固溶体型合金树枝晶组织形成过程研究

IMR OpenIR

	固溶体型合金树枝晶组织形成过程研究
	张显飞
学位类型	博士
导师	赵九洲
	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	固溶体型合金凝固树枝晶来流元胞自动机 Solid Solution Alloy Solidification Dendritic Growth Forced Flow Cellular Automaton
摘要	"树枝晶是一种最常见的固溶体型合金凝固组织。树枝晶的形态及微观尺度对合金材料的力学性能具有决定性影响，其形成过程一直是材料凝固研究领域的重要内容之一。由于树枝晶形态复杂，不可能用解析模型描述其整个形成过程。数值模拟可以耦合求解浓度场、温度场和枝晶生长动力学，模拟整个枝晶形态及生长过程。与其它数值模拟方法相比，元胞自动机模型具有较高的计算效率，已广泛应用于模拟树枝晶形成过程。考虑到目前关于多元合金三维树枝晶生长的元胞自动机模型还很少且不完善，本文建立了合金树枝晶生长的元胞自动机模型，模拟研究了二元和三元合金凝固组织形成过程。具体研究内容如下：建立了二元合金树枝晶生长的二维元胞自动机模型，根据界面处溶质守恒关系计算固液界面生长速度。对丁二腈-乙醇过冷熔体中树枝晶生长的计算结果与LGK模型预测结果吻合很好。研究了丁二腈-2.5 wt.%乙醇定向凝固一次枝晶间距选择过程，对定向凝固一次枝晶间距上下限的形成机制进行了细致分析。结果表明，定向凝固一次枝晶间距上下限由固液界面稳定性决定，凡是影响固液界面稳定性的因素都会影响一次枝晶间距上下限。建立了多元合金树枝晶生长的三维元胞自动机模型，根据界面处溶质守恒关系计算固液界面生长速度，考虑了合金元素之间相互作用对溶质扩散的影响，研发了直接耦合合金热力学计算、溶质扩散系数计算和枝晶生长动力学过程计算的模拟方法。将元胞自动机模拟结果与已有分析模型预测结果和文献中的实验结果进行对比分析，证明了该模型可定量描述三元合金三维树枝晶生长过程。采用不同的溶质扩散系数处理方式，计算了Al-Cu-Mg合金枝晶生长过程，结果表明溶质扩散系数对模拟结果有显著影响，因此，在三元或多元合金凝固过程模拟中，必须考虑合金元素之间相互作用对溶质扩散的影响。开展了Al-11.6 wt.%Cu-0.85 wt.%Mg合金定向凝固实验，结果表明，Al-11.6 wt.%Cu-0.85 wt.%Mg合金定向凝固一次枝晶间距和二次枝晶间距与凝固速度间均满足指数函数关系。用所建的三维元胞自动机模型预测该合金定向凝固枝晶组织的形成过程，计算的定向凝固一次枝晶间距和二次枝晶间距与实验结果吻合得很好；进而模拟研究了Mg含量对定向凝固一次枝晶间距的影响。结果表明，定向凝固一次枝晶间距随着Mg含量的增加而增大，定向凝固二次枝晶间距随着Mg含量的增加而减小。对Al-Cu-Mg合金等轴晶组织形成过程进行了研究，考察了冷却速率、形核特性对凝固组织的影响。结果表明：随着冷却速率的增大，形核率增大；在给定冷却速率条件下，随着形核接触角的增大，异质形核形核率减小；随着形核质点密度的增大，异质形核形核率增大。用三维元胞自动机模型模拟研究了来流对Al-Cu合金过冷熔体中等轴晶和定向凝固树枝晶生长的影响。结果显示，迎流侧枝晶生长速度大于背流侧枝晶生长速度；迎流侧二次枝晶发达，背流侧二次枝晶生长受到抑制；与来流方向垂直的枝晶向来流方向偏转；随着来流速度增大，枝晶偏转角度增大，尖端不对称性增强。考察了来流速度大小对迎流枝晶尖端选择参数的影响。结果表明，与热扩散控制的纯物质树枝晶生长不同，即使来流速度很小，其对由溶质扩散控制的合金三维枝晶尖端选择参数也有明显影响。不同过冷度和不同界面能各向异性条件下模拟结果表明，来流对枝晶尖端选择参数的影响与熔体过冷度和界面能各向异性有关，在给定过冷度条件下，流动对枝晶尖端选择参数的影响随着界面能各向异性的增大而增强；对于给定的合金，流动对枝晶尖端选择参数的影响随着过冷度的增大而增强。合金三维枝晶尖端Péclet数随着来流速度的增大而增大，其趋势与二维Oseen-Ivantsov解预测结果相似。"
其他摘要	"Dendritic structures are the most frequently observed solidification structure of the solid solution alloys. The morphology and microstructural scales of dendrites determine the mechanical properties of alloys. The dendritic growth attracts thus a lot of attentions. Analytical solutions are not possible to describe the morphology and the growth process due to the complex interface topology associated with dendritic growth. The numerical simulations have emerged as an important tool in the study of dendritic growth. The entire dendrite morphology and the growth process can be reproduced by numerical calculation of growth kinetic coupled with the solutions of solute and thermal fields. Compared with other methods, cellular automaton (CA) has a higher calculation efficiency. It has been used to simulate the microstructural evolution during solidification of the pure materials and alloys. However, up to now there are few reports on the three-dimensional (3-D) modeling and simulation of dendritic growth in multi-component alloys. In order to understand the practical process of microstructure evolution in an alloy we have developed a CA model for binary and multi-component alloys. The CA model is used to simulate the dendritic growth in binary and ternary alloys. The main contents are as follows: A two-dimensional (2-D) cellular automaton model is constructed to simulate the dendritic growth in binary alloys. The solid/liquid (S/L) interface velocity is calculated based on the solute conservation relationship at the S/L interface. The CA model is first verified by comparing with the results from the theoretical predictions of the dendritic growth in an undercoold melt of succinonitrile-2.5 wt.% ethanol, and is then applied to simulate the dendritic growth in a directionally solidified succinonitrile-2.5 wt.%ethanol. The selection mechanism of the upper and lower limits of primary dendrite spacing is analyzed in details. The results indicate that the stability of the S/L interface determines the upper and lower limits of primary dendrite spacing and that all the factors affecting the stability of the S/L interface influence the upper and lower limits of the primary dendrite spacing. A three-dimensional (3-D) CA model for multi-component alloys is developed. The S/L interface velocity is calculated based on the solute conservation at the S/L interface. The effect of interactions between alloying elements on the diffusion coefficient of solutes in the solid and liquid phases are considered. The model is solved by fully coupling the kinetic calculations with the thermodynamic calculations. The numerical algorithm is developed. The CA model is verified by comparing with the theoretical predictions and experiments for binary and ternary alloys. The results demonstrate that the 3-D CA model can be used to study the dendritic growth in multi-component alloys quantitatively. Free dendritic growth in an Al-Cu-Mg alloy melt at a constant temperature were calculated by treating the diffusion coefficient in different ways. The results demonstrate that it is essential to couple the diffusion matrix calculation with other calculations in order to show the practical process of dendritic growth in a melt of multi-component alloys. Directional solidification experiments are carried out with an Al-11.6 wt.%Cu-0.85 wt.%Mg alloy. The primary and secondary dendrite spacings are measured. The results demonstrate that the primary and secondary dendrite spacings can be expressed as an exponential function of the growth rate, respectively. The 3-D CA model is applied to simulate dendritic growth during the directional solidification of the Al-11.6 wt.%Cu-0.85 wt.%Mg alloy. The primary and secondary dendrite spacings are also calculated. The calculated primary and secondary dendrite spacings are in favorable agreement with the experimental results. The effect of Mg content on the primary and secondary dendrite spacings in the directionally solidified Al-Cu-Mg alloy is discussed. The results indicate that the primary dendrite spacing increases and the secondary dendrite spacing decreases with the increase in Mg content. The 3-D CA model is applied to study the equiaxed grain growth of an Al-11.6 wt.%Cu-0.85 wt.%Mg alloy. The effects of the cooling rate and the nucleation properties on microstructure formations are calculated. The results indicate that the nucleation rate increases with the increase in the cooling rate. For a given cooling rate, the nucleation rate decreases with the increase in the nucleation contact angle. For a given cooling rate, the nucleation rate increases with the increase in the nucleation site density. The 3-D CA is fully coupled with the numerical solution of momentum to predict the evolution of dendritic growth during solidification of alloys in the presence of melt flow. The dendritic growths in an undercooled melt and in a directionally solidified Al-4 wt.%Cu alloy with a forced flow are simulated. The results show that a forced flow affects the 3-D dendritic growth of alloys significantly. The growth of primary and secondary arm in the upstream direction is much greater than that in the downstream direction. The growth direction of the primary arm perpendicular to the flow direction tilts into the upstream direction. The dendrite tip of the primary arm perpendicular to the flow direction shows an asymmetric morphology. The degree of tilt and the asymmetry of tip become stronger with the increase in the forced flow velocity. With the increase of the flow velocity the growth velocity of the upstream dendrite tip increases, and the radius and the selection parameter of upstream dendrite tip decrease. For a given melt undercooling, the effect of forced flow on the selection parameter of upstream dendrite tip becomes stronger with the increase in the anisotropy of the S/L interfacial energy. For a given alloy or anisotropy of the S/L interfacial energy, the effect of the forced flow on the selection parameter of upstream dendrite tip also becomes stronger with the increase in the melt undercooling. The Péclet number of the 3-D dendrite tip increases with the increase in the flow velocity. This trend is in agreement with the prediction of the Oseen-Ivantsov solution for the 2-D dendritic growth.
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64410
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	张显飞. 固溶体型合金树枝晶组织形成过程研究[D]. 北京. 中国科学院金属研究所,2012.