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金属学报  2019, Vol. 55 Issue (5): 664-672    DOI: 10.11900/0412.1961.2018.00427
  本期目录 | 过刊浏览 |
定向凝固糊状区枝晶粗化和二次臂迁移的实验和模拟
方辉1,薛桦1,汤倩玉1,张庆宇1,潘诗琰2,朱鸣芳1()
1. 东南大学材料科学与工程学院江苏省先进金属材料高技术研究重点实验室 南京 211189
2. 南京理工大学材料科学与工程学院 南京 210094
Dendrite Coarsening and Secondary Arm Migration in the Mushy Zone During Directional Solidification:
Hui FANG1,Hua XUE1,Qianyu TANG1,Qingyu ZHANG1,Shiyan PAN2,Mingfang ZHU1()
1. Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, China
2. School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
引用本文:

方辉,薛桦,汤倩玉,张庆宇,潘诗琰,朱鸣芳. 定向凝固糊状区枝晶粗化和二次臂迁移的实验和模拟[J]. 金属学报, 2019, 55(5): 664-672.
Hui FANG, Hua XUE, Qianyu TANG, Qingyu ZHANG, Shiyan PAN, Mingfang ZHU. Dendrite Coarsening and Secondary Arm Migration in the Mushy Zone During Directional Solidification:[J]. Acta Metall Sin, 2019, 55(5): 664-672.

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摘要: 

采用透明合金原位观察实验和元胞自动机(CA)模拟,对丁二腈-丙酮(SCN-ACE)合金在定向凝固过程中糊状区的枝晶形貌演化进行了分析研究。实验和模拟均观察到了由于小枝晶臂重熔、相邻枝晶臂从尖端或根部合并的3种枝晶臂粗化模式,以及由于温度梯度区域熔化(TGZM)效应所引起的二次枝晶臂向高温方向的迁移现象。结果表明,枝晶臂的迁移速率随温度梯度的提高而加快;随保温时间的延长,枝晶臂的迁移速率降低。实验值和解析解吻合良好。通过模拟证实了必须有熔化效应才能实现枝晶臂迁移和小枝晶臂重熔的粗化模式。此外,熔化效应对由相邻枝晶臂合并引起的粗化模式也有显著的促进作用。

关键词 定向凝固枝晶温度梯度区域熔化元胞自动机    
Abstract

Directional solidification is a common and important process in both scientific research and industrial practice. Dendrites are the most frequently observed microstructures in the directional solidification. It is known that dendrite coarsening in mushy zones is an unavoidable phenomenon that influences microstructures and thereby properties significantly. Moreover, the presence of temperature gradients during directional solidification leads to temperature gradient zone melting (TGZM), which yields dendrite arm migration towards higher temperatures. In the present work, the evolution of dendrite microstructures in the mushy zone during directional solidification is investigated through in situ experiments and cellular automaton (CA) simulations for a transparent succinonitrile-acetone (SCN-ACE) alloy. The phenomena of dendrite coarsening and the secondary dendrite arm migration toward high temperature direction due to TGZM have been observed by both experiment and simulation. Dendrite coarsening is found to be caused by three modes, including the remelting of small dendrite arms, and dendrite arm coalescence through the advancement of interdendritic grooves and joining of dendrite arm tips. The experimental measurements indicate that the average migration velocity of the secondary dendrite arm increases with increasing the temperature gradient. For a fixed temperature gradient, dendrite arm migration becomes slower with time. The experimental data agree reasonably well with the analytical predictions. The present CA model involving the mechanisms of both solidification and melting can effectively reproduce the typical features of secondary dendrite arm migration and dendrite coarsening as observed in experiments. The simulation results show that the local liquid concentrations near the lateral side of big arms and in the "valleys" between side arms are relatively higher than that at the tips of small arms. This drives solute diffusion and leads to the dissolution of small arms, the growth of thick arms, and advancement of interdendritic groove bases. However, at the groove between two relatively narrow and long adjacent side arms, the solute diffusion is obstructed. In this case, dendrite arm coalescence through joining arm tips together with an entrapped liquid droplet in the solid can be observed. The role of melting for microstructure evolution in mushy zones is investigated by comparing the simulation results using CA models with and without melting effect. It is demonstrated that remelting is one of the dominant mechanisms for dendrite arm migration and dendrite coarsening by the mode of small dendrite arm remelting. Moreover, remelting also promotes dendrite coarsening by the mode of dendrite arm coalescence.

Key wordsdirectional solidification    dendrite    temperature gradient zone melting    cellular automaton
收稿日期: 2018-09-10     
ZTFLH:  TG113.12,TG111.4  
基金资助:国家自然科学基金项目(51371051);国家自然科学基金项目(51501091);中央高校基本科研业务费专项资金项目(2242016K40008);东南大学优秀博士论文培育基金项目(YBJJ1627)
作者简介: 方 辉,女,1995年生,博士生
图1  透明材料定向凝固原位观察实验装置示意图
ParameterUnitValue
Gibbs-Thomson coefficient, Γ℃·m6.48×10-8
Diffusion coefficient in liquid, Dlm2·s-11×10-9
Diffusion coefficient in solid, Dsm2·s-11×10-12
Partition coefficient, k-0.1
Liquidus slope, ml℃·%-1 (mass fraction)-2.8
Melting point of the pure solvent, Tm58.081
表1  本工作采用的物性参数[23,34]
图2  SCN-2.0%ACE合金在温度梯度G=9 ℃/mm、抽拉速率Vp=0条件下保温不同时间时二次枝晶臂迁移的原位实验观察照片
图3  温度梯度区域熔化(TGZM)原理示意图

Temperature gradient

℃·mm-1

Average migrating velocity / (μm·s-1)

Experiment

Analytical

solution

Relative

error / %

70.61±0.0640.7417.6
80.72±0.0760.8515.3
90.85±0.0940.9611.5
100.89±0.0731.0716.8
110.94±0.0761.1820.3
表2  SCN-2.0%ACE合金在不同温度梯度条件下枝晶臂迁移速率的实验值和解析解比较
图4  SCN-2.0% ACE合金在Vp=7.8 μm/s、G=7和9 ℃/mm时,$\tilde{y}_{0}=0.94$位置的枝晶臂迁移速率随时间(t)变化的实验值和解析解比较
图5  SCN-2.0%ACE合金在G=9 ℃/mm条件下保温时,相邻枝晶臂合并的原位实验观察照片
图6  SCN-2.0%ACE合金在G=9 ℃/mm条件下保温时,枝晶臂粗化的原位实验观察照片
图7  利用包含熔化和凝固效应的CA模型模拟的SCN-2.0%ACE合金在G=9 ℃/mm条件下保温不同时间后的枝晶臂迁移、粗化及合并的形貌
图8  不包含熔化效应的CA模型模拟的SCN-2.0%ACE合金在G=9 ℃/mm条件下保温不同时间后的枝晶形貌
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