凝固过程显微偏析预测数值模拟研究的发展现状

显微偏析现象对于合金是非常重要的，因为它在很大程度上影响着合金的性能和后续加工，对它的数值模拟研究已受到许多研究的关注，本作回顾并介绍了显微偏析数值模拟的发展现状，指出了现有模型的不足和以后的发展方向。     

Simulation of Microsegregation in Multicomponent Alloys During Solidification

A phase‐field model is applied to the simulation of microsegregation and microstructure formation during the solidification of multicomponent alloys. The results of the one‐dimensional numerical simulations show good agreement with those from the Clyne–Kurz equation. Phase‐field simulations of non‐isothermal dendrite growth are examined. Two‐dimensional computation results exhibit different dendrites in multicomponent alloys for different solute concentrations. Changes in carbon concentration appear to affect dendrite morphology. This is due to a larger concentration and a lower equilibrium partition coefficient for carbon. On the other hand, changes in phosphorus concentration affect the dendrites and interface velocity in multicomponent alloys during solidification when phosphorus content is increased from 10^?3 mol% P. With additional manganese, the solidification kinetics slow down; dendrite morphology, however, is not affected. The potential of the phase‐field model for applications pertaining to solidification has been demonstrated through the simulations herein.     

Prediction of Columnar to Equiaxed Transition during Diffusion-Controlled Dendritic Alloy Solidification

A model is presented to predict the columnar to equiaxed transition (CET) in alloy castings. The model is based on a multiphase approach and accounts for heat and solute diffusion, as well as for grain nucleation, growth, and morphology. The model equations are applicable to both columnar and equiaxed dendritic solidification, thus offering an efficient single-domain formulation. A fixed grid, fully implicit finite-difference procedure is employed in the numerical solution, and a novel front tracking technique is incorporated that is also implicit in nature and readily applies to multidimensional situations. Calculations are performed for one-dimensional (1-D) and two-dimensional (2-D) castings of Al-Cu and Sn-Pb alloys. The calculated CET positions are compared with previous measurements in a (1-D) ingot cast under well-controlled conditions, and good agreement is found. The effects of various casting parameters on the CET are numerically explored.     

Microsegregation Model with Local Equilibrium Partition Coefficients During Solidification of Steels

A microsegregation model with local partition coefficients and temperature‐dependent diffusion coefficients based on the model of Ohnaka (I. Ohnaka, Trans. Iron Steel Ins. Jpn. 1986, 26, 1045) is proposed. In this model, multicomponent alloy effects and precipitations are considered, and the peritectic reaction can be indicated using the thermodynamic library ChemApp. The proposed model is validated by comparing the results to other models from the literature and to measured data. Local partition coefficients are calculated, and their significant influence on microsegregation prediction is shown. The characteristic temperatures determined using the proposed model are in good agreement with measured values.     

On the Mechanism of Natural Convection and Equiaxed Structure during Dendritic Solidification Processes

A new technique has been developed to generate dendritic‐equiaxed structures in aluminium alloy casting processes, not only to improve the mechanical properties but also to study the effect of crystal structure on the chemical and physical properties of alloys to be cast. The investigation combined laboratory experimental work, metallographic examination and mathematic modelling. The laboratory experimental work involved different superheats for Al‐4.5%Cu alloy in cast ingots. Measurements of temperature distributions were conducted to verify the solidification model. A metallographic study combined macro and micro structural evolution of cast ingot samples. Two‐dimensional mathematical models of fluid flow and heat transfer were developed to characterise the natural convection streams and thermal fields. The model predictions were compared to temperature and isotherms measurements where a good agreement was found. The formation of cast structure and columnar, equiaxed transition (CET) and macro segregation phenomena were studied and discussed, based not only on the theories of nucleation but also on the thermal effects in the mushy and liquid zones.     

Variations of Microsegregation and Second Phase Fraction of Binary Mg-Al Alloys with Solidification Parameters

A systematic experimental investigation on microsegregation and second phase fraction of Mg-Al binary alloys (3, 6, and 9 wt pct Al) has been carried out over a wide range of cooling rates (0.05 to 700 K/s) by employing various casting techniques. In order to explain the experimental results, a solidification model that takes into account dendrite tip undercooling, eutectic undercooling, solute back diffusion, and secondary dendrite arm coarsening was also developed in dynamic linkage with an accurate thermodynamic database. From the experimental data and solidification model, it was found that the second phase fraction in the solidified microstructure is not determined only by cooling rate but varied independently with thermal gradient and solidification velocity. Lastly, the second phase fraction maps for Mg-Al alloys were calculated from the solidification model.     

Microsegregation and Secondary Phase Formation During Directional Solidification of the Single-Crystal Ni-Based Superalloy LEK94

A multicomponent phase-field method coupled to thermodynamic calculations according to the CALPHAD method was used to simulate microstructural evolution during directional solidification of the LEK94 commercial single-crystal Ni-based superalloy using a two-dimensional unit cell approximation. We demonstrate quantitative agreement of calculated microsegregation profiles and profiles determined from casting experiments as well as calculated fraction solid curves with those determined in differential thermal analysis (DTA) measurements. Finally, the role of solidification rate on dendrite morphology and precipitation of the secondary phases is investigated and a new measure of the dendrite morphology is presented to quantify the effect of back diffusion on the amount of secondary phases.     

Effect of Cooling Rate on Microsegregation During Solidification of Superalloy INCONEL 718 Under Slow-Cooled Conditions

Metallurgical and Materials Transactions B - The solidification sequence, microstructural evolution, solid-liquid interface variation, interdendritic segregation, and elemental distribution of...     

Reconstruction of Three‐Dimensional Dendritic Structures based on the Investigation of Microsegregation Patterns

The shape of growing dendrites during the solidification of cast steel depends on many different parameters. This includes the casting technique, the applied cooling conditions or the casting geometry. Alloying elements such as carbon, silicon or manganese have a major influence on the solidification behaviour and therefore on the morphology of the growing dendrites. In this paper, a method is presented which enables the reconstruction of three‐dimensional dendritic structures by using concentration maps of a certain alloying element. For this purpose, a sample of a conventionally produced St52 continuous casting steel has been investigated. To determine the required two‐dimensional maps, an electron probe microanalyzer (EPMA or “microprobe”) with wavelength dispersive spectrometers (WDS) has been used. After the EPMA measurements, a commercial software tool has been utilized to process the concentration maps to three‐dimensional structures. This processing includes an iterative numerical method which allows estimating the curvature effect on the measured concentration patterns and therefore on the dendritic shape.     

Effects of Inclusion Precipitation,Partition Coefficient,and Phase Transition on Microsegregation for High-Sulfur Steel Solidification

Metallurgical and Materials Transactions B - A microsegregation model coupled with inclusion precipitation, variable partition coefficient, and diffusion coefficient of the solute was developed to...     

Microsegregation behavior during solidification and homogenization of AerMet100 steel

Cast AerMet100 exhibits mechanical properties comparable to the wrought properties of competing ultrahigh-strength steels; however, the segregation behavior had not been quantified under casting conditions. A microsegregation profile of the as-cast ingot was simulated and homogenization treatments were predicted using Thermo-Calc and DICTRA software. Experimental composition analysis confirmed the simulated microsegregation profile, and the homogenization process occurred at a slightly faster rate than predicted. The convergence of theory and experiment demonstrates the feasibility of designing cast alloy compositions assisted by computer simulation to minimize segregation without casting a large experimental matrix of test ingots.     

Recalescence and Segregation Phenomena During Equiaxed Dendritic Solidification of Fe-C Alloy

Metallurgical and Materials Transactions B - Recalescence and segregation are two characteristic phenomena for the equiaxed dendritic solidification of alloys. The present work developed a...     

A Multi Scale Method for Thermoelectric MHD in Dendritic Solidification

A fully coupled transient 3-dimensional multi-scale model has been developed to predict the evolution of dendritic growth in alloys.The motivation to use such a method is to both reduce computational costs and increase the size of the computational domain.The model consists of a mixture of finite volume and finite difference solvers integrated within a novel multi-scale method that solves the three sets of equations(electromagnetism,heat transfer/solidification and fluid dynamics)on appropriate length and time scales.A locus-based method,allows for mesh ref’mement in regions of interest localised around the interface,to improve accuracy without incurring significant computational overheads.This method has facilitated modelling the evolution of complex 3-dimensional structures on a single processor within a reasonable amount of time.As a demonstration,the model is applied to a super-cooled dendritic solidification problem,in the presence of a constant high magnetic field.     

Coarsening kinetics during solidification of Ni-Al-Ta dendritic monocrystals

Several dendritic monocrystals of nickel-rich Ni-Al-Ta alloys were directionally solidified at about 0.25 m/h⁻¹ under a gradient of 8 × 10⁻³ K/m⁻¹. The solid-liquid interface was fossilized at a given moment by rapidly quenching the remaining liquid. In some specimens crystal pulling was interrupted for various lengths of time prior to quenching. The quenched solid-liquid interfaces were used for a convenient and rapid evaluation of: 1) isothermal coarsening kinetics of the dendritic solid at a temperature between the liquidus and the eutectic temperatures and; 2) dendrite coarsening kinetics during solidification. It was found that extension to the ternary Ni-Al-Ta system of a model previously developed for binary systems predicted isothermal dendrite coarsening kinetics in close agreement with experimental results. Agreement for coarsening kinetics during solidification was less good. An increase in tantalum or aluminum contents slowed down coarsening, yielding finer microstructures. At equal atomic percental increase in concentration, the effect of tantalum was more significant than that of aluminum.     

连铸坯枝晶凝固的重要微观结构特征参数的研究进展

连铸钢水凝固时结晶方式包括柱状晶和等轴晶对铸坯质量有显著影响。连铸过程合金成分、铸坯过热度、尺寸，冷却速度和温度梯度等技术参数均影响一次、二次枝晶臂间距、柱状晶至等轴晶转变边界位置和糊状区渗透率等结构特征参数。在已有研究的基础上总结了微观结构特征参数的半解析计算公式，在对连铸时宏观流场、温度场和浓度场分布的微观．宏观耦合数值模拟时可采用合适的半解析计算公式计算这些特征参数。