1.School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China 2.Advanced Energy Engineering Science, Kyushu University, Fukuoka 8168580, Japan 3.Research Institute for Applied Mechanics, Kyushu University, Fukuoka 8168580, Japan
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 51071021, 51471026) and the National Magnetic Confinement Fusion Program of China (Grant No. 2014GB120000).
Received Date:06 August 2019
Accepted Date:28 August 2019
Available Online:01 November 2019
Published Online:05 November 2019
Abstract:The development of electrically insulating coatings is extremely important for the lithium/vanadium (Li/V) blanket of the fusion reactor. However, Li/V cladding materials suffer many problems such as tritiumpermeation and material corrosion. Thus, it is very important to find suitable insulating, tritium-resistant and corrosion-resistant coatings. So, the " V-alloy/Ti/AlN” bilayer coating was proposed by our group in previous study for the first time. In this paper, the evolution of the hardness, irradiation defects and microstructure of the Ti-clad V-4Cr-4Ti composite material after Fe10+ implantation are studied by transmission electron microscopy (TEM) and nanoindentation. According to the characteristics of the composition and microstructure, V-4Cr-4Ti/Ti composite material can be divided into four zones: V-4Cr-4Ti matrix, interface I (the interface near V-4Cr-4Ti matrix), interface II (the interface near Ti matrix), and Ti matrix. The nanoindentation results show that radiation hardening occurs in all regions during irradiation. The radiation hardening in the interface is lower than in the V-4Cr-4Ti and Ti matrix. Thus, the interface of heterogeneous material exhibits fine resistance to radiation hardening. The experimental values of hardness are much higher than the values calculated by the dispersed barrier hardening model. One reason for the discrepancy is that the theoretical values are calculated under the hypothesis of the uniform loop distribution. Actually, a large number of dislocation loops accumulate and tangle with each other in the samples. In addition, the formation of the precipitates is also one of the key factors. The TEM results show that the irradiation defects in the interface are low in density, large in size, and uniform in distribution. As a contrast, high density, small size and twisted dislocation loops are observed in irradiated V-4Cr-4Ti and Ti matrix. These results indicate that the interface can play a critical role in the resistance to irradiation damage. Few tiny Ti-rich precipitates appear in the V-4Cr-4Ti matrix, while there are large quantities of Ti precipitates in the interface after irradiation. Moreover, the number and size of precipitates in the interface I are larger than those in the interface II due to the formation of a few V-rich precipitates in the interface I. The formation of precipitations changes the proportion of V/Ti, which leads to the transformation from β-Ti to α-Ti in the interface. Keywords:V-4Cr-4Ti/Ti/ interface of heterogeneous materials/ precipitates/ irradiation defects
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3.1.辐照前后V-4Cr-4Ti/Ti界面的硬度变化
辐照前后V-4Cr-4Ti/Ti界面及两侧基体的SEM形貌和成分分析如图2所示, 离子辐照方向为垂直于照片. 从图2可以看出辐照前样品在界面处具有明显的过渡区. 能谱仪(energy dispersive spectrometer, EDS)的结果表明界面处元素分布有明显的扩散过渡区, 但辐照前后元素分布无明显变化, 即辐照过程对界面的元素的宏观分布无明显影响. 通过课题组前期工作发现, 扩散界面通过微观结构和成分的不同可以分为明显的左右两部分: 界面I和界面II. 界面I (靠近V-4Cr-4Ti基体一侧的界面)呈宽度均匀的浅色带状区域; 界面II (靠近钛基体一侧的界面)由细长而密集的针状组织组成[1]. 因此, 本文采取相同的分区方法, 将整个样品划分成V-4Cr-4Ti基体(以下简称钒基体)、界面I、界面II和钛基体[1], 如图2(a)所示. 图2(b)中白色长方形区域为采用FIB法制备TEM样品的取样位置, 1#为文中钒基体取样位置; 2#为界面取样位置, 其中2#左侧区域对应文中界面I取样位置, 2#右侧区域对应文中界面II取样位置; 3#为文中钛基体取样位置. 图 2 V-4Cr-4Ti/Ti界面区域及两侧基体在(a) 辐照前和 (b) 辐照后的SEM形貌图及对应的EDS元素线扫描分析结果; 图中的白色长方形区域为FIB的取样位置 Figure2. The SEM morphology and EDS line analysis of V-4Cr-4Ti/Ti samples: (a) Before and (b) after irradiation; the positions of FIB samples are marked with white rectangles.
由于在辐照过程中, 材料中会引入大量的缺陷和尺寸很小的缺陷团, 阻碍了位错的运动, 从而引起辐照硬化[23]. 为了对比辐照前后界面及两侧基体硬度变化, 采用纳米压痕技术进行了硬度测试, 结果如图3所示, 其中$\Delta { HV}$为辐照前后硬度差值. 从图3可以看出在辐照前界面II具有最高的硬度(547 HV), 界面I和钒基体则硬度相对较低(389 HV和336 HV), 钛基体区域硬度最低(276 HV). 辐照后钒基体和钛基体硬化明显, 界面I和II处的硬化程度较低. 界面I硬度增加了136 HV, 辐照硬化率为35%; 界面II硬度增加了55 HV, 辐照硬化率为10%; 钒基体硬度增加了208 HV, 辐照硬化率为62%; 钛基体硬度增加了247 HV, 辐照硬化率为90%. 从辐照硬化的角度来考虑, 辐照过程中产生的缺陷和析出相是引起材料硬化的重要因素[23—26], 因此接下来将对辐照缺陷和析出物分别进行讨论. 图 3 V-4Cr-4Ti/Ti界面及两侧基体区域的硬度分布 Figure3. Vickers hardness distribution across the interface of the V-4Cr-4Ti/Ti.
23.2.V-4Cr-4Ti/Ti界面的辐照缺陷 -->
3.2.V-4Cr-4Ti/Ti界面的辐照缺陷
图4所示为辐照后样品各区域在近似双束条件下辐照损伤最大位置附近的TEM形貌图, 由于TEM样品厚度均为100 nm左右, 因此可以直接以缺陷的表观面密度来反映缺陷密度. 从图4可以看出辐照之后钒基体和钛基体的缺陷密度要远高于界面区域, 且位错相互缠结严重, 形成了大范围的位错聚集, 而在界面处位错的密度较低而且分布较为均匀, 但位错尺寸较大. 图 4 V-4Cr-4Ti/Ti界面区域及两侧基体辐照后的TEM形貌图 (a) 钒基体; (b) 界面I; (c) 界面II; (d) 钛基体 Figure4. The TEM images of V-4Cr-4Ti/Ti after irradiation: (a) V-4Cr-4Ti; (b) interface I; (c) interface II; (d) Ti.
表1通过纳米压痕所得硬化实验值与采用DBH模型对辐照硬化进行的估算值 Table1.Experimental hardness values by nanoindentation and the estimated hardness values calculated by the DBH model
为确定辐照前后析出物的成分, 对析出物进行EDS成分分析, 结果如图7和图8所示. 可以看出辐照前样品只有很少的点状富钛析出物(如图7所示); 而辐照后钒基体中包含一些点状析出物组成的细长的针状富钛析出物(如图8所示); 界面I出现了一些棒状的Ti析出物和富V析出物(如图7和图8所示); 而界面II则主要为Ti的析出物(如图7所示). 界面I富V析出物的出现可能由于界面I靠近钒基体一侧, 钒含量较高, 导致了钒、钛在辐照过程中同时析出. 而两界面位置析出物的生成意味着在辐照过程中在界面I区和II区均发生了Ti的脱溶, 形成了Ti的相对富集. 图 7 辐照前后析出物EDS分析结果 (a) 辐照前界面; (b) 辐照后钒基体; (c) 辐照后界面I; (d) 辐照后界面II Figure7. EDS analysis of the V-4Cr-4Ti/Ti: (a) Interface before irradiation; (b) V-4Cr-4Ti after irradiation; (c) interface I after irradiation; (d) interface II after irradiation.
图 8 辐照后钒基体和界面I处析出物EDS面扫描分析结果 (a)—(d) 钒基体; (e)—(h) 界面I Figure8. EDS-mapping analysis of the V-4Cr-4Ti and interface I after irradiation: (a)?(d) V-4Cr-4Ti; (e)?(h) interface I.