晶界网络特征分布对奥氏体不锈钢晶间应力腐蚀开裂影响的三维显微研究

Intergranular stress corrosion cracking (IGSCC) is a major failure form of austenitic stainless steels during service in nuclear reactors. Grain boundary (GB) engineering is a potential technique to improve the IGSCC resistance of austenitic stainless steels because of the formation of high proportion of low-∑ (∑≤29) coincidence site lattice (CSL) grain boundaries which have higher SCC resistance. However, many results shown that the improvement of IGSCC resistance after GB-engineering is less than ideal. A possible reason is that the GB-network was not optimized in spite of the formation of high proportion of low-∑ CSL boundaries after GB-engineering. The GB-network must be studied in three-dimensional (3D) space. Now the GB-network was studied less due to the difficulty of 3D-characterization. In this project, a GB-engineered austenitic stainless steel sample and the sample after SCC will be characterized by using 3D-EBSD (electron backscatter diffraction) and 3D-OM (optical microscope). Subsequently, the quadruple-junctions, grain-clusters and connectivity of random boundaries will be investigated to reveal the characteristics of 3D GB-network of GB-engineered material. The 3D GB-network along the paths of intergranular cracks after SCC will be investigated to reveal the effects of GB-network characteristics on IGSCC. Eventually the 3D model that IGSCC propagates along GB-network would be constructed. These results will not only provide scientific and theoretical basis for GB-engineering technique but also provide technical support for service performance improvement of austenitic stainless steels in nuclear reactors.

晶间应力腐蚀开裂是核反应堆用奥氏体不锈钢长期服役过程中发生的主要失效形式之一。通过晶界工程技术在材料中形成大量耐腐蚀的低∑重位点阵(CSL)晶界，并优化低∑CSL晶界在晶界网络中的分布，有可能显著提高奥氏体不锈钢的抗晶间应力腐蚀开裂能力。但目前对晶界网络特征分布认识不足，晶界工程处理没能达到预期效果。晶界网络问题只有在三维空间中才能被研究清楚。本项目将使用三维显微表征技术(3D-EBSD和3D-OM)对晶界工程处理的奥氏体不锈钢及其在压水堆环境中进行应力腐蚀开裂后的试样进行表征，研究四叉界角、晶粒团簇和随机晶界网络连通性，揭示晶界工程处理材料的晶界网络特征分布；对沿应力腐蚀裂纹的三维晶界网络进行研究，揭示晶界网络特征分布对晶间应力腐蚀开裂的影响，建立晶间应力腐蚀开裂沿晶界网络扩展的三维模型。本研究成果能够丰富晶界工程理论，为利用晶界工程技术提高奥氏体不锈钢在反应堆中的服役性能提供技术支持。

晶界是奥氏体不锈钢的重要显微组织特征，对材料性能有重要影响，尤其是在高温和侵蚀性溶液环境下，晶界是材料性能的薄弱位置，这也是核反应堆用奥氏体不锈钢长期服役过程中容易发生晶间应力腐蚀开裂(IGSCC)的主要原因。已有研究发现，晶界工程(GBE)技术通过在材料中引入大量低∑重位点阵(CSL)晶界，能够提高材料的抗晶间破坏能力，原因被归结为：高比例的低∑CSL晶界、大尺寸的晶粒团簇和打断随机晶界网络连通性，但是这些结论都是基于对材料的二维截面组织观察得出的，并没有充分了解材料的三维晶界网络特征。. 本项目以奥氏体不锈钢316/316L为研究对象，通过GBE技术制备出含有大量低∑CSL晶界(>70%)的大尺寸样品，在模拟轻水堆冷却剂环境中开展SCC试验，利用EBSD和3D-EBSD技术分析GBE处理前后材料的晶界网络特征及对IGSCC扩展路径的影响规律。得出GBE处理材料的三维晶界网络特征：①形成了一些形貌复杂的超大尺寸孪晶界(∑3)，孪晶界的面积比得到显著提高，但数量比并没有得到显著提高；②含有2个孪晶界的三叉界角和含有2-3个孪晶界的四叉界角比例得到一定程度提高；③形成了一些形貌复杂的大尺寸的晶粒团簇，随机晶界网络路径变得更加复杂曲折。对沿IGSCC裂纹的晶界网络特征进行三维分析，得出GBE技术提高材料的抗IGSCC的机理模型：①∑3晶界的抗晶间开裂敏感性明显低于随机晶界（约1.5%），∑9和∑27晶界也表现出较低的开裂敏感性；②三叉界角和四叉界角的开裂模式受所含孪晶界数量与排布的影响，含有2个孪晶界的三叉界角和含有3个孪晶界的四叉界角能阻止裂纹穿过，含有2个孪晶界的四叉界角也表现出很强抗开裂能力；③大尺寸晶粒团簇是GBE处理提高材料抗晶间开裂能力的重要原因，尽管GBE处理没能阻止IGSCC扩展，但形成了大尺寸的形貌复杂的晶粒团簇，晶间开裂只能沿晶粒团簇边界蜿蜒曲折向前扩展，有时路径太过曲折而无法扩展，形成裂纹桥，提高了材料的抗晶间开裂能力。本研究成果能够丰富GBE理论，为利用GBE技术提高奥氏体不锈钢在反应堆中的服役性能提供技术支持。