X射线衍射层析成像及其合金晶粒三维定量研究

发布时间：2018-03-06 10:29

  本文选题：X射线衍射层析成像　切入点：X射线三维成像　出处：《中国科学院研究生院(上海应用物理研究所)》2017年博士论文　论文类型：学位论文

【摘要】：硬多晶材料,例如金属、合金和陶瓷,构成了多数现代工业的基础。这些材料的物理、化学和机械性能在很大程度上取决于它们在晶粒尺度上的微结构及其相互作用。因此,对材料内微结构的表征成为了材料学研究领域的核心课题,同时,也是联系材料设计与制造工艺的直接桥梁。传统的结构表征方法在晶粒尺度上都存在一些局限性,例如:基于电子显微学的表征方法只能用于晶粒的二维分布研究,且需要事先对样品进行切片处理;基于实验室光源的X射线衍射方法主要用于表征平均结构特性;显微CT成像技术虽然是三维、无损的表征方法,但由于单相晶粒间的电子密度差异较小,从而难以获得有效的衬度,无法获得晶粒取向及三维空间分布信息。因此,迫切需要发展一种方法来实现材料内部晶粒的三维、无损、定量乃至动态微结构表征。本论文基于上海光源X射线成像线站建立了一种X射线衍射层析成像方法,可以用于材料内部晶粒的位置、尺寸、形貌、相、晶体学取向以及平均应变张量等信息的三维原位无损研究。本论文取得的创新性研究成果如下:基于上海光源X射线成像线站,成功搭建了X射线衍射层析成像实验平台,完成了软件和硬件的调试,并利用立方晶系和六方晶系等标准样品验证了该方法的可行性。系统研究了各种因素对衍射层析成像质量的影响,并提出了相关优化措施,具体包括:光子通量密度是造成精修操作中晶粒数量损失的主要原因;中值滤波可以有效抑制椒盐噪声的影响,改善寻峰的结果;优值方法可以大幅提高校准的精度,结合并行化运算可以提高计算效率;对比度增强方法可以提高近场图像的质量,改善寻峰和重建的结果。提出了一套不需要先验知识来识别晶体第二相的方法,不仅可以原位无损的对第二相进行识别,同时还可以获得各相晶粒的位置、尺寸、形貌和取向等信息。模拟研究和实验研究结果验证了该方法的可行性。模拟研究中建立了六个由不同晶系的主、次相组合的模型,结果表明该方法对于第二相的识别具有很好的普适性。实验研究中,成功识别出VI系铝合金样品中的未知第二相为γ-Fe相,电子能谱分析和X射线显微CT的结果进一步验证了识别结果的正确性。实验研究了合金晶粒信息与二次加热温度的关系。利用X射线衍射层析成像成功获取合金内部晶粒的三维定量信息,包括:空间分布、体积、表面积、球度、取向、晶界取向差和织构等。对不同二次加热温度下VI系铝合金样品的定量分析结果表明:当温度为640℃时,晶粒拥有最适合于触变成形工艺的尺寸、形貌和取向:较大的晶粒数量、小而均匀的晶粒尺寸、较高的球度和随机均匀分布的取向。
[Abstract]:Hard polycrystalline materials, such as metals, alloys and ceramics, form the basis of most modern industries. The physical, chemical and mechanical properties of these materials depend to a large extent on their microstructures at grain size and their interactions. The characterization of microstructure in materials has become the core subject in the field of materials science, and it is also a direct bridge between material design and manufacturing process. The traditional methods of structure characterization have some limitations in grain size. For example, the characterization method based on electron microscopy can only be used to study the two-dimensional distribution of grains, and the sample needs to be sliced in advance, and the X-ray diffraction method based on laboratory light source is mainly used to characterize the average structural characteristics. Although microscopic CT imaging is a three-dimensional, nondestructive characterization method, it is difficult to obtain effective contrast because of the small difference of electron density between single phase grains. Therefore, the information of grain orientation and three-dimensional spatial distribution can not be obtained. There is an urgent need to develop a method to characterize the three-dimensional, non-destructive, quantitative and even dynamic microstructure of the grains inside the material. In this paper, an X-ray diffraction tomography method is established based on the X-ray imaging line station of Shanghai Light Source. It can be used for the position, size, morphology and phase of grain in the material. Three-dimensional in situ nondestructive study of crystallographic orientation and average strain Zhang Liang. The innovative research results obtained in this paper are as follows: based on the X-ray imaging line station of Shanghai Light Source, an experimental platform for X-ray diffraction tomography has been successfully built. The software and hardware are debugged, and the feasibility of the method is verified by using cubic crystal system and hexagonal crystal system. The influence of various factors on the quality of diffraction tomography is systematically studied, and the relevant optimization measures are put forward. The results include: photon flux density is the main cause of grain number loss in finishing operation; median filter can effectively suppress the effect of salt and pepper noise and improve the result of peak finding; the accuracy of calibration can be greatly improved by the best value method. Parallel operation can improve the computational efficiency, contrast enhancement method can improve the quality of near-field images, and the results of peak finding and reconstruction can be improved. A method for identifying the second phase of crystals without prior knowledge is proposed. Not only can the second phase be identified in situ without damage, but also the position and size of each phase grain can be obtained. The results of simulation and experimental studies show that the method is feasible. In the simulation study, six models composed of main and secondary phases of different crystal systems have been established. The results show that the method has good universality for the recognition of the second phase. In the experimental study, the unknown second phase in VI aluminum alloy sample is successfully identified as 纬 -Fe phase. The results of electron spectrum analysis and X-ray microCT further verify the correctness of the identification results. The relationship between the grain information and the secondary heating temperature of the alloy is experimentally studied. The alloy is successfully obtained by X-ray diffraction tomography. Three dimensional quantitative information of internal grains, It includes: spatial distribution, volume, surface area, sphericity, orientation, grain boundary orientation difference and texture. The quantitative analysis results of VI aluminum alloy samples at different secondary heating temperatures show that: the temperature is 640 鈩，

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