金属单相凝固组织演化的定量相场模拟与实验验证

IMR OpenIR

	金属单相凝固组织演化的定量相场模拟与实验验证
	陈云
学位类型	博士
导师	李殿中
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料加工工程
关键词	定量相场模拟金属单相凝固组织实验晶体生长 Quantitative Phase-field Model Solidification Metal Microstructure Of Primary Phase Experiment Crystal Growth
摘要	"掌握和控制凝固过程组织形成对获得高质量的铸件具有重要意义。近年来，为再现金属凝固过程组织演化特点，揭示凝固组织形成机制，计算机数值模拟已经成为国内外很多学者所采用的研究方法。相场方法作为一种有效的数值模拟方法，克服了其它数值方法的缺点，能够很好地模拟凝固过程复杂界面的演化过程。本研究采用自适应网格的有限元法求解相场模型控制方程，采用定量模拟与实验紧密结合的研究方式，主要进行了以下几方面工作： (1) 镍的过冷熔体中自由枝晶生长的相场模拟。首先基于热力学定律推导了纯物质相场模型，比较了目前广泛使用的两种相场模型在模拟纯物质自由枝晶生长方面的差异。然后研究了不同的模型参数，如界面宽度、计算域大小、扰动幅度等对相场模拟的枝晶形貌的影响。最后将相场模拟结果分别与经典凝固理论和实验数据进行比较。 (2) Al-4wt.%Cu合金定向凝固初始过渡阶段组织演化相场模拟分别与凝固理论和原位实时观察实验的定量比较。研究结果表明，相场模拟的定向凝固初始过渡平面凝固阶段与Warren-Langer理论模型预测一致，确定了相场模拟能够准确反映扩散控制的凝固过程。在与实验数据进行定量比较时发现，由于实验中自然对流的存在，相场模拟结果只与平面凝固初期阶段吻合很好，而在平面凝固后期，自然对流较强，模拟与实验差别较大。平界面失稳后，模拟的初期胞状晶间距与实验中流动跟胞晶生长方向平行的区域内的胞晶间距差别较大，而与实验中流动跟胞晶生长方向垂直的区域内的胞晶间距吻合较好。 (3) Al-4wt.%Cu合金定向凝固初始过渡阶段凝固组织形貌演化的相场模拟与海藻状组织形成机制。当冷却速度和温度梯度满足成分过冷的平界面失稳判据时，界面形状的演化随着温度梯度和冷却速度的变化而变化。通常认为平界面失稳后，界面由平面状向胞状（Cellular），再由胞状向树枝状（Dendritic）转变，但研究发现了两种新的转变模式：胞状或者树枝状向海藻状（Seaweed）的转变。。根据模拟结果，研究了海藻状组织的形成机制，并对其生长动力学进行了表征。 (4) 流动对凝固组织生长影响的模拟。在前述扩散控制相场模拟与原位实时观察实验比较的结果中，已经表明流动对固液界面演化过程影响的重要性。因此，为进一步深化对实际凝固过程的认识，改进相场模型，耦合流体动力学Navier-Stokes方程，模拟了存在自然对流作用时，Al-4wt.%Cu合金定向凝固初始过渡阶段固液界面演化过程，并将模拟结果与原位实时观察实验进行比较。由于流动传热的影响，枝晶尖端生长速度受流动速度和流动方向影响明显。关键词：定量相场模拟，金属，单相凝固组织，实验，晶体生长"
其他摘要	"The understanding of the formation mechanism of microstructural patterns and shapes during solidification of alloys is significant to improve practical properties of most engineering metallic objects. Since the numerical simulation offers researchers a convenient way to visualize the evolution process during microstructure formation, it is a helpful tool for deeper understanding of the microstructure formation mechanisms, optimizing and exploiting novel materials. As one of the numerical simulation methods to elucidate the complex microstructure evolution during solidification, The phase-field approach has emerged as a powerful method to simulate the pattern formation during solidification. Amounts of phase-field simulations have been performed by researchers worldwidely to simulate microstructure formation during solidification, however, most of these simulations just qualitatively or semi-quantitatively convinced that the phase-field model can be employed to reveal the microstructure evolution, but can not provide the exact growth dynamics of solid. And thus quantitative phase-field modeling in combination with experimental validations of simulations are obviously necessary and crucial to precisely visualize the microstructure evolution process. Based on this research interest, the thesis consists of following aspects. (1) Free dendrite growth from highly undercooled melts of nickel was studied by phase-field model. The phase-field model of pure substance was first derived based upon the thermal dynamics laws and then a comparison between two widely used phase-field models was conducted. The influence of different parameters of the model on the shape of dendrite, such as the interface width, the calculation domain size and the amplitude of the noise, was investigated. Finally, the phase-field simulations were compared with classical solidification theory and experimental data. The comparison demonstrated that the phase-field simulations agreed well with predictions by the analytical model and quantitative agreements were achieved between simulations and experiment at low undercoolings, however, at high undercoolings (above 180K), the simulations and analytical predictions overestimated the reported experiment data because the linear and isotropic interface kinetic relationship is out of function. In simulations with noise, four coarsening modes of secondary arms were found, some of them have been observed in solidification experiments of organic substance. (2) The initial transient during directional solidification on Al-4wt.% Cu alloy was simulated by a quantitative phase-field model, and then simulation was compared with the related analytical theory and in situ and real-time observations by means of synchrotron radiation X-ray radiography. The simulated velocity of the planar interface and solute profile ahead of the solidification front in the liquid are close to the predictions of Warren-Langer model of the initial planar solidification transient, but in fair quantitative agreement with experimental results only at early stage of planar solidification. After the accelerated flat interface lost its stability, a transition to cellular solidification was initiated. The initial cell spacing predicted by phase-field simulation agreed well with experimental observation in the region where the cell growth direction was perpendicular to fluid flow, whereas discrepancy was obvious in the corners where the fluid flow was parallel to growth. An analytical relation well describing the wavelength of initial cell spacing was obtained by comparing phase-field simulation data with analytical expressions. The gravity-driven natural convection in experiment resulted in misfits between phase-field predictions and experimental observations in the late stage of planar solidification, onset and development of morphological instability. (3) In the phase-field simulations, once the smooth solidification front loses morphological stability in the initial solidification transient, the evolution of the non-planar solid-liquid interface microstructure varies with the processing control parameters. It was found that the solid-liquid interface shape changed through transitions from flat to cellular, cellular to dendritic, cellular or dendritic to seaweed depending on the values of the applied cooling rate and temperature gradient. The seaweed microstructure and the cell/dendrite to seaweed transitions modes were observed and verified in the in situ observed solidification experiments. Cell/dendrite to seaweed transition and oscillatory seaweed growth dynamics during initial solidification transient in directional growth of Al-4wt.% Cu alloy were characterized by means of phase-field simulation. The simulations reveal that, above a critical value of tip velocity, cell/dendrite tips are undergoing Mullins-Sekerka morphological instability resulting in tip splitting and transition from cells or dendrites to seaweeds. In seaweed growth, the oscillation period is scaling with the characteristic time of solute diffusion. The evolution obtained in numerical simulations is in agreement with the growth dynamics observed by in situ and real-time synchrotron X-ray radiography. (4) The effects of melt flow on the solid-liquid interface evolution were studied by phase-field model in corporation of fluid flow dynamics. The initial transient directional solidification with consideration of thermo-solutal convection effects on Al-4wt.%Cu alloy was studied and compared with real-time experiments. The results denoted that the phase-field simulation with consideration of natural convection reproduced the growth morphology and dynamics observed in experiment. To obtain the influence of fluid flow effects on dendrite growth, the growth velocity of a free dendrite of nickel at the upstream direction was investigated with varied imposed inlet velocities. Through analysis of these simulation results, a dimensionless flow Péclet number involving with the inlet velocity, the thermal (solute) diffusion length and the thermal (solute) diffusion coefficient, was proposed for the solidification of pure substances and alloys. It is demonstrated that when the flow Péclet number is larger than unity, the dendrite growth is controlled mainly by momentum, and thus the melt flow effects on the dendrite growth will become important. Then this conclusion is turned out to be robust for the dendrite growth of alloys affected by thermo-solutal convections in transient directional solidification. Key words: Quantitative phase-field model, solidification, metal, microstructure of primary phase, experiment, crytsal growth"
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64442
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	陈云. 金属单相凝固组织演化的定量相场模拟与实验验证[D]. 北京. 中国科学院金属研究所,2012.