电场对定向凝固类包晶合金凝固组织的影响
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Effect of Electric Field on Solidification Structure of Directionally Solidified Peritectic Alloys
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通讯作者: 王国田,guotianw@139.com,研究方向为定向凝固微观组织控制与力学性能改善
收稿日期: 2021-04-24 修回日期: 2021-11-11
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Corresponding authors: WANG Guotian, Tel:
Received: 2021-04-24 Revised: 2021-11-11
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作者简介 About authors
王国田,男,1978年生,博士
为研究直流电流对亚包晶合金凝固过程的影响,选用与亚包晶合金都存在相似凝固过程的AMPD-4.1%SCN透明亚包晶模拟物为研究对象。使用显微镜感光器件(CCD)和智能通讯测温仪表对实验过程进行实时拍照和温度记录,研究了亚包晶透明模拟物在电场作用下的结晶过程和晶体生长规律。结果表明:在电场的作用下,由电迁移效应使定向结晶的亚包晶模拟物的初生β相颗粒逐渐向正极方向迁移,使凝固界面前沿的液相成分与包晶点的成分(0.05%SCN,原子分数)接近,从而促进包晶反应的进行;电场的作用使电流偏聚产生的焦耳热效应和溶质富集引起的成分过冷,使定向凝固的枝晶尖端产生特殊分裂的生长形貌,使枝晶尖端分裂,枝晶间距减小。
关键词:
It is known that the AMPD-4.1% SCN (Aminomethyl Propanediol-4.1% Succinonitrile) transparent hypoperitectic polymer alloy and the Ni3Al hypoperitectic alloy present the similarity in directional solidification behavior, therefore, the former was selected as the reference material to simulate the effect of DC current on the directional solidification process of the later one. During the experiment, the real-time photo shooting and real-time temperature recording were carried out by the microscope photosensitive device (CCD) and an intelligent communication temperature measuring instrument. The results show that under the action of electric field, the primary β phase particles of the directionally crystallized subperitectic alloy migrate to the positive pole, which facilitates the peritectic reaction by making the composition of liquid phase near peritectic point at the frontier of solidification interface. The special growth morphology of dendrite tip splitting and the dendrite spacing decreasing of directionally solidified dendrite tip under the action of electric field may be mainly caused by Joule heating effect caused by electric field and supercooling caused by solute enrichment. Last but not least, results of the similar experiment for Ni3Al-based alloy Ni-20Al-10Fe-0.2B proved fairly well the above observed growth morphology of the AMPD-4.1% SCN transparent hypoperitectic polymer alloy.
Keywords:
本文引用格式
王国田, 王强, 郭建华, 丁宏升, 孙宏喆.
WANG Guotian, WANG Qiang, GUO Jianhua, DING Hongsheng, SUN Hongzhe.
随着航空航天技术的发展燃气涡轮发动机的涡轮前温度和推重比不断提高,对材料的耐高温性能提出了越来越高的要求。Ni3Al亚包晶金属间化合物的熔点高、密度低并具有良好的导热和抗氧化性能,是最有应用前景的高温结构材料[1~3]。但是,多数Ni3Al亚包晶金属间化合物的凝固组织粗大、没有明显的方向性、相匹配不合理和相形态不好。电场的作用可使晶粒和增强相的尺寸以及枝晶间距减小,并促进晶间对流[4]。电场还能减弱微观偏析使凝固组织更为均匀和择优取向生长,改善材料的组织形貌和使力学性能提高。例如,电场能使Al-4.5%Cu(质量分数)合金[5,6]和Pn-Sb-Sn合金[7~13]的凝固组织细化和性能提高。但是,上述研究主要针对低熔点合金[14~17]。同时,包晶金属间化合物的熔点高,难以实时原位观察其凝固过程。本文研究熔点低、与亚包晶金属间化合物的凝固过程类似、都有亚包晶反应的透明有机物凝固组织的演变规律,模拟电场作用对定向凝固亚包晶合金凝固过程和凝固组织的影响,并探讨电场影响金属凝固的机理。
1 实验方法
由于金属凝固温度高,很难对其进行原位观察,现有的实验也局限于低熔点合金。由相似理论首先类
图1
Experimental materials | /Ω·m | Melting entropy/ J·mol-1·K-1 | Melting point/K |
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AMPD-SCN | 470 | 56.6 | 365 |
再次,对于Ni3Al合金在凝固过程中首先析出的是NiAl相,对NiAl杰克逊因子
图2给出了物理模拟实验装配图。物理模拟实验平台充分考虑了原型实验的载荷与实验过程的相似性。在定向凝固过程中施加的是直流电流产生的电场,液相接电源正极,固相接电源负极。为了保证物理相似模拟物的熔化,将热端加热到105℃,将冷端温度调节到90℃,用冷端温度控制面板保持温度的稳定。在实验中调节热端和冷端的温度产生一定的温度梯度,以实现定向凝固。实验过程为:将熔区(9)置于显微镜的载物台上并校准显微镜,使用专用软件将电脑与显微镜链接以采集图像。将温度控制系统接通电源,将热端设置在熔点温度以上,用温度调节装置将冷端设置在包晶反应温度以下。温度恒定后打开循环泵,用循环泵和温度控制装置使熔区的冷端和热端温度恒定。实验用物理模拟物熔化后开启电源,使热端接直流电源正极,冷端接负极。调节电流的数值,观察所加的电场对定向凝固亚包晶模拟物结晶过程和组织的影响。
图2
图2
物理模拟的装配图
Fig.2
Assembly drawing of physical simulation experiment 1 (10) heat conducting plate; 2 glass plates; 3 (11) wiring post; 4 (12) heat conducting block; 5 pipes; 6 (15) hot (cold) end circulating pump; 7 (13) heater and thermocouple; 8 (14) heating oil; 9 melting zone
2 实验结果和分析
2.1 电场对结晶过程的影响
图3
图3
AMPD-4.1%SCN凝固初期的组织形态
Fig.3
Microstructure of AMPD-4.1%SCN at early stage of solidification
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图4
在1.2 mA 直流电流的作用下亚包晶模拟物定向凝固结晶生长的形貌
Fig.4
Directional solidification growth morphology of hypoperitectic simulator under action of 1.2 mA DC current (a) 0 s; (b) 2 s; (c) 4 s; (d) 6 s; (e) 8 s; (f) 10 s
2.2 电场对包晶凝固组织的影响
调整冷端和热端的温度差使温度梯度为4 K·mm-1。图5给出了电流强度为0 mA时亚包晶模拟物定向凝固生长形貌,可见与图3中的结晶形貌相似。图6给出了在1.2 mA直流电流产生的电场的作用下AMPD-4.1%SCN模拟物定向结晶初期两相结晶生长形貌的演变。实验中电场的加载方式相同,冷端与热端的温度梯度为4 k·mm-1,电流的方向为正极接液相,负极接固相。可以看出,包晶
图5
图5
亚包晶模拟物定向凝固生长形貌
Fig.5
Directional solidification growth morphology of hypoperitectic simulator (a) 0 s; (b) 3 s; (c) 6 s; (d) 9 s
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图6
在1.2 mA电流专用下亚包晶模拟物的定向结晶形貌
Fig.6
Directional crystallization morphology of hypoperitectic simulator with current intensity of 1.2 mA (a) 0 s; (b) 9 s
2.3 电场对凝固界面的影响
图7
图8
图8
直流电流影响枝晶尖端形貌演变的原理
Fig.8
Schematic explanation to the influence of DC on the evolution of dendrite tip morphology
图9给出了枝晶尖端演变的原理图。由于电场对溶质分布的影响,电流的偏聚使中心部位的溶质难以向两侧扩散而堆积,使固-液界面枝晶前沿出现成分过冷,枝晶尖端由圆滑平直的界面向下凹趋势发展,即溶质原子的堆积引起成分过冷,使枝晶中部的生长变缓,枝晶尖端形成下凹的形貌。电场的作用使这些新形成的分枝继续产生电流偏聚和溶质堆积,循环往复上述凝固过程。因此,在电场作用下的定向凝固过程中,产生枝晶尖端特殊的凝固-分裂形貌的原因,是电场引起电流偏聚所产生的焦耳热效应和溶质的堆积引起的成分过冷。
图9
图10
图10
在电流作用下Ni-20Al-10Fe-0.2B定向凝固的最初阶段和枝晶分裂阶段
Fig.10
Directional solidification of Ni-20Al-10Fe-0.2B under electric current (a) initial solidification stage; (b) dendrite splitting stage
3 结论
(1) 在电场作用下定向结晶亚包晶合金模拟物的初生β相呈颗粒状向正极方向迁移,使溶质浓度更接近包晶反应浓度(0.05% SCN),从而促进包晶反应的进行。
(2) 电场的电迁移效应使固-液界面溶体流动改变溶质原子的扩散系数而使
(3) 电场影响定向凝固过程,枝晶尖端特殊的凝固-分裂形貌主要是电场作用引起电流偏聚产生的焦耳热效应和溶质的富集引起的成分过冷引起的。
(4) 电场对亚包晶Ni3Al基合金Ni-20Al-10Fe-0.2B的影响再现了枝晶尖端分裂生长的形貌,亚包晶模拟物与实际包晶合金的凝固过程相似。
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