薄片贝氏体微观组织形貌及其精细结构

利用透射电子显微镜研究了钢中贝氏体转变初期的微观组织形貌及精细结构。首次观察到贝氏体的薄片状形貌及｛１１１｝ｆ面缺陷。贝氏体具有应力应变促发形核特征,｛１１１｝ｆ面缺陷为母相奥氏体的塑性协调亚结构并可作为贝氏体铁素体／奥氏体的相界面。实验结果表明贝氏体初期转变可能具有滑移切变特征。     

钢中贝氏体长大方式的TEM研究

利用ＴＥＭ考察了钢中贝氏体铁素体／奥氏体相界面台阶结构及奥氏体精细结构。用ＴＥＭ温台发现在贝氏体增厚过程中新形成的贝氏体中存在奥氏体预存孪晶；在贝氏体铁素体／奥氏体相界面存在台阶结构，但台阶阶面可对应于母相奥氏体中孪晶面或层错面，表明台阶的阶面为共格的滑移界面。因而贝氏体铁素体／奥氏体相界面可通过界面台阶沿面缺陷进行保守滑移，贝氏体长大具有滑移切变特征。根据实验结果提出了贝氏体二次层错切变模型及组织形貌示意图。     

钢中贝氏体长大方式的TEM研究

利用ＴＥＭ考察了钢中贝氏体铁素体／奥氏体相界面台阶结构及奥氏体精细结构。用ＴＥＭ温台发现在贝氏体增厚过程中新形成的贝氏体中存在奥氏体预存孪晶；在贝氏体铁素体／奥氏体相界面存在台阶结构，但台阶阶面可对应于母相奥氏体中孪晶面或层错面，表明台阶的阶面为共格的滑移界面。因而贝氏体铁素体／奥氏体相界面可通过界面台阶沿面缺陷进行保守滑移，贝氏体长大具有滑移切变特征。根据实验结果提出了贝氏体二次层错切变模型及组     

贝氏体相变机制与块状转变

应用JEM-2010高分辨电镜和Quanta-400型环境扫描电镜,运用试验与综合分析的方法,研究了纯铁的块状转变和钢中的贝氏体相变.通过对相变的形核、长大,贝氏体亚单元和组织的形成的综合研究和分析,认为贝氏体相变与块状转变存在亲缘关系.依靠随机涨落,形成贫碳区,贝氏体铁素体在贫碳的奥氏体中形核.Fe原子和替换原子通过热激活跃迁、界面扩散或切变等方式,重复产生亚单元.在亚单元边界处的富碳奥氏体中析出碳化物,或成为残留奥氏体.贝氏体相变机制具有过渡性,即切变-扩散整合机制.     

多元微合金化空冷贝氏体钢

中碳和中高碳Ｍｎ－Ｓｉ－８系多元微合金化贝氏体钢，锻后空冷可获得以贝氏体或贝氏体／马氏体为主，含碳化物和少量残留奥氏体的组织。加入微量元素，使奥氏体晶粒和显微组织细化，形成的高硬度碳化物弥散分布，提高硬度，韧性和耐磨性。     

Cu—Zn—Al合金贝氏体及马氏体表面浮突的扫描隧道显微镜研究

首次用扫描隧道显微镜（ＳＴＭ）观察了ＣｕＺｎＡｌ合金的贝氏体和马氏体表面浮突，发现贝氏体浮突是由许多亚单元组成的浮突群，单个亚单元浮突呈“Ｖ”型，不同于马氏体相变不变平面应变的“Ｎ”型浮突，从而说明贝氏体不可能切变形成。同时在实验结果的基础上，提出了贝氏体激发形核－台阶生长的形成机制     

贝氏体相变的切变-扩散整合机制

运用试验与综合分析的方法,通过对贝氏体相变的形核、长大,贝氏体铁素体的亚结构、相变动力学的综合研究和分析,认为贝氏体相变机制具有过渡性,既非切变机制,也非台阶扩散机制,并提出了切变-扩散整合机制.依靠涨落、碳原子的扩散,形成贫碳区.贝氏体铁素体在贫碳的奥氏体中形核.系统自组织功能使相界处和奥氏体中的Fe原子和替换原子通过热激活跃迁、界面扩散或切变等方式,重复产生亚单元.在亚单元边界处的富碳奥氏体中析出渗碳体或ε-碳化物,也可以成为残留奥氏体.     

贝氏体和马氏体浮突的扫描隧道显微镜研究

本文首次用扫描隧道显微镜（ＳＴＭ）研究了Ｆｅ－Ｃ－Ｃｒ钢中贝氏体和马氏体浮突的形态，利用ＳＴＭ所独具的优异的纵向分辨率，发现贝氏体浮突实际上是贝氏体的亚片条，亚单元及超亚单元造成的表面浮突所组成的浮突群，完全不符合马氏体浮突的不变平面应变特征，说明了贝氏体相变不可能按切变机制进行。     

贝氏体铁素体取向关系及残留奥氏体膜

使用高分辨电子显微镜(TRTEM)观察分析了中碳和中高碳贝氏体钢贝氏体铁素体-奥氏体取向关系及贝氏体铁素体条束内残留奥氏体.贝氏体钢中上贝氏体-奥氏体界面同时存在K-S关系和N-W关系或单独存在K-S关系.标定结果证明,[100]α1∥[110]γ∥[111]α2,(011)α1//(111)γ∥(110)α2,以及[111]α∥[110]γ,(110)α//(111)γ.透射电镜观察发现,贝氏体铁素体内有形貌及尺度不同的残留奥氏体膜存在,它们把贝氏体铁素体分割或包围成不同层次的结构单元,以残留奥氏体膜为分界面确定了贝氏体铁素体不同层次的精细结构单元及其尺度.贝氏体铁素体条束由亚片条、亚单元和基本单元(或称为超亚单元)组成.多数亚片条,亚单元和基本单元尺度分别在25～80nm、25～80nm和50～30nm不等.     

贝氏体相变的过渡性特征

对35CrMo钢等多种材料奥氏体化后进行贝氏体等温淬火,得到少量贝氏体+马氏体+残留奥氏体的整合组织。应用JEM-2100透射电镜和QUANTA-400环扫电镜等设备观察了贝氏体组织的形貌和亚结构。综合分析了贝氏体相变的特征。研究表明,过冷奥氏体随着温度的降低具有转变贯序,贝氏体相变在组织形貌、亚结构方面具有过渡性。贝氏体相变动力学具有交叉性、重叠性、变温性和等温性的过渡性特征。贝氏体相变中铁原子和置换原子以非协同热激活跃迁方式位移,表明贝氏体形核长大具有过渡性。     

应力对钢中贝氏体相变的影响

外加应力使贝氏体相变形核率增大，等温孕育期缩短，即使所加应力远低于母相的屈服强度．由于钢中γ→α γ′的形核驱动力较大(约为kJ／mol数量级)，贝氏体相变的嘭胀应变能很小，过小的外加应力对形核率的影响甚微．考虑在外加应力的影响下，会使界面能量有所下降，也可能发生碳原子的再分布，偏聚在晶界或其它缺陷，甚至碳化物析出都会显著地增大形核率和缩短孕育期，有待进一步实验给予证明．无应力下，贝氏体相变动力学可以用Avrami的等温相变方程来表述；应力下则符合应力下铁索体及珠光体相变的动力学模型(经修改的Avrami方程)．形变奥氏体促发贝氏体相变，但随后会发生奥氏体的力学稳定化，其机制可能和马氏体相变时的奥氏体力学稳定化不完全相同，仅形变形成的位错阻碍贝氏体以一定位向长大，使相变动力学迟缓．贝氏体相变时奥氏体力学稳定化的模型有待建立。     

In situ observation and electron backscattered diffraction analysis of granular bainite in simulated heat-affected zone of high-strength low-alloy steel

In this study, the formation of granular bainite (GB) which may form in the heat-affected zone of double-sided double arc welding joint was in situ observed. The crystallographic characteristics of GB were also compared with those of microstructure mainly comprising lath bainite (LB). The results show that bainite packets exist at the initial stage of GB transformation and can be distinguished by the distribution of retained austenite and martensite–austenite constituents. The bainite blocks of GB are larger than those of LB which makes both the length and fraction of high-angle grain boundaries in GB less than those of LB which partly lead to the brittleness of granular bainite.     

Austenite to bainite phase transformation in the heat-affected zone of a high strength low alloy steel

The austenite to bainite phase transformation was investigated in a low alloy structural steel after simulated welding heat treatment, by means of light microscopy, electron backscatter diffraction and transmission electron microscopy. Upper bainite packets result from the growth of groups of laths having close crystallographic orientations but highly misoriented habit planes. Self-accommodation of the transformation eigenstrain was evaluated for various bainite configurations using a micromechanical model. The observed pairs of variants seem to help limiting plastic strain in the austenite phase, thus enhancing growth of the bainite phase during cooling.     

Study of the isothermal transformation of ductile iron with 0.5% Cu by electrical resistance measurement

A computer-controlled system for measuring electrical resistance has been developed and used to study the isothermal transformation of austenite in a ductile iron (3.31 % C, 3.12 % Si, 0.22 % Mn, 0.55 % Cu). The ability of the technique to follow the isothermal decomposition of austenite was established by measurements on an AISI4340 steel. The times at which the austenite decomposed to primary ferrite, pearlite, and bainite were accurately detected. In the ductile iron, the formation of pearlite and of bainite was easily detected, and an isothermal transformation diagram was constructed from the results. The temperature range for the formation of bainite is especially important in producing austempered ductile iron (ADI) and was mapped. An initial stage of decomposition of austenite to ferrite and high-carbon austenite is followed by a time delay; then the high-carbon austenite decomposes to bainite. The formation of ADI requires austempering to a structure of ferrite and high-carbon austenite, then quenching to retain this structure, thus avoiding the formation of bainite. This is achieved by isothermal transformation into the time-delay region. For the ductile iron studied here, this time region was about 2.6 h at 400 °C and increased to 277 h at 300 °C.     

A study of microstructure,transformation mechanisms and correlation between microstructure and mechanical properties of a low alloyed TRIP steel

Differently heat treated samples of a low alloyed TRIP steel have been investigated using electron diffraction techniques in SEM and TEM. Aim was, first, to discriminate the microstructure constituents, austenite, ferrite, bainite, and martensite, second to gain information on the γ–α phase transformation mechanisms and third to correlate the mechanical properties and the microstructure of the samples. Bainite always occurs in conjunction with an orientation gradient in the surrounding ferrite matrix. It consists of fine lamellae of ferrite and austenite which show a sharp Kurdjumov–Sachs orientation relationship with each other. This was interpreted in terms of a displacive bainite formation mechanism. The microstructure is formed by growth of γ-grains during intercritical annealing and shrinking of these grains during the subsequent cooling without nucleation of new α-grains. The transformation first occurs reconstructively into ferrite and then, at lower temperature, displacively into bainite. The mechanical properties of differently heat treated samples are most strongly influenced by the amount and distribution of carbon in the retained austenite and by the degree of recovery in bainite and austenite.     

TiNbV微合金钢焊接接头HAZ晶粒长大及相变原位观察

采用激光共聚焦显微镜原位观察方法,研究了大热输入用TiNbV微合金钢在模拟焊接热循环作用下焊接热影响区(HAZ)晶粒长大过程及相变的规律. 热循环过程中加热温度升高至860 ~ 980 ℃时,发生由铁素体和珠光体向奥氏体的转变,1 100 ℃时,奥氏体晶粒开始有明显长大的趋势,1 300 ~ 1 400 ℃时,晶粒以合并长大方式迅速长大;冷却过程中温度降低至1 400 ~ 1 350 ℃时,晶粒以晶界迁移方式缓慢长大,660 ~ 580 ℃时,发生奥氏体迅速向贝氏体转变,焊接HAZ主要由贝氏体与铁素体组成,贝氏体的尺寸是由奥氏体晶粒大小决定的. 热循环高温停留时间延长,奥氏体与贝氏体的形成、终了、转变温度区间均有下降. 结果表明,组织中先共析铁素体含量先降低后增加,贝氏体含量降低,多边形铁素体消失,先共析铁素体含量增加,冷却组织趋于均匀粗大. 焊接过程中,选择合适的高温停留时间可提高组织中IAF的含量,提高力学性能.     

LEDGE STRUCTURE ON BROAD FACES OF FERRITE PLATES IN LOWER BAINITE

The ferrite/austenite interfacial structure in lower bainite has been studied in ahypereutectoid steel by means of electron microscopy.It is found that the mobile growthledges and superledges and their three-dimensional morphologies exist on the broad faces offerrite plates.Structural ledges were also observed on the broad faces of lower bainite.Basedon TEM observations,the viewpoint that the growth ledges can also evolve from structuralledges is proposed.These observations provide evidences to the ledge growth mechanism oflower bainite.     

合金元素对钢热处理的影响

分析了合金元素对奥氏体的形成速度和奥氏体晶粒大小的影响,对珠光体转变的形核率和核长大速度的影响,对贝氏体转变开始温度Bs及转变速度的影响,对马氏体转变温度Ms和马氏体分解的影响,对碳化物形成和二次硬化效应的影响。     

MIDRIB AND LEDGE OF BAINITE IN A Cu-Zn-Al-Mn ALLOY

The bainitic transformation in a Cu-Zn-Al-Mn alloy has been examined with TEM.The re-sults show that the stacking faults and also the bainitic midrib are found at the beginning ofbainite formation in the alloy.The stacking fault planes can pass continuously through themidrib in bainite.The growth ledges occur at the broad faces and rims of bainite.The broadand narrow faces of th ledges are parallel to the fault plane and habit plane respectively.Boththeir Miller indices are {110}_(B2).The moving direction of ledges is parallel to the fault plane.It may be deduced that the bainite in alloy are initially formed by shear and the process ofgrowth are go verned by propagation o f fault planes.