2-25. Three-dimensional neutron imaging of hydrogen distribution in titanium alloys during thermal hydrogen processing

Lixia Yang, Danqi Huang, Linfeng He, Yaoqi Wang, Lei Zhao, Xuejing Shen, Yunhai Jia, Haizhou Wang

1. China Iron and Steel Research Institute Group, 2. University of Science and Technology Beijing, 3. China Institute of Atomic Energy, 4 Beijing Aviation Manufacturing Engineering Research Institute

Abstract: As lightweight and high strength structural materials, titanium alloys have broad application prospect in aerospace industry. Thermohydrogen processing (THP) offers an effective method for optimizing microstructure and improving the workability and mechanical properties of titanium alloys via high temperature hydrogenation treatment. However, study on hydrogen distribution in titanium alloys during THP remains unclear. It is of important academic and practical significance for the process control and material application to deep learning the mechanism of hydrogen distribution and microstructure evolution.

       Neutron imaging technology has been used to hot issue research on material defect determination, light element distribution characterization, phase transition and microstructure evolution, due to neutron features of strong penetrability, light element identification and nondestructive testing. In this work, the hydrogen distribution in titanium alloys during THP was observed by neutron tomography. The quantitative characterization method of hydrogen distribution in titanium alloys was introduced. Cold neutrons were provides by the research BER II to picture the samples with a spatial resolution in the reconstructed three-dimensional model of ~50 μm. Full field of view microstructure were characterized by high throughput SEM. Combined with deep learning and statistical analysis methods, quantitative characterization of microstructure was realized. The element distribution, microstructure evolution and corresponding mechanical properties were studied by EPMA, micro-XRD and micro-Vickers hardness test separately.

       Three dimensional hydrogen distribution of titanium alloys during THP by neutron tomography was showed in Fig.1. It revealed that hydrogen went into the specimens from every direction and hydrogen content decreased from the surface to center of specimens. Aluminum and vanadium, separately as stabilizing elements of α phase and β phase, redistributed due to different hydrogen contents. Alumium content increased and vanadium content decreased in β phase with increasing hydrogen content. Micro-XRD results displayed that the diffraction peak of α phase widened with increasing hydrogen content. It was attributed to the grain refinement of α phase. The peak of β phase moved to low-angle region due to hydrogen entered the interstitial position of lattice. Nanoscale TiH2 with FCC crystal structure was identified by EBSD in TKD mode. Compared with hydrogen free specimen, the micro-hardness of hydrogenated specimens decreased at first and then increased with rising hydrogen content. This is owing to the increasing of β phase content and formation of TiH2.

       In conclusion, three dimensional hydrogen distribution in titanium alloy during THP was characterizated by neutron tomography. Hydrogen content decreased from surface to center of the specimen. As hydrogen content increasing, Al content increased and V content decreased in β phase, in addition, β phase increased and TiH2 formed. It is of important significance for the process improving and applying to deep understanding the mechanism of hydrogen distribution and microstructure evolution.
Key words: Hydrogen distribution; Thermohydrogen processing; Neutron tomography; Statistical analysis; Compisition-microstructure-property relationship

图 1 热氢处理后钛合金中三维中子成像氢分布

Fig. 1 Three dimensional neutron imaging hydrogen distribution of titanium alloy during THP

    基于三维中子成像的钛合金热氢处理中氢分布研究

杨丽霞, 黄丹琪, 贺林峰, 王耀奇, 赵雷, 沈学静, 贾云海, 王海舟
1 . 钢铁研究总院;2 . 北京科技大学;3 . 中国原子能科学研究院;4 . 北京航空制造工程研究所 

摘要:钛合金作为轻量化高强结构材料，在航空领域具有广泛的应用前景。热氢处理技术通过 高温气相充氢为钛合金提供了一种优化组织结构、改善加工及服役性能的有效方法。值得注意 的是，关于热氢处理后钛合金中氢分布的研究尚不完善，深入研究钛合金热氢处理中氢分布及组织结构演变对于工艺的精准控制及材料的进一步推广具有重要的学术与应用价值。

      由于中子具有穿透性强、能分辨轻元素、无损等特性，中子成像技术已广泛应用于材料缺 陷探测、轻元素分布、相变结构演变及疲劳裂纹扩展等热点问题研究中。本文采用三维中子成 像技术表征了钛合金热氢处理中氢分布，建立了钛合金中氢分布的定量表征方法，该工作在德 国亥姆霍兹 BERII 反应堆的冷中子谱仪上开展，谱仪空间分辨率为 50 μm。采用高通量扫描电 镜对热氢处理后钛合金进行了全视场组织形貌表征，融合深度学习及统计分析技术，实现了组 织结构分布状态的定量表征。采用电子探针、微区 X 射线衍射技术及显微维氏硬度试验分别研究了热氢处理后钛合金的元素分布、组织结构及力学性能演变。

       热氢处理后钛合金中氢分布的三维中子成像结果如图 1。可以看出，热氢处理中氢从各个方向进入钛合金试样，且从样品表面至中心位置氢含量逐渐降低。钛合金中 Al 和 V 分别作为 α 相和 β 相的稳定化元素，随着氢含量的升高，β 相中 Al 含量升高而 V 含量降低，α 相中元素含 量变化不大。组织结构表征结果显示，随着氢含量的升高，β 相含量也升高，微区 XRD 衍射峰 向低角度偏移;而 α 相衍射峰发生宽化，这是由于 α 相晶粒发生细化。TKD-EBSD 结果显示形 成了纳米尺度 FCC 结构的 TiH2 氢化物。显微维氏硬度分析结果显示，随着氢含量的升高，硬度 先降低后升高。这是由于氢加入引起的 β 相含量的升高及氢化物形成导致的氢脆。

      综上所述，本文首次利用三维中子成像技术直接表征了钛合金热氢处理中氢分布:从样品表面至中心位置氢含量逐渐降低。随着氢含量的升高，钛合金中元素分布、相含量分布及相的 组成发生了明显的变化，并由此可调控钛合金的性能分布状态。钛合金热氢处理中氢分布及组 织性能状态的研究对于热氢处理工艺的改进及进一步广泛应用具有重要的意义。 
关键词:氢分布; 热氢处理; 三维中子成像; 统计分析; 成分-组织-性能相关性