【作者】 黄亚敏；

【导师】 潘春旭；

【作者基本信息】 武汉大学 ， 凝聚态物理， 2010， 博士

【摘要】 材料的微结构与性能之间存在密切的关系。对材料微结构在高温下的“原位”观察,以及材料微区残余应力的测试等,对于研究材料的失效机理具有重要意义。但是长期以来,这方面的研究工作一直是研究热点,也是研究难点。近年来,随着材料测试表征技术和理论的不断发展和完善,为我们解决上述问题提供了可能。本论文综合应用电子背散射衍射(electron backscatter diffraction, EBSD)技术、纳米压痕(nano-indentation)技术和电子显微镜等,对在工程中广泛应用的奥氏体不锈钢及其焊接接头在1200℃超高温下的组织转变和焊接界面处微米级微区的残余应力分布等进行了系统的研究和分析,以期为奥氏体不锈钢在复杂工况下的应用和失效分析提供科学依据。全文共分九章。第一章为绪论,介绍了本工作的来源、意义和所做的主要工作,综述了国内外有关EBSD技术和纳米压痕技术在材料科学中的研究现状与进展。为了使读者更好阅读本论文,以及体现论文的系统性,第二章简要介绍了EBSD和纳米压痕技术的基础原理、设备结构、数据获取原理及可靠度分析。第三章为本文所用的实验材料和方法。详细介绍了奥氏体不锈钢材料及其焊接接头材料的金相试样实现方法,另外还对其形貌和微结构的表征技术以及性能测试手段进行了介绍。第四章重点介绍了一种新的“原位跟踪(in-situ-tracking)"观察和测量技术。与传统常规方法和严格的“原位”分析技术相比,“原位跟踪”观测法即能够实现对材料特定部位显微组织、化学成分和性能等的连续观察与研究,又不需要增加特殊的,甚至是昂贵的专用实验设备,为我们进行更多材料微结构-性能分析提供了有效手段。第五章利用SEM和EBSD“原位跟踪”观测技术,观察和研究了奥氏体不锈钢在1200℃超高温下的回复与再结晶过程,特别是研究了材料在高温下的晶粒取向以及晶界性质的转变过程。发现奥氏体不锈钢显微组织在1200℃超高温下的“回复与再结晶”过程较常规高温更快,这与材料中位错的快速运动和相互作用有关,晶粒长大符合位错模型机制。第六章研究了奥氏体不锈钢焊接熔池“液—固界面”处的结晶生长关系,从实验上证实了熔池结晶的竞争生长机制,即焊缝组织中形成<100>织构。另外,还通过EBSD“原位跟踪”观测还研究了焊接界面附近显微组织在超高温下的转变规律以及取向演变特征。第七章研究了奥氏体不锈钢在1200℃超高温下表面氧化膜的形成及其脱落机理。认为奥氏体不锈钢表面氧化膜具有多膜层结构,氧化环境及氧化膜—基体界面处的元素分布和扩散对氧化膜的结构与性能有直接影响：氧化膜的失效行为主要表现为氧化层中裂纹的产生和不致密氧化层的脱落。第八章采用具有高空间分辨率和测量精度的纳米压痕技术,首次实现了对奥氏体—铁素体异种钢焊熔合区附近微米级范围内的残余应力测量。结果显示,在焊态接头中,熔合区附近区域的残余应力整体为压应力,且最大压应力在熔合区中；焊后热处理能有效释放接头中残余应力,对母材热影响区组织最为明显。第九章是全文总结。最后简要的列出了作者在博士生期间的科研情况和公开发表的论文等。更多还原

【Abstract】 Generally, material microstructure is closely related with its property. "In-situ" observing the material microstructure during high temperature, and measuring micrometer-scale residual stress of materials etc., are important to analyze the failure mechanism of materials. For a long time, however, these studies are research hotspot, but also are research difficulties. In the recent years, with the development and improvement of material characterization techniques and theories, it is possible to overcome the above difficulties. In the present work, the microstructural transformation during 1200℃super-high temperature and the micrometer-scale residual stress at the weld interface of austenitic stainless steels and welded joints widely used in the engineering are examined systematically using electron backscatter diffraction (EBSD) technique, nano-indentation technique, and scanning electron microscopy (SEM) etc.. It is respected to provide scientifical basis to the application of austenitic stainless steels serviced under complicated conditions and its failure analysis.This dissertation consists of nine chapters. Chapter one is an introduction, in which the origin and support of the project, significance and the major author’s work are introduced briefly. The research status and development of both EBSD and nano-indentation techniques in materials science are reviewed.In order to better understand this dissertation, and reflecting the system of dissertation, in chapter two the fundamental knowledge, the system structure, the principle of data acquisition, and data dependability of EBSD and nano-indentation techniques are introduced briefly.Chapter three illustrates the experimental materials and methods used in this dissertation, on the emphasis of the metallographic samples preparation of austenitic stainless steels and welded joints. In addition, the characterization techniques for morphologies and microstructures, measurements of properties are also demonstrated.In chapter four, a novel "in-situ-tracking" approach for evaluating microstructural variations using SEM, EDS and EBSD is introduced. Comparing with the regular methods and the strict "in-situ" techniques, the present "in-situ-tracking" approach provides a possibility to achieve the examination in series on the variations of microstructures, compositions and properties at a fixed place during treatments simply and economically. It is an effective approach for studying more materials microstructure-property.In chapter five, the "in-situ-tracking" approach using SEM and EBSD is proposed to examine the "recovery and recrystallization" process of austenitic stainless steel, especially the grain orientation evolution and grain boundary variation, during 1200℃super-high temperature. It is found that comparing to the regular high temperature service (below 900℃), the present "recovery and recrystallization" process is accelerated due to dislocation fastened movement and intensive interaction. The grain growth mechanism still meets the well-accepted dislocation model of subgrain combination.In chapter six, the crystallographic relationship of the "liquid-solid" interface of the weld pool of austenitic stainless steel is investigated. It has experimentally demonstrated the reasonability of the competitive growth resulting from the solidification of weld pool. That is to say, the<100> texture has been formed in the weld metal. Additionally, the microstructural transformation and grain orientation evolution at the weld interface during high temperature are studied using the EBSD "in-situ-tracking" method.In chapter seven, the formation and failure mechanism of oxidation scale on the stainless steel substrate surface served at 1200℃with corrosion environment are analyzed. The results indicate that the scale on the stainless steel substrate surface consisted of poly-film layers, depending on the oxidation environment, and the elements distribution and diffusion at the scale-substrate interface. The formation of cracks in the scale and the fall off of the non-compact scale are the main problem at the failure behavior of stainless steel oxidation scale. In chapter eight, the nano-indentation technology with higher resolution and measuring precise is firstly adopt to measure micrometer-scale residual stress around the fusion bound. The results suggest that in the as-welded joint, the residual stresses around the fusion bound are entirely compressive, and the maximum compressive stresses occurrs in the fusion bound; after post-weld heat treated (PWHT), the compressive stresses around fusion bound are reduced by thermal stress relief, especially the heat-affected zone (HAZ).Chapter nine is the conclusions of all the research works mentioned in this dissertation. The last part of the dissertation lists author’s published papers finished during the doctorate study period.更多还原