approach of simulating the dendritic growth is incorporated with the kinetic-based lattice Boltzmann method (LBM) for numerically solving the melt flow and solute transport. Two sets of distribution functions are used in the LBM to model the convective-diffusion phenomena during dendritic growth. After validating the model by comparing the numerical results with the theoretical solutions, it is applied to simulate the single and multi dendritic growth of Al-Cu alloys without and with a forced convection. The typical asymmetric growth features of convective dendrite are reproduced and the dendritic morphology is strongly influenced by melt convection. The simulated convective multi dendritic features by the present model are also compared with that by the CA-NS model. The present model is found to be more computationally efficient and numerically stable than the CA-NS model. [ABSTRACT FROM AUTHOR]

A two-dimensional quantitative cellular automaton (CA) model is employed to simulate dendrite arm fragmentation and dendrite arm coarsening in mushy zones. The phenomenon of dendrite arm fragmentation of an Al-Cu alloy during heating is well represented by the CA simulation, and it is analyzed in detail by comparing the local actual concentration and local equilibrium concentration. The CA simulations for the dendritic microstructures of SCN-ACE alloys during isothermal holding in a mushy zone reproduce the typical dendrite coarsening features as observed in experiments. 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. [ABSTRACT FROM AUTHOR]

A two-dimensional (2D) lattice Boltzmann method (LBM)-cellular automaton model is presented to investigate the dendritic growth of binary alloys in the presence of natural convection. The kinetic-based LBM is adopted to calculate the transport phenomena by the evolution of distribution functions of moving pseudo-particles. To numerically solve natural convection thermal and solute transport simultaneously, three sets of distribution functions are employed in conjunction with the lattice Bhatnagar–Gross–Krook scheme. Based on the LBM calculated local temperature and concentration at the solid/liquid interface, the kinetics of dendritic growth is determined according to a local solute equilibrium approach. Thus, the physics of a complete time-dependent interaction of natural convection, thermal and solutal transport, and dendritic growth during alloy solidification is embedded in the model. Model validation is performed by comparing the simulated results with literature data and analytical predictions. The model is applied to simulate dendritic growth in binary alloys under the influence of natural convection. The effects of Rayleigh numbers and initial undercooling on dendrite growth are investigated. The results show that natural buoyancy flow, induced by thermal and solutal gradients under gravity, transports the heat and solute from the lower region to the upper region. The dendritic growth is thus accelerated in the downward direction, whereas it is inhibited in the upward direction, yielding asymmetrical dendrite patterns. Increasing the Rayleigh number and undercooling will enhance and reduce, respectively, the influence of natural flow on the dendritic growth. [ABSTRACT FROM AUTHOR]