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金属学报  2019, Vol. 55 Issue (2): 202-212    DOI: 10.11900/0412.1961.2018.00053
  本期目录 | 过刊浏览 |
杨燕, 杨光昱(), 罗时峰, 肖磊, 介万奇
西北工业大学凝固技术国家重点实验室 西安 710072
Microstructures and Growth Orientation of Directionally Solidification Mg-14.61Gd Alloy
Yan YANG, Guangyu YANG(), Shifeng LUO, Lei XIAO, Wanqi JIE
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
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采用电子背散射衍射(EBSD)和元胞自动机有限元(CAFE)方法研究了Mg-14.61Gd合金在温度梯度G=30 K/mm和抽拉速率v=10~200 μm/s条件下的定向凝固组织和生长取向。研究发现,Mg-14.61Gd合金纵向凝固组织呈单一方向的α-Mg枝晶生长形貌,随着v的增加,枝晶界面生长方式由凸前生长向平齐生长转变,枝晶间距减小。当v从10 μm/s增至100 μm/s时,α-Mg枝晶的生长取向由<1120>和<1010>转变为<1120>,其与凝固热流的偏离角(θ)由11.0°减小至5.7°,热流是影响生长取向的主导因素;当v从100 μm/s增至200 μm/s时,α-Mg枝晶的生长取向仍为<1120>,但θ却逐渐增大至10.6°,此时,晶体的各向异性占主导。研究表明,CAFE模型可以合理预测定向凝固镁合金的晶粒组织和生长取向。

关键词 Mg-14.61Gd合金定向凝固EBSDCAFE模型生长取向    

As one of the most promising heat-resistant magnesium alloys, Mg-Gd series alloy has a wide application prospect in the industrial fields of aerospace, cars, and rail transit. There have been extensive researches on the performance improvement of Mg-Gd series alloys. As known, dendrites are the common solidification microstructures of castings of magnesium alloys, and solidification conditions have a significant effect on dendrite morphologies and growth orientation, which could strongly affect the mechanical properties of castings, thus it is critical to study the grain growth regularity for predicting the performance of magnesium castings. However, there are few studies on numerical simulation of dendrite growth process and growth orientation of magnesium alloys. Solidification behavior of magnesium alloys can be scientifically studied via directional solidification technology, and cellular automaton finite element (CAFE) method should be effective to simulate the dendrite growth process of magnesium alloys. In present work, microstructures and growth orientation of directionally solidified Mg-14.61Gd alloy under the temperature gradient G=30 K/mm and the withdrawal rate v=10~200 μm/s were investigated by EBSD measurement method and CAFE numerical simulation method. It was found that α-Mg primary phase presented unidirectional dendritic morphologies on longitudinal cross-section. The growth interface appearance of α-Mg changed from the protruding forward growth to the flat growth gradually and the dendritic arm spacing decreased gradually with the increasing v. when v increased from 10 μm/s to 100 μm/s, the main growth orientation of α-Mg changed from <1120> and <1010> to <1120>, and the deviation angle (θ) from solidification heat flow direction reduced from 11.0° to 5.7°, the reason for this lied mainly in the change of the heat flux. Further increasing v up to 200 μm/s, the main growth direction of α-Mg was still in <1120>, but the value of θ increased to 10.6°, and the anisotropy of the crystal was the dominant factor then. It was proved that the CAFE numerical simulation model could predict the grain structure and growth orientation reasonably for Mg alloy.

Key wordsMg-14.61Gd alloy    directional solidification    EBSD    CAFE model    growth orientation
收稿日期: 2018-02-05     
ZTFLH:  TG113.1  
基金资助:资助项目 国家自然科学基金项目Nos.51771152、51227001,国家重点研发计划项目No.2018YFB1106800及凝固技术国家重点实验室自主研究课题项目No.138-QP-2015

作者简介 杨 燕,女,1992年生,硕士生


杨燕, 杨光昱, 罗时峰, 肖磊, 介万奇. Mg-14.61Gd合金的定向凝固组织及生长取向[J]. 金属学报, 2019, 55(2): 202-212.
Yan YANG, Guangyu YANG, Shifeng LUO, Lei XIAO, Wanqi JIE. Microstructures and Growth Orientation of Directionally Solidification Mg-14.61Gd Alloy. Acta Metall Sin, 2019, 55(2): 202-212.

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图1  Bridgman定向凝固装置及试样模型网格示意图
Interface heat transfer coefficient Temperature Process condition
Alloy/Mold: 1000[22,23] Alloy: 740 Heat zone enclosure temperature: 740 ℃
Alloy/Pull rod: 500[22,23] Mold: 740 Heat zone enclosure emissivity: 0.9[23]
Mold/Pull rod: 500[22,23] Graphite heater: 740 Cooling zone enclosure temperature: 20 ℃
Mold/Liquid Ga-In-Sn alloy: 2000 Pull rod: 20 Cooling zone enclosure emissivity: 0[23]
Mold emissivity: 0.7[23,24]
表1  Bridgman定向凝固装置及试样模型的边界条件
Parameter Symbol Value Unit
Slope of liquidus line m -2.513 K%-1 (mass fraction)
Melting point Tm 629
Enthalpy ΔH 883.6 kJkg-1
Thermal conductivity κ 39 Wm-1K-1
Partition coefficient k 0.101
Diffusion coefficient D 1.233×10-9 cm2s-1
Gibbs-Thomson coefficient Γ 1.1×10-7 mK
表2  Mg-14.61Gd合金的热物性参数
Parameter Symbol Value Unit
Critical value of average nucleation supercooling degree ΔTˉ 6 K
Total supercooling degree ΔT 1 K
Maximum nucleation density nmax 5×107 m-2
Fitting polynomial coefficient a2 8.145×10-7 ms-1K-2
Fitting polynomial coefficient a3 5.871×10-7 ms-1K-3
表3  形核及简化KGT模型计算所用参数
图2  不同换热系数下Mg-14.61Gd合金在温度梯度G=30 K/mm和抽拉速率v=100 μm/s生长条件时的冷却曲线和凝固温度梯度
图3  G=30 K/mm、不同v下定向凝固Mg-14.61Gd合金的XRD谱
图4  G=30 K/mm、不同v下定向凝固Mg-14.61Gd合金横向及纵向显微组织的OM像
图5  G=30 K/mm、v=10 μm/s时定向凝固Mg-14.61Gd合金的晶粒组织、EBSD像及α-Mg相的反极图与极图
图6  G=30 K/mm、v=100 μm/s时定向凝固Mg-14.61Gd合金的晶粒组织、EBSD像及α-Mg相的反极图与极图
图7  G=30 K/mm、v=200 μm/s时定向凝固Mg-14.61Gd合金的晶粒组织、EBSD像及α-Mg相的反极图与极图
v Growth orientation of θ
μms-1 α-Mg dendrite
10 [112?0], [101?0] 11.0°
100 [112?0] 5.7°
200 [112?0] 10.6°
表4  采用EBSD方法测定的G=30 K/mm和不同v下定向凝固Mg-14.61Gd合金的α-Mg枝晶生长取向
图8  采用CAFE方法模拟得到的G=30 K/mm和不同v下定向凝固Mg-14.61Gd合金的晶粒组织和α-Mg相的<100>极图
v (φ1, Φ, φ2) Growth orientation of α
μms-1 α-Mg dendrite
10 (22.6, 148.3, 145.1) [2?111?] 10.886°
(283.9, 6.3, 284.8) [01?10]
40 (91.5, 73.8, 166.7) [1?1?20] 8.016°
(115.7, 141.9, 224.5) [3?122]
100 (261.9, 78.7, 277.4) [12?10] 6.709°
(87.2, 158.3, 86.1) [12?10]
150 (275.7, 90.8, 273.8) [12?10] 8.835°
(261.8, 78.7, 277.3) [12?10]
200 (277.8, 62.2, 95.8) [12?10] 10.167°
(52.56, 8.6, 303.2) [12?10]
表5  采用CAFE方法模拟得到的G=30 K/mm时不同v下定向凝固Mg-14.61Gd合金的α-Mg枝晶生长取向
图9  Mg-14.61Gd合金晶粒生长取向偏差随v的变化曲线
图10  枝晶生长取向、最优长大方向与热流方向之间的关系示意图
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