【作者】 吴静；

【导师】 张文征；

【作者基本信息】 清华大学 ， 材料科学与工程， 2011， 博士

【摘要】 对板条马氏体的深入认识是许多高强度钢组织控制、设计钢成分和热处理工艺的重要理论基础。马氏体理论研究的一个重要方面是马氏体相变晶体学，这是理解马氏体形貌和相关性能的核心。本论文应用透射电镜(TEM)对Fe-20Ni-5.4Mn(wt%)合金中板条马氏体两相位向关系、惯习面、内部位错组态和界面结构开展系统地研究。采用菊池花样定量分析得到同一板条内两相位向关系不存在本征变化，平均位向关系为(111)f偏离(011)_b1.3°，[10-1]f偏离[11-1]_b1.6°。板条马氏体两侧界面一侧平直，另一侧不规则，这可能是由于相变过程中板条朝着不规则一侧界面移动导致的结果。通过双直立法和(或)迹线+直立法确定平直一侧界面平均取向为(0.380.390.34)f|(-0.010.470.43)_b。通过不同衍射矢量的中心暗场像确定板条马氏体内部存在4套<111>_b/2（近）螺位错，奥氏体内部观察到[011]f/2和[10-1]f/2两组（近）螺位错或具有较大螺位错分量的混合位错。界面上观察到一组间距为6~15nm的[10-1]f/2|[11-1]_b/2混合位错。应用热台原位TEM观察到马氏体逆相变过程中界面在~550C以台阶方式在板条一侧时断时续运动，最高速率达到0.79μm s-1。本工作还对板条马氏体表面浮凸进行系统地定量表征。采用原子力显微镜(AFM)结合背散射衍射(EBSD)观察到该合金中板条马氏体浮凸呈帐篷型。借鉴双面金相位移合成法合成单面样品浮凸的平均位移矢量为[0.370.43-0.82]f，最大切变角为29.8。应用AFM观察到最大浮凸角为22.4，小于合成得到的最大切变角，这可能由于惯习面不垂直于自由表面所致。马氏体表象理论被用于解释板条马氏体相变晶体学。当输入(-110)[110]_b切变系统时计算得到的结果与本文实验观察到的两相位向关系、惯习面取向和浮凸位移矢量方向和大小较为吻合。(-110)[110]_b可以等效为_bcc中两个常见滑移系(-110)_b[111]_b和(-110)_b[11-1]_b组合而成。本工作还基于二维倒空间O线性质和正空间位移分析，推导了界面含有一组平行矢量的不变线系统中各晶体学参量的解析表达式，可以用于解释前人在马氏体相变中所观察到的实验结果。更多还原

【Abstract】 In-depth knowledge of lath martensite is an essential theoretical_basis to thecontrolling of the microstructure of many high strength steels and designing ofcomposition of steels and heat treatment processes. An important aspect of theoreticalstudy of martensite is on the crystallography of martensitic transformation, as the corefor understanding the properties and morphology of martensite.In this study, transmission electron microscopy (TEM) has_been applied for theinvestigation of orientation relationship (OR), interfacial structure and internaldislocations of lath martensite in Fe-20Ni-5.4Mn(wt%) alloy. It was found that the OR_between the martensite and austenite is close to G-T OR. The average ha_bit plane is(0.380.390.34)f|(-0.010.470.43)_b. Four sets of pure screw <111>_b/2dislocations, and two sets of [011]f/2and [10-1]f/2(nearly) screwdislocations have_been identified within martensite and austenite, respectively.The interface consists of a single set of parallel mixed-type dislocations havingBurgers vector [10-1]f/2|[11-1]_b/2. The spacing of the interface dislocationsvaries from6.0to15nm. Frame-_by-frame analysis of the austenite/lathmartensite interface during in situ heating using TEM is used to providedirection information on the mechanisms of the martensite interface motion. Itreveals that the interface migrated on only one side of the lath_by a ledgemechanism displaying start-stop growth_behavior; the highest velocity o_bservedwas0.79μm s-1.The shape strain accompanying surface relief, such as the magnitude and directionof the displacement vector, has also_been investigated in a quantitative way. Themorphology of the relief was studied_by the optical microscopy (OM) and the atomicmicroscopy (AFM). The crystallographic orientations of the matrix grain and the lathwere measured_by the electron_backscattered diffraction (EBSD), respectively, whichwas used to determine the orientation of the ha_bit plane, and the OR. Com_bining thedata from EBSD and AFM, it is concluded that the relief is produced_by a single_bcccrystal, which exhi_bits a tent-shaped relief. Based on an EBSD analysis, the OR and theha_bit plane are consistent with those got_by TEM. The largest shear angle for the relief is calculated to_be29.8, and the directions of com_bined displacement vector arescattered around [11-2]_b. The o_bserved maximum surface title angle is22.4, which issmaller than the calculated value. Considering the ha_bit plane is not perpendicular to thepre-polishing surface, the measured smaller value of tile angles is reasona_ble.The phenomenological crystallographic theory of martensitic transformation has_been applied to lath martensite. Experimental o_bservations on OR, ha_bit plane, theinterface dislocations and the shape strain of surface relief are in agreement with thetheory_by using the lattice invariant shear (-110)[110]_b. This mode is seen to_beequivalent to two equal amounts of shears in [111]_band [11-1]_bdirectionscontained in (-110)_b, which are the most common deformation mode in_bccstructure.This study also provides analytical expressions of crystallographic features_based on derivations in_both reciprocal and direct spaces in two dimensions (2D)for the phase transformations involving an invariant line strain with a pair ofparallel Burgers vectors in the ha_bit plane. Compared to existing2D models,this approach is more straightforward and simpler, and could_be applied tointerpret results in martensitic transformation from earlier studies.更多还原