二次枝晶取向对镍基高温合金晶粒竞争生长行为的影响

采用精密铸造法制备板状双晶和三晶试样,研究晶粒的二次枝晶取向对晶粒竞争生长的影响。结果表明,对于一次枝晶取向相同的双晶,随二次晶界角增大,晶界位置始终位于板状试样的中间,2个晶粒之间几乎不存在竞争生长。对于一次枝晶取向不同的三晶,汇聚晶粒和发散晶粒的竞争生长情况不同。在汇聚生长的情况下,择优取向枝晶与非择优取向枝晶互相阻挡,导致晶界向择优取向晶粒倾斜,这与经典的WaltonChalmers模型不一样。并且二次取向不影响汇聚晶粒的竞争生长。在发散生长的情况下,择优取向枝晶与非择优取向枝晶都能分枝出新的枝晶,使晶界向非择优取向晶粒倾斜,这与Walton-Chalmers模型一样。此外,二次取向不影响发散晶粒的竞争生长。     

相场法模拟对流对枝晶生长的影响

基于Tong相场模型发展得到的单相场控制多晶粒相场模型,采用相场、流场及温度场三场耦合的方法,对纯镍凝固过程中的枝晶生长进行了数值模拟.时比研究了有无对流情况下,单个枝晶不同择优取向的枝晶形貌和枝晶尖端生长速度;模拟了多个晶粒的竞争生长,再现了凝固过程中的枝晶生长过程.     

对流作用下镁合金凝固组织演变的数值模拟

基于改进元胞自动机模型和流场传输模型,模拟对流作用下的镁合金等轴晶和柱状晶组织演变过程。采用改进元胞自动机模型模拟具有密排六方结构的镁合金的枝晶生长;采用投影法求解流场传输模型耦合质量守恒方程、动量守恒方程和溶质扩散方程。不仅模拟了镁合金中单枝晶、多枝晶和柱状晶在对流作用下的生长规律,还对不同入流速度下枝晶凝固前沿的溶质分布进行定量分析。模拟结果表明:迎流端枝晶生长较快,二次枝晶臂较为发达;背流端生长缓慢,二次枝晶臂较细小或没有二次晶臂。对流作用还会改变等轴枝晶扩散层的分布,在背流端扩散层呈拖曳特性。因此,对流作用对镁合金凝固组织的演变有重要影响。     

强制对流作用下纯Ni枝晶生长的相场法研究

基于耦合流场的Wheeler模型,采用有限差分法,对纯镍单枝晶和多枝晶生长过程进行模拟,研究了强制对流对枝晶形貌、温度分布及枝晶臂尖端生长行为的影响。结果表明,在流速为14.76 m/s的垂直强制对流作用下,单枝晶生长时逆流枝晶臂生长迅速,其尖端稳态生长速度比纯扩散时增加了47.14%;顺流枝晶臂生长缓慢,其尖端稳态生长速度比纯扩散时减小了49.72%。模拟结果与Ivanstov理论及试验结果一致。多枝晶生长时,受强制流动、晶体结构特征和生长空间的共同作用,仅有指向逆流侧外部区域的优先生长方向上的枝晶臂生长受到促进,主枝和侧枝发达,其他优先生长方向的枝晶臂生长均受到抑制,主枝和侧枝退化。     

三维枝晶生长数值模拟及实验验证(英文)

建立了用于模拟立方晶系合金三维枝晶生长的改进元胞自动机模型。该模型将枝晶尖端生长速率、界面曲率和界面能各向异性的二维方程扩展到三维直角坐标系,从而能够描述三维枝晶生长形貌演化。应用本模型模拟在确定温度梯度和抽拉速度条件下三维柱状晶生长过程的一次臂间距调整机制和不同择优取向柱状晶之间的竞争生长。使用NH4Cl?H2O透明合金进行凝固实验,模拟结果和实验结果吻合较好。     

Mg-Gd合金凝固过程中枝晶生长的相场模拟

基于Karma定量相场模型对Mg-6Gd(wt%)合金等温凝固过程中单枝晶和多枝晶的生长规律进行了模拟。定量研究了相场参数和溶质偏析对凝固过程中枝晶演化的影响。结果表明,各向异性强度越小,单枝晶尖端的失稳现象越明显,极易生成二次枝晶臂。同时讨论了各向异性强度与枝晶尖端半径和移动速率之间的关系,其结果与微观可解理论吻合较好。空间步长过大会导致枝晶形貌的变异,过小则计算效率低,合理的范围在(0.2～0.8)W_0(W_0为界面厚度)内。多枝晶生长模拟结果表明,枝晶臂弯曲是由于多枝晶在等温条件下的竞争生长所致。同时,大量溶质在枝晶间富集和分离,抑制枝晶的生长。     

枝晶生长的相场法模拟

采用相场法模拟了等温过程中枝晶长大过程。结果表明:晶粒经历了由球形经星形向复杂枝晶转变的过程;过冷度的增加使晶粒的生长速度增加。利用本模型成功模拟出各种不同择优取向的枝晶生长形貌。     

CA-LBM模型模拟自然对流作用下的枝晶生长

建立了一个基于二维的元胞自动机-格子Boltzmann方法(cellular automaton-lattice Boltzmann method,CA-LBM)的耦合模型,对自然对流作用下枝晶的生长行为进行模拟研究.本模型采用CA方法模拟枝品的生长,采用LBM对自然对流及由对流和扩散控制的溶质和热传输进行数值计算.通过计算方腔自然对流问题对模拟自然对流、溶质和热传输的LB模型进行了验证.应用所建市的CA-LBM耦合模型模拟研究了合金中单枝晶和多枝晶在自然对流作用下的生长规律,并将单枝晶上游尖端的稳态生长模拟数据与解析模型的预测结果进行了比较.结果表明,模拟结果与理论预测值吻合良好,自然对流会对枝晶的生长产生重要影响.     

强制对流作用下镁合金枝晶生长的相场法数值模拟

基于Wheeler等提出的纯扩散相场模型,针对hcp晶系镁合金建立耦合温度场、溶质场和流场的相场模型,研究强制对流作用下镁合金凝固过程中单枝晶和多枝晶的生长行为。结果表明:在流速为17.19m/s的垂直强制对流作用下,流体逆流侧3个方向的枝晶臂明显比流体顺流侧3个方向的枝晶臂生长得快,逆流方向枝晶臂的稳态生长速度比纯扩散时的增加78.0%,顺流方向枝晶臂的稳态生长速度比纯扩散时的减小53.7%;各枝晶的臂长存在很大差异,枝晶形貌的六重对称性被严重破坏;在多枝晶生长时,相向生长的枝晶互相影响并竞相生长,不同枝晶的枝晶臂彼此抑制,最终形成不对称的枝晶形貌。     

定向凝固过程中NH_4Cl-H_2O枝晶生长的数值模拟

建立了一种改进的元胞自动机模型(MCA),通过考虑成分过冷、曲率过冷、择优取向系数、温度梯度和抽拉速度等因素,模拟了不同温度梯度方向、不同择优取向及不同抽拉速度对柱状晶形态的影响.模拟结果较好地刻画了温度梯度和生长方向夹角的变化对一次枝晶臂间距的影响,不同择优生长取向的柱状晶竞争生长过程以及柱状晶尖端分叉机制.为了验证模型的可靠性,使用NH_4Cl-H_2O透明合金进行了定向凝固实验,实验结果和模拟结果吻合较好.     

Phase-field modeling of dendritic growth of magnesium alloys with a parallel-adaptive mesh refinement algorithm

A two-dimensional phase field (PF) model was developed to simulate the dendritic solidification in magnesium alloy with hcp crystal structure.By applying a parallel-adaptive mesh refinement (Para-AMR) algorithm,the computational efficiency of the numerical model was greatly improved.Based on the PF model,a series of simulation cases were conducted and the results showed that the anisotropy coefficient and coupling coefficient had a great influence on the dendritic morphology of magnesium alloy.The dendritic growth kinetics was determined by the undercooling and equilibrium solute partition coefficient.A significant finding is acquired that with a large undercooling,the maximum solute concentration is located on both sides of the dendrite tip in the liquid,whereas the maximum solute concentration gradient is located right ahead of the dendrite tip in the liquid.The dendrite tip growth velocity decreases with the increase of the equilibrium solute partition coefficient,while the variation trend of the dendrite tip radius is the opposite.Quantitative analysis was carried out relating to the dendritic morphology and growth kinetics,and the simulated results are consistent with the theoretical models proposed in the previously published works.     

Convection effect on dendritic growth using phase-field method

Phase-field model was employed to quantitatively study the effect of convection on pattern selection and growth rate of 2D and 3D dendrite tip,as well as the effect of the different convection velocity on the dendritic growth.The calculated results show that crystal is asymmetric in the priority direction of growth under flow.The dentritic growth is promoted in the upstream region and suppressed in the downstream region.Convection can cause deviation in the dendrite growth direction and the preferred direct...     

Numerical simulation of the effect of fluid flow on solute distribution and dendritic morphology

Abstract

The presence of bulk and interdendritic flow during solidification can alter the microstructure, potentially leading to the formation of defects. In this paper, a numerical model is presented for the direct simulation of dendritic growth in the presence of fluid flow in both liquid and mushy zones. The Navier–Stokes equations are solved for multiphase flow using a projection method. The energy conservation and solute diffusion equations are solved via a combined stochastic nucleation approach and finite difference solution to simulate dendritic growth. The predicted microstructures illustrate typical asymmetric dendritic growth behaviour under forced convection, which is consistent with prior similar simulations of a single dendrite during unconstrained growth (both 2D and 3D). The micromodel was coupled with a macromodel to investigate the effects of forced fluid flow on equiaxed dendritic growth and micro-segregation during vacuum arc remelting.     

Effects of magnetic fields on crystal growth

Abstract

The effects of a constant uniform magnetic field on thermoelectric currents during dendritic solidification were investigated using a two-dimensional enthalpy based numerical model. Using an approximation for three-dimensional unconstricted growth, the resulting Lorentz forces generate a circulating flow influencing the solidification pattern. Under the presence of a strong magnetic field secondary growth on the clockwise side of the primary arm of the dendrite was encouraged, whereas the anticlockwise side is suppressed due to a reduction in local free energy. The preferred direction of growth rotated in the clockwise sense under an anticlockwise flow. The tip velocity is significantly increased compared with growth in stagnant flow. This is due to a small recirculation at the tip of the dendrite; bringing in colder liquid and lowering the concentration of solute.     

A three-dimensional cellular automaton model of dendrite growth with stochastic orientation during the solidification in the molten pool of binary alloy

A three-dimensional (3D) cellular automaton model is developed for the prediction of dendrite growth with stochastic orientation during solidification process in the molten pool of binary alloy. An angle-information transfer method is proposed for improving cellular automaton technique to simulate the growth of the dendrites whose preferred growth direction owns stochastic misorientation with respect to the direction of the coordinate system. Dendrite morphologies and solute distributions of single dendrite growth and multi-dendrite growth are able to be obtained by the simulation using present model. The model is also employed to study the difference between two-dimensional simulation and 3D simulation on solute segregation and dendritic growth. Using the established model, 3D multi-columnar dendrites with stochastic crystallographic orientations can be obtained efficiently, and the competitive growth and impinging of dendrites can be reproduced in practice. The simulation results agree well with the experimental results.     

Ti-50.0Ni单晶形状记忆合金组织与记忆性能研究

在自制的定向凝固炉中,用带引晶器的石墨模壳制备出Ti-50.0at%Ni单晶.利用光学显微镜(OM)、扫描电镜(SEM)及其配置的能谱(EDS)仪研究合金的铸态组织,通过X射线衍射(XRD)和极图分析确定单晶晶粒取向,利用自制模具对合金的记忆性能进行评价.结果表明:试样横截面与纵截面均呈枝状晶分布特征,枝晶生长方向与定向凝固方向基本一致,未出现晶界,单晶横截面法向与[111]取向夹角为15°.由于消除了晶界对马氏体相变的不利影响,同时[111]取向是TiNi合金能获得较大可恢复应变的择优取向,与普通铸造的TiNi多晶相比,材料的形状记忆最大可恢复应变和记忆疲劳性能都得到一定提高.     

A three-dimensional cellular automaton model for simulation of dendritic growth of magnesium alloy

A numerical model based on the cellular automaton method for the three-dimensional simulation of dendritic growth of magnesium alloy was developed. The growth kinetics was calculated from the complete solution of the transport equations. By constructing a three-dimensional anisotropy model with the cubic CA cells, simulation of dendritic growth of magnesium alloy with six-fold symmetry in the basal plane was achieved. The model was applied to simulate the equiaxed dendritic growth and columnar dendritic growth under directional solidification, and its capability was addressed by comparing the simulated results to experimental results and those in the previously published works. Meanwhile, the three-dimensional simulated results were also compared with that of in two dimensions, offering a deep insight into the microstructure formation of magnesium alloy during solidification.     

Numerical modeling of solidification morphologies and segregation patterns in cast dendritic alloys

A comprehensive stochastic model for simulating the evolution of dendritic crystals during the solidification of binary alloys was developed. The model includes time-dependent calculations for temperature distribution, solute redistribution in the liquid and solid phases, curvature, and growth anisotropy without further assumptions on the nucleation and growth of dendritic crystals. Stochastic procedures previously developed by Nastac and Stefanescu (Modell. Simul. Mater. Sci. Engng, 1997, 5(4), 391) for simulating dendritic grains were used to control the nucleation and growth of dendrites. A numerical algorithm based on an Eulerian–Lagrangian approach was developed to explicitly track the sharp solid/liquid (S/L) interface on a fixed Cartesian grid. Two-dimensional mesoscopic calculations (i.e. at the dendrite tip length scale) were performed to simulate the evolution of columnar and equiaxed dendritic morphologies including the formation of the columnar-to-equiaxed transition. The effects of solutal and curvature undercoolings on the evolution of both the dendrite morphology and segregation patterns during the solidification of binary alloys were analyzed in detail.     

Phase-field simulation of dendritic growth for binary alloys with complicate solution models

1 INTRODUCTION The formation of complex microstructures during solidification of metals and alloys have fas- cinated researchers in materials science and related areas for so many years , especially the formation of dendritic structures . In many commercial al- loys , microstructural features that determine the mechanical integrity of a cast ingot ,such as solute segregation, grain size and porosity , all depend critically on the morphologies and velocities of in- dividual or arrays of gr…