摘要
相变晶体学是定量描述相变后材料组织的知识。双相不锈钢相变系统是具有表面浮突现象的扩散型相变,对其复杂的形貌和相变晶体学特征的定量分析,有助于人们加深对沉淀相变晶体学规律的理解,相变过程中的组织形貌为分析和控制组织提供工具。
本论文工作深入研究了00Cr25Ni7Mo3双相不锈钢合金系统奥氏体在铁素体基体表面沉淀析出的相变晶体学,综合应用扫描电子显微镜、背散射电子衍射、原子探针研究了表层奥氏体沉淀相的三维形貌和表面浮突现象,实验发现针状形貌是表面沉淀相的基本单元,复杂形貌的分枝均呈针状。应用透射电子显微镜对表层有代表性的3种针状沉淀相与基体的位向关系、长轴方向、界面取向等进行了表征。定量分析结果表明丰富多变的表面沉淀相的形貌及其晶体学特征与样品表面取向密切相关。
本工作将表面因素引入O线模型,提出表面约束判据。运用扩展O线模型结合位移分解的方法对3种针状沉淀相的晶体学做出了合理的定量解释,特别是解释了表面特有的现象比如两相中相关柏氏矢量偏离、界面位错与长轴方向偏离以及带有“缓冲层”的双惯习面现象等。扩展O线方法具有普适性,可以用于其它约束条件的相变系统中。
本工作的分析表明,在表层沉淀相与基体之间可以构成V型双惯习面,这是表层特有的三维形貌。双惯习面可以以不同的方式实现,例如孪晶对片条的基体/沉淀相1/沉淀相2/基体的结构,以及带有缓冲层片条的基体1/沉淀相/基体2的三明治结构。本工作还提出了基体倾转参与组织演化的模型,解释了“缓冲层”结构产生的原因,证实了相变系统在相变应力作用下由有理向无理位向关系转变的组织演化猜测。
奥氏体析出时会在表面形成单倾或者双倾性的表面浮突。本工作应用背散射电子衍射技术和原子探针技术定位观测了表面浮突的结构。发现双倾型浮突由一对呈孪晶关系的片条组成而单倾型的浮突是单晶片条。
Crystallography of the phase transformation is a key aspect for understanding the microstructure, and is essential to theory-based materials design. In the present work the phase transformation near surface of a duplex stainless steel is investigated for understanding the complex morphology. The theoretical model has been extended to give reasonable explanations of the observed crystallographic features. This in-depth study has enriched the knowledge of the precipitate crystallography.
3D morphology and the crystallography in 00Cr25Ni7Mo3 has been systematically measured with scanning electron microscope (SEM), electron backscattering diffraction (EBSD) and atomic force microscope (AFM). The basic unit of the precipitates transformed near the surface is for single needle as well as for complex dendrite shaped precipitates. The detailed crystallographic features of the needle shaped precipitates vary with the surface orientation. Three types have been identified, according to the orientation relationship (OR), the long axis and the interface orientation, as measured with transmission electron microscope (TEM): the OR for the first type is near the K-S OR, and the Burgers vector is the near parallel close packed direction (CPD), and a pair of twin precipitates is often observed; the second is also near K-S OR, while the Burgers vector is not the near parallel CPD; the third is near N-W OR.
Surface effect has been taken into consideration to modify the O-line model and a new criterion is proposed: the deviation of the invariant line from the surface should be minimized (Smin). The O-line solution of the Smin provide good explanation of the typical phenomena only observed on the surface, such as the deviation of the corresponding Burgers vectors, the deviation of the interfacial dislocation and the long axis, and the double habit plane with“buffer matrix”and so on. The“buffer matrix”was proposed to be the result of recovery in the local matrix nearby the interface, during the evolution of microstructure from a rational OR to the irrational OR.
The“double habit planes”structure is a unique phenomenon in the surface layer between the precipitates and the matrix, which can be achieved in different way: such as the structure matrix / precipitate 1 / precipitate2 / matrix of the twining laths and the the structure matrix1 / precipitate / matrix2 of the lath with“buffer matrix”. The precipitation of austenite on the surface is associated with the surface relief.
A tracking measurement combining EBSD with AFM reveals the structure of the surface relief: the double-tilt surface relief is related with the twinning austenite pair and the single-tilt surface relief is related with the single precipitate, no matter the OR is near K-S or near N-W.
引文
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