3-13. Application of the Synchrotron radiation based μ-XRF, μ-XRD and μ-XANES integrated technology-High-throughput characterization on alloys for molten salt environment

Xiang-Xi Ye, Li Jiang, Zhijun Li*

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201800

Abstract: Synchrotron radiation has the advantages of high brightness, wide energy spectrum, high collimation, high penetration, short pulse, which is in accordance with the characteristics of high-throughput characterization, i.e. multi-scale and multi-performance data of one sample are measured at the same time. The synchrotron radiation based μ-XRF, μ-XRD and μ-XANES techniques are suitable for in-situ characterization with high spatial resolution at the micrometer scale. Traditional synchrotron radiation based μ-XRF, μ-XRD and μ-XANES cannot be used at the same time, which therefore cannot guarantee the consistency of the measured region. In 2018, we developed a set of high-precision multi-dimensional sample control platform (patent application number: 201811348861.3, 201811348863.2) at the hard X-ray micro-focusing beamline (BL15U1) at Shanghai Synchrotron Radiation Facility (SSRF, China). Through preliminary studies, the integrated characterization technique can simultaneously characterize the chemistry, microstructure (phase) and chemical valent state of the same local region (2×5μm²) of a sample, improving the measurement efficiency by more than 100%.

Recently, we applied this characterization technique to investigate the chemistry, phase structure and valence state of nickel-based superalloys, stainless steels and bimetal composite plates which are the candidate structural alloys for high temperature molten salt environment (including molten salt and atmosphere), to reveal the structural evolution and corrosion mechanism.

1. We applied the integrated characterization technique, combined with SEM-EDS、XRD、EPMA, to investigate the oxidation of Ni-(5–30 wt.%) W-6 wt.% Cr alloys at 850 °C in air, among which Ni-25W-6Cr alloy shows the best oxidation resistance. Because W acts as secondary getter, an external continuous oxide scale (NiCr₂O₄) is formed, which is responsible for the excellent oxidation resistance of these alloys. When the W content reach above 25 wt. %, the excessive W can facilitate the growth of CrWO₄, preventing the formation of NiCr₂O₄, breaking the continuity of the chrome oxide scale and creating “breakaway” for the diffusion of oxygen and metallic ion. The existence of CrWO₄ has not been mentioned in the previous studies on the oxidation of Ni-W-Cr alloys. In addition, the valence state of W in the oxide is aliovalent (non-six-plus), which is between 4+ and 6+, suggesting the WO_x (x=2–3) should be formed in the initial oxidation of W by reacting mixtures of W+WO₃. This indicates the excessive W can also change the valent state of the W which may affect the oxidation behavior.

2. We investigated the corrosion of carburized 316ss, as well as 316ss, in molten NaCl-KCl-MgCl₂ salts at 700 °C for 400h. Carburized 316ss show much severer intergranular corrosion than 316ss, which is caused by the Cr-rich M₇C₃ carbides. Almost all the carbides are M₇C₃ ones in the outermost 50 μm alloy surface and a mixture of M₇C₃ and M₂₃C₆ carbides distributes in the inner alloy bulk. It should be noted that the M₂₃C₆ carbides were measured using μ-XRD, which were not detected by SEM-EDS and EBSD at lab. (Corrosion Science under review)

3. The microstructure, chemistry and lattice constant of the 316H-GH3535 composite plate after thermal fatigue of 500 cycles (700 °C to room temperature, simulating molten salt reactor start-stop) were characterized. Obvious cracks were found at the interface between 316H and GH3535. The μ-XRD results show obvious lattice distortion exists at the interface between 316H and GH3535, and the residual stress at the interface is the source of cracking. Due to the large strain, the same area cannot be resolved by EBSD.

The synchrotron radiation based μ-XRF, μ-XRD and μ-XANES integrated technology developed in this work enables high-throughput characterization of the chemistry, microstructure (phase) and chemical valent state, and therefore rapidly obtain the composition-structure-property relationships. It provides a basis for revealing the degradation mechanism of materials and design of new materials.

Keywords: High-throughput characterization; Synchrotron radiation; Molten salt corrosion; Oxidation

同步辐射微束荧光、衍射和吸收谱的集成表征技术的应用-高温熔盐环境用合金材料的高通量表征

叶祥熙，蒋力，李志军*

中国科学院上海应用物理研究所，上海，201800

摘要：同步辐射具有高亮度、宽能谱、高准直、高穿透、短脉冲等优势，极为符合高通量表征的特征，即一次测量快速同时获取样品的多尺度的多种性能数据。其中同步辐射的微束荧光、微束衍射和微束吸收谱技术适合于微米尺度的高时空分辨率的原位表征。为了解决传统同步辐射微束荧光、衍射以及吸收谱三种方法只能逐一使用耗时且无法保证三者表征的区域的一致性的问题，我们于2018年在上海光源硬x射线微聚焦线站（BL15u1）上自主研制一套高精度多维度样品控制平台（专利申请号：201811348861.3、201811348863.2），解决了同步辐射微束荧光、衍射、吸收谱表征过程中的时间同步性和空间一致性问题，通过初步的实验表征，该集成表征技术可多参量的表征样品同一局部区域（2×5μm²）的组织成分、相结构以及化学价态等性能数据，将原先同时应用同步辐射微束荧光、衍射和吸收谱三种方法的使用效率提高100%以上。

最近我们采用集成表征技术对镍基高温合金、不锈钢以及双金属复合板在高温熔盐环境（包括熔盐和大气）环境下服役后的合金成分、相结构以及价态等进行了一系列表征，揭示其在高温腐蚀和热应力的综合工况之下组织结构演化和腐蚀机理。

1. 采用集成表征方法，结合SEM-EDS、XRD、EPMA等方法，系统表征了Ni-（0-30 wt.%）W-6Cr合金在850 ^oC高温氧化100 h后表面形成的氧化物分布，发现Ni-25W-6Cr合金表现出最佳的耐氧化性能，在W含量低于25 wt.%时，Ni-xW-6Cr合金的抗氧化性能随W含量增加而增强是因为W引起的第三组元效应导致氧气的内扩散减弱从而促进Cr的外氧化形成致密的Cr₂O₃和NiCr₂O₄；而过量的W（30 wt.%）会竞争消耗Cr₂O₃抑制NiCr₂O₄形成，同时其形成的疏松的CrWO₄可以在氧化层中为氧气和金属离子的扩散创造通道，并且CrWO₄也是首次在Ni-W-Cr合金氧化反应中被发现。同时发现W的氧化物的价态并且+6价，而是介于+4价和+6价之间，这也可能影响合金的氧化行为（Corros. Sci., 149 (2019) 87-99）。

2. 对渗碳以及未渗碳316H不锈钢在700℃高温熔融氯盐（NaCl-NaCl-MgCl₂）腐蚀400h后的样品进行了表征，发现渗碳后316H不锈钢的晶界Cr被严重腐蚀，这是由于晶界处富Cr的C₇C₃和Cr₂₃C₆被严重腐蚀导致的，Cr₇C₃主要出现在不锈钢表面50-100μm区域，在合金的内部以Cr₇C₃和Cr₂₃C₆在晶界处混合分布为主。而采用实验室XRD仅能发现腐蚀后的渗碳316H存在Cr₇C₃型碳化物，少量的Cr₂₃C₆无法被表征出来。

3. 对316H-GH3535复合板热疲劳500次循环后（700℃至室温，模拟熔盐堆启停）的样品的组织成分以及应力（应变）进行表征，发现在316H和GH3535的界面处有明显的裂纹，微区荧光成分和微区X射线衍射表征发现在316H和GH3535的界面处有明显的晶格畸变，界面处的残余应力是产生裂纹的原因之一。而采用EBSD对同一区域进行表征，由于该区域应变大，出现无法解析的情况。

本工作发展的微束同步辐射荧光、衍射和吸收谱的集成光子表征技术，能够实现对材料的成分、组织结构及元素价态的高通量表征，从而能够快速建立材料成分-结构-性能映射关系，为揭示材料的失效机理以及新材料的设计提供依据。

关键词：高通量表征；同步辐射；熔盐腐蚀；氧化