【作者】 叶飞；

【导师】 张文征；

【作者基本信息】 清华大学 ， 材料科学与工程， 2004， 博士

【摘要】 本论文工作应用透射电子显微镜系统地研究了Ti–7.26 wt%Cr合金中β(bcc)/α(hcp)相变的晶体学现象,包括位向关系、三维形貌和位错结构。综合应用光学显微镜、扫描电子显微镜、原子力显微镜研究了α相析出时发生的表面浮突现象。还运用O点阵理论并结合该理论开拓了近重合位置(NCS)模型,建立了三维界面位错结构模型,合理地解释了观察结果。本工作精确测量了位向关系相对于理想的Burgers位向关系的偏离,以及各个界面的取向和位错方向。惯习面垂直于一系列的Δg矢量,含有一套平行排列的位错,间距约为11 nm,其Burgers矢量为[1 –1 1]β/2([2 –1 –1 3]α/6),用这个Burgers矢量可以更好地解释塑性变形时体内和界面位错之间的位错反应现象。侧面含有两套平行排列的位错,方向与惯习面位错方向相同。一套间距约为9.4 nm,其Burgers矢量为[1 0 0]β([2 –1 –1 0]α/3)。另一套间距约为1.8 nm,Burgers矢量为[1 1 1]β/2([1 1 –2 0]α/3)。端面上位错网不是由惯习面和侧面上的位错简单环绕相交形成,而是会产生新的位错。惯习面的位错与侧面上大间距位错和小间距位错在端面上发生位错反应形成位错的Burgers矢量分别为[1 1 –1]β/2([2 –1 –1 –3]α/6)和[0 1 0]β([0 1 –1 –1]α/2)。在实验结果的基础上,构造了α相的三维形貌和位错结构模型。本工作运用O线模型定量解释了位向关系和惯析面位错结构。结合O线模型,发展了NCS模型,用水纹图对NCS团簇的分布进行了定量分析,建立了可能的侧面位错结构模型。本工作还建立了无理界面原子尺度的台阶–弯折结构模型,并构造了惯习面和侧面的台阶–弯折结构。本工作发展和应用的模型和方法具有普适性,可以用于其他相变系统的晶体学分析。α相析出时在样品表面会形成单倾或双倾型的表面浮突。本工作应用背散射电子衍射技术测得产生浮突的位移在不变线方向附近。在相变晶体学研究结果的基础上应用相变位移场分解模型对表面浮突的产生进行了分析。模型对单倾型浮突有较合理的解释,但是现有模型仍不能对双倾型浮突进行圆满的解释。更多还原

【Abstract】 The crystallography of proeutectoid α (hcp) precipitates in β (bcc) matrix in a Ti–7.26 wt% Cr alloy has been studied systematically using transmission electron microscope. The orientation relationship (OR), three-dimensional (3D) interfacial structures were examined. The surface relief effect associated with α precipitates were also studied using optic microscope, scanning electron microscope and atomic force microscope. The O-lattice theory and the near coincidence sites (NCS) model which was developed based on the O-lattice theory, were used to explain the experimental results.The OR, interface normal and dislocation direction were measured precisely. The OR was found to deviate slightly from the ideal Burgers OR. The habit plane was found to be normal to a particular set of Δg’s. A set of dislocations, about 11 nm apart, were observed on the habit plane of the precipitates. The Burgers vector of the dislocations is [1 –1 1]β/2([2 –1 –1 3]α/6). The dislocation reaction during plastic deformation could be explained by this Burgers vector. Two sets of dislocations, which are parallel to the dislocations on the habit plane, were observed on the side facet. One set of dislocations, about 9.4 nm apart, have a Burgers vector [1 0 0]β([2 –1 –1 0]α/3) which has not been determined by previous investigations. The other set of dislocations, about 1.8 nm apart, have a Burgers vector [1 1 1]β/2([1 1 –2 0]α/3). The dislocations on the habit plane and side facet loop around the α plate and form the network of dislocations on the end face. Another kind of dislocations was observed due to the dislocation reaction between the dislocations on the habit plane and the dislocations on the side facet in larger spacing or in smaller spacing. The Burgers vector of it is [1 1 –1]β/2([2 –1 –1 –3]α/6). Based on the observation, a 3D model of morphology and dislocation structures of α plate was constructed.The observed OR and dislocation structure on the habit plane was explained by an analytical O-line model. Based on the O-line model, NCS <WP=6>model was developed and the positions of NCS clusters were defined quantitatively by Moiré planes. Then the dislocation structure on the side facet could be explained based on the developed NCS model. A step-kink structure of irrational interface on atomic scale was constructed. Then, the step-kink structures of habit plane and side facet were predicted. Based on the NCS model, from the slabs near the interfaces, the atomic step-kink structures were observed and consistent with the prediction. The models and methods developed and used in the present work could be readily extended to analyze the crystallography of other phase transformation system.Single and double tilt surface relives were both observed. The displacement vector for surface relief was determined to be near the invariant line direction using electron back scattering diffraction method. Based on the results of the crystallography, the surface relief effects were analyzed by the model of decomposition of the transformation displacement field. The single tilt surface relief could be explained by this model. However, the double tilt surface relief could not be explained by the existing models.更多还原