强界面能各向异性下二元Ni-Cu合金枝晶生长过程的相场法模拟

基于Wheeler模型和Eggleston修正强界面能各向异性的方法,建立耦合溶质场和温度场的相场模型,模拟强界面能各向异性下Ni-Cu合金枝晶生长过程.结果表明:在强界面能各向异性作用下,界面方向枝晶生长不连续且枝晶出现棱角;由于枝晶尖端温度梯度和溶质梯度较大,枝晶生长迅速.当界面能各向异性强度低于临界值时,枝晶尖端生长速度随界面能各向异性强度的增加而增大;当界面能各向异性强度等于临界值时,枝晶尖端生长速度下降4.34％;当界面能各向异性强度大于临界值时,枝晶尖端生长速度随界面能各向异性强度的增加先增大到极大值后逐渐减小.当无量纲热过冷度较小时,晶体平衡形貌为类矩形;随着无量纲热过冷度的增加,晶体平衡形貌向枝晶转变,枝晶尖端生长速度先呈幂指数增加,然后呈线性增加,枝晶生长由扩散控制转变为动力学控制.     

三维枝晶各向异性生长的相场模拟

研究了三维枝晶生长相场模型中各向异性问题,采用数值计算模拟了各种程度的各向异性。在界币各向异性系数变化中,平衡晶体形貌从光滑过渡到出现角部之间有一临界值,三维的临界值为0．3。小于临界值时,枝晶生长稳定,界面光滑;大于临界值时界面不稳定,出现耳子,生长不连续。随着各向异性系数的增大,在相同条件下枝晶生长得越快。     

HCP材料强界面能各向异性的相场模型

基于Karma模型和Eggleston修正强界面能各向异性的方法,建立HCP材料的强界面能各向异性相场模型。采用有限差分法对控制方程进行数值求解,模拟研究HCP材料的枝晶生长行为。结果表明：枝晶形貌呈现出明显的六重对称性,界面方向不连续,导致在主枝和侧枝的尖端出现棱角。当界面能的各向异性强度低于临界值（1/35）时,枝晶尖端稳态生长速度随着各向异性强度的增加而增加;当界面能各向异性强度值超过临界值时,尖端稳态生长速度降低0.89%;当进一步增加各向异性强度值时,尖端稳态速度增加且在各向异性强度值为0.04时达到极大值,随后减小。     

Fe-C合金枝晶生长的相场模拟

采用相场法模拟了Fe-C0.5mol%合金等温凝固中的枝晶生长过程,研究了各向异性模数、各向异性强度和过冷度对枝晶生长的影响.结果表明,在二维平面上,枝晶沿<100>方向生长时,形成四重对称的枝晶形貌;枝晶沿<110>方向生长时,形成六重对称的枝晶形貌.随着各向异性强度增加,枝晶生长速度增加,晶粒由类球状向枝晶转变.随着过冷度的增加,枝晶生长速度增大,溶质扩散层减小,枝晶变的粗大.     

Al-0.035Ni合金凝固过程微观组织的相场法数值模拟

采用相场法对Al-0.035Ni合金在凝固过程中的微观组织进行数值模拟。分别模拟时间步长、各向异性系数和过冷度下Al-0.035Ni合金的枝晶生长过程;分析了Al-0.035Ni合金中过冷度对溶质分布的影响。结果表明:随着时间步长的不断增大,晶粒生长由最初的近圆形向枝晶状形貌转变,沿晶轴方向的枝晶生长速率明显快于其他方向。当各向异性系数较小时晶体生长缓慢,晶核在各个方向上的生长速度并没有太大的差异;随着各向异性系数的增大,晶粒在主轴方向上的生长优势越发明显。合金等轴枝晶周围的热扩散层由于受过冷度的影响,其枝晶尖端具有不同的生长速率;当增大过冷度时,枝晶尖端的生长速率随之增大,尖端形核半径呈减小趋势。此外,过冷度对合金枝晶内的溶质偏析影响不容忽视,会在枝晶内形成明显的溶质梯度。     

来流对Al-Cu合金三维树枝晶生长的影响

建立了模拟二元合金树枝晶生长的三维元胞自动机模型,以Al-4%Cu(质量分数)为模型合金,模拟了合金过冷熔体中树枝晶的生长过程,研究了来流对枝晶生长的影响.结果表明,来流对合金过冷熔体中三维树枝晶生长影响显著,迎流侧枝晶尖端生长速度随来流速度的增大而增大,枝晶尖端半径随来流速度的增大而减小;随着来流速度的增大,枝晶尖端选择参数减小;在给定过冷度条件下,随界面能各向异性的增大,来流对枝晶尖端选择参数的影响增强;对于给定的合金(或界面能各向异性),来流对枝晶尖端选择参数的影响随着过冷度的增大而增强.     

晶粒生长中高界面能各向异性的相场模拟

建立了一个能够模拟纯物质的计算有效的相场模型,该模型考虑了强动力学各向异性和高各向异性界面能。采用数值计算模拟了各种程度的各向异性（强动力学和高各向异性界面能）。在界面各向异性系数变化中,平衡晶体形貌从光滑过渡到出现角部,这之间有一临界值。小于临界值时,生长速度随界面能各向系数的增加单调增加;各向异性系数大于临界值时,随界面能各向系数的增加生长速度减小。在过冷度的变化过程中,生长速度从受热扩散控制转变到动力学控制,在热扩散控制时,生长速度随过冷度增加而增加,枝晶尖端半径随温度增加而减小;在动力学控制时生长速度和枝晶尖端半径随过冷度增加而增加,这与传统的微观可解理论认为R^2V=CONSTANT有所违背。     

基于自适应有限元相场模型模拟强制流动条件下的枝晶生长(英文)

应用投影算法与相场法相结合的数学模型,采用基于非均匀网格的自适应有限元法求解该模型,并对强制流动作用下镍过冷熔体中枝晶生长行为进行模拟。模拟结果表明,强迫对流的引入导致枝晶生长的不对称性。当流速小于临界值时,流动对枝晶的不对称生长影响较小;当流速达到或超过临界值时,枝晶生长的控制因素逐渐从热扩散过渡到对流。随着流速的增大,流动法向的一次枝晶臂朝逆流方向倾斜角度增大。而枝晶生长对熔体流动具有明显的影响。随着枝晶尺寸的增大,在顺流区域产生涡流效应,涡流区逐渐扩大并在枝晶尖端出现重熔现象。此外,非均匀网格的自适应有限元方法的CPU耗费时间比均匀网格方法降低一个数量级,并且加速比与计算域尺寸成正比。     

用相场法模拟纯铝的枝晶生长

利用相场法微观组织数值模拟技术,对纯铝物质的枝晶生长过程进行了仿真.研究了初始晶核半径、空间步长和各向异性对纯铝枝晶生长的影响.通过计算结果表明,在保证初始晶核半径不被熔化前提下,初始晶核半径不影响枝晶形貌;空间步长取值应在0.2d0～0.6d0范围内;各向异性趋于增大枝晶尖端生长速度,减小枝晶尖端半径.     

强迫对流作用下各向异性对枝晶生长的影响

基于Tong相场模型,采用相场、流场及温度场三场耦合的方法,对纯金属镍凝固过程中的枝晶生长数值模拟。结果表明:强迫对流的存在明显地改变了枝晶的生长行为,枝晶呈现不对称生长,在逆流方向上生长速度最快,水平方向生长次之,而在顺流方向受到了抑制。在各向异性系数较小时,枝晶各个方向上主枝都比较粗大,随着各向异性系数的增大,各向主枝都逐渐变细,各向生长速度明显加快,但当各向异性大于0.07时,枝晶形貌发生了变异失真。     

相场法模拟动力学各向异性对过冷熔体中晶体生长的影响

采用相场模型定量模拟了动力学各向异性作用下过冷熔体中的晶体生长过程.模拟结果表明,仅存在动力学各向异性时,各向异性系数大小对生长方式选择起着决定性作用.当各向异性较低时,固相以分形方式生长,在生长过程中不存在占优势的生长方向,同时也不存在稳态生长状态;而当各向异性系数大于0.02时,固相以枝晶方式沿〈110〉方向生长.进一步研究表明,枝晶生长稳定性系数随各向异性值的增加而增加,而与动力学系数取值无关.     

Influence of Bridgman solidification on microstructures and magnetic behaviors of a non-equiatomic FeCoNiAlSi high-entropy alloy

The non-equiatomic FeCoNiAlSi alloy is prepared by the Bridgman solidification (BS) technique at different withdrawal velocities (V = 30, 100, and 200 μm/s). Various characterization techniques have been used to study the microstructure and crystal orientation. The morphological evolutions accompanying the crystal growth of the alloy prepared at different withdrawal velocities are nearly the same, from equiaxed grains to columnar crystals. The transition of coercivity is closely related to the local microstructure, while the saturation magnetization changes little at different sites. The coercivity can be significantly reduced from the equiaxed grain area to the columnar crystal area when the applied magnetic field direction is parallel to the crystal growth direction, no matter what is the withdrawal velocity. In addition, the alloy possesses magnetic anisotropy when the applied magnetic field is in different directions.     

Effects of donor concentration and oxygen partial pressure on interface morphology and grain growth behavior in SrTiO3

Change in interface morphology with ionic vacancy concentration and the correlation between interface structure and grain growth behavior in strontium titanate (SrTiO₃) have been investigated using SrTiO₃ single crystals and powder compacts. Under experimental conditions where SrTiO₃ contains a negligible amount of ionic vacancies, the shape of the single crystal embedded in matrix grains was well faceted, showing a strong anisotropy in interfacial energy. However, as strontium or oxygen vacancies increased with the addition of an Nb₂O₅ donor dopant or reduction of oxygen partial pressure, the faceted shape changed to a smoothly curved rough one indicating that an interface roughening transition occurred and, as a result, the anisotropy in interfacial energy was considerably reduced. Grain growth behavior was also strongly dependent on the interface structures; while normal grain growth occurred when the interfaces were rough, abnormal grain growth behavior was observed in the samples with faceted interfaces. It appears, therefore, that the ionic vacancies in SrTiO₃ can cause the interface roughening transition and change the resultant grain growth behavior.     

Phase field method simulation of faceted dendrite growth with arbitrary symmetries

A numerical simulation based on a regularized phase field model is developed to describe faceted dendrite growth morphology. The effects of mesh grid, anisotropy, supersaturation and fold symmetry on dendrite growth morphology were investigated, respectively. These results indicate that the nucleus grows into a hexagonal symmetry faceted dendrite. When the mesh grid is above 640×640, the size has no much effect on the shape. With the increase in the anisotropy value, the tip velocities of faceted dendrite increase and reach a balance value, and then decrease gradually. With the increase in the supersaturation value, crystal evolves from circle to the developed faceted dendrite morphology. Based on the Wulff theory and faceted symmetry morphology diagram, the proposed model was proved to be effective, and it can be generalized to arbitrary crystal symmetries.     

自适应有限元方法凝固问题模拟的网格各向异性研究

建立了一套液-固平衡时晶体形态计算方法,并以其对自适应有限元方法凝固模拟的网格各向异性进行了研究。模拟结果表明该方法具有良好的收敛性;网格各向异性随空间步长的增大而增大,并与初始晶核大小无关;弱各向异性下网格各向异性引起的误差必须校正,而当各向异性系数大于0.03时可以忽略。     

YBCO的生长取向及定向凝固机制研究

利用定向凝固技术制备了取向良好的YBCO棒材,其a-b晶面与提拉方向的夹角小于5°.为研究YBCO生长的各向异性及其定向凝固机制,分别通过空淬及炉冷的方式获取了熔体生长的YBCO晶粒形貌及固液界面形貌.YBCO晶粒呈现片层状结构,其长厚比约为2～5.晶体生长的固液界面清晰反映出了外延生长的特征.基于上述结果,讨论了YBCO的定向凝固机制,认为YBCO的定向凝固是由择优生长及外延生长两阶段构成,这很好地解释了本文的试验结果.     

Influence of ordering kinetics on dendritic growth morphology

The effect of interfacial ordering kinetics on growth morphology is examined for undercooling solidification of intermetallics. The focus is on NiAl, which has been found to solidify into completely irregular seaweed-like morphologies at growth velocities of ～2 m s^?1. The transition from regular dendritic to seaweed growth in this material is simulated using a hybrid deterministic–probabilistic model of microstructure evolution, taking account of the interfacial kinetic effects. Modelling shows that seaweed formation in intermetallic forming systems can be attributed in part to transient entrapment of the chemical and orientational disorder at the solid/liquid interface. Both types of disorder trapping give rise to strongly preferential depression of the interfacial temperature, while orientational disorder trapping also reduces the definitiveness of crystal orientation at the interface. These effects counterbalance the role of crystallographic anisotropy in promoting growth into preferred directions, and hence lead to relatively easy breakdown of regular dendritic patterns in highly ordered materials.     

Numerical simulation of facet dendrite growth

Numerical simulation based on phase field method was performed to describe the solidification of silicon. The effect of anisotropy, undercooling and coupling parameter on dendrite growth shape was investigated. It is indicated that the entire facet dendrite shapes are obtained by using regularized phase field model. Steady state tip velocity of dendrite drives to a fixed value when γ≤0.13. With further increasing the anisotropy value, steady state tip velocity decreases and the size is smaller. With the increase in the undercooling and coupling parameter, crystal grows from facet to facet dendrite. In addition, with increasing coupling parameter, the facet part of facet dendrite decreases gradually, which is in good agreement with Wulfftheory.     

Effect of texture on phase-transformation strain in CuZnAl shape memory sheets

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