魏氏渗碳体相变造成的长程应变场的表征和建模

Widmanstätten precipitates are commonly observed in many metallic structural materials of technical importance, such as steels and titanium alloys. Their formation is often accompanied with a surface relief effect, indicating a long-range strain field due to the precipitation transformation. This strain field is an essential input for the computation or simulation of the related microstructure. However, quantitative description of this strain field remains a long-standing unsolved problem in the study of precipitation transformations. In order to develop a theoretical method for quantitative description of the long-range strain field around the Widmanstätten precipitates, this project will make an in-depth investigation of proeutectoid Widmanstätten cementite in a high manganese steel. The experimental work includes the applications of scanning electron microscope (SEM) combined with atomic force microscope (AFM) to measure the surface relief of the Widmanstätten cementite precipitates. This measurement will provide the experimental data of the long-range strain field carrying crystallographic information. Transmission electron microscope (TEM) and SEM will be utilized for quantitative characterization of the transformation crystallography and interfacial structure, to provide basic data for the study of the interface migration. Molecular dynamics and the approach of phase-transformation-field decomposition will be employed to analyze shear-coupled interface migration and to calculate the corresponding long-range strain field. By integration of experimental and computational investigations, a reliable model will be built, which should be quantitatively consistent with the observations of the surface relief effect and other experimental results of Widmanstätten cementite. The results of this project will not only enrich quantitative knowledge about Widmanstätten cementite, but also improve the understanding of the surface relief effect associated with Widmanstätten precipitates in a general system and promote the development of the phase transformation theory.

魏氏沉淀相存在于许多重要金属结构材料中(例钢和钛合金)，其形成过程往往伴随表面浮凸的产生，说明该相变会造成长程应变场。这个应变场是组织计算模拟的重要输入，但关于它的定量描述是沉淀相变研究中长期未解决的问题。本项目旨在通过对高锰钢中先共析魏氏渗碳体的深入研究，发展定量描述魏氏沉淀相周围长程应变场的理论方法。实验研究将采用扫描电镜和原子力显微镜相配合来测量魏氏渗碳体的表面浮凸，获得携带晶体学信息的长程应变场的实验数据。还将应用透射和扫描电镜对魏氏渗碳体的相变晶体学和界面结构进行精确表征，提供界面迁移建模的基本数据。采用分子动力学模拟和相变应变场分解理论，定量分析界面迁移-切变耦合过程，计算伴随界面迁移的长程应变场。结合实验和计算结果，建立能够与魏氏渗碳体表面浮凸等实验结果定量吻合的模型。研究结果不仅将丰富关于魏氏渗碳体的定量认知，也将促进对一般魏氏沉淀相表面浮凸的理解，并将推动相变理论的发展。

本项目系统地研究了高锰钢中先共析魏氏渗碳体的形貌、相变晶体学、界面迁移模式、表面浮凸效应，提高了对化合物沉淀相形貌和相变晶体学的深入认识。利用透射电子显微镜和扫描电子显微镜表征了渗碳体的形貌、晶体学和界面结构，发现魏氏渗碳体以片状和板条状形貌为主，其中片状渗碳体与奥氏体保持精确的Pitsch位向关系，板条状渗碳体所对应的位向关系分散在F-E位向关系附近。片状渗碳体的宽面主要是由惯习面和不规则生长台阶构成，惯习面上含周期性分布的二次位错，表征了生长台阶和位错的柏氏矢量。原位透射电镜揭示了片状渗碳体宽面的迁移是通过生长台阶的连续移动实现，并且宽面延伸的尖端会向奥氏体基体不断释放位错。板条状渗碳体的较宽界面上存在不规则分布的台阶以及取向基本平行的位错，定量表征了位错的线方向及柏氏矢量。同时，还发现了前人未曾报道的位向关系，定量表征了界面上两组位错。基于择优界面结构奇异性的法则，建立了一个分析择优位向关系和择优界面的集成几何分析方法，包括在倒易空间根据g和 Δg 列分布考查潜在的择优位向关系，在正空间进行GMS团簇分布的图像分析、应用CSL/DSCL模型和O点阵理论计算错配应变场、不变线和二次位错结构。根据这个方法完善了对Pitsch位向关系下惯习面取向、宽界面上位错结构和生长台阶实验结果的定量分析与解释，并统一解释了F-E和新位向关系下界面择优取向和位错结构。结合聚焦离子束技术、扫描电子显微镜和原子力显微镜等方法测量了伴随片状渗碳体形成的表面浮凸，根据界面结构模型，估算界面迁移伴随的长程应变场，初步解释了表面浮凸的实验结果。

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