准下贝氏体的孪晶及形成机制

透射电镜实验证实在含硅钢准下贝氏体组织中存在孪晶亚结构,贝氏体中脊亦存在孪晶。中脊与残余奥氏体孪晶是贝氏体铁素体切变共格长大时协调均匀切变形成的变形挛晶。     

下贝氏体中碳化物的析出

通过3组实验证实了下贝氏体碳化物是在α/γ界面的奥氏体一侧析出,并向奥氏体中生长,同时也在贝氏体亚单元之间析出.在形态上,碳化物存在于铁素体内,也可跨越α/γ或α/α界面;在整个相变过程中,铁素体与碳化物的长大呈相互竞争机制,碳化物从α/γ界面的γ一侧析出,但铁素体的长大速度远高于碳化物,二者长大的结果是铁素体将碳化物包围,导致碳化物似乎是由铁素体中析出的假象;用热力学及台阶理论对实验结果做了分析,在实验证据及理论分析的基础上,提出了下贝氏体碳化物的析出及长大模型;下贝氏体碳化物是由富碳残余奥氏体中析出而不是由碳过饱和的铁素体中析出,其实质是确定新形成的贝氏体铁素体中是否含有过饱和碳,这直接涉及贝氏体相变机制的类型.     

贝氏体相变理论新进展及学术分歧

介绍了贝氏体相变领域切变和扩散控制台阶长大理论的核心内容：论述了贝氏体机变理论的主要研究成果及争论焦点：涉及贝氏体定义，转变不完全性，溶质拖蓝及类拖效应，表面浮突的形态，特征及本质，相变产物的晶体学，贝氏体铁素体初始碳含量，贝氏体“中脊”，下贝氏体碳化物来源及形成模型等。     

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。     

Fe－C合金贝氏体铁素体的相变基元与类马氏体形貌贝氏体的形成

本文从多方面研究观察再次证实贝氏体铁素体相变基元的存在，相变基元为切变相变基元，并提出其形态示意图。在Ｆｅ－Ｃ合金中，本文观察到正三角型、交叉型、“Ｎ”型和蝴蝶型类马氏体形貌贝氏体。在交叉型贝氏体中，发现基元相对滑移的、相互脱节的、形成“束腰”的和互换生长取向的交叉型贝氏体，交叉效应直接体现于相变基元的活动行为，相应提出其活动行为示意图。导致交叉的应力场是切变相变剪切应力集中应力场，贝氏体交叉的形成是切变应力－应变诱发贝氏体转变的结果。在应变诱发贝氏体转变时，相变驱动力的补偿与奥氏体所发生的真实应变的平方成正比，两贝氏体铁素体碰撞非常猛烈，承认其形成方式为切变型可得到说明。     

Fe-0.3C-3Mn-2Ni-2Si中贝氏体表面浮突效应的原子力显微镜研究

利用原子力显微镜(AFM)对Fe-0.3C-3Mn-2Ni-2Si(质量分数,％)合金中315℃等温贝氏体相变的表面浮突效应及相应区域的微观组织进行了对比观察.结果表明,伴随贝氏体相变的帐篷型表面浮突效应完全是新相长大过程中的形状应变所引起;贝氏体铁素体最小结构单元的表面浮突为帐篷型,不同于单倾型马氏体浮突,不可能以切变方式形成,它的两浮突角在8-10°之间.扩散台阶机制可以解释上述现象.     

巨型台阶的透射电镜观察及其可动性探讨

用透射电镜观察了下贝氏体铁素体宽面上三维形态的巨型台阶，发现巨型台阶可在异相前沿堆积而成尺寸更大的巨型台阶。导致巨型台阶堆积的相为另一片下贝氏体铁素体或下贝氏体碳化物。实验观察到下贝氏体碳化物在巨型台阶前沿奥氏体内析出，而并非在下贝氏体铁素体内析出。有关巨型台阶的堆积与下贝氏体碳化物来源的实验结果为下贝氏体铁素体扩散控制台阶长大机制提供了重要的实验证据。     

MORPHOLOGY AND PHASE TRANSFORMATION OF GRANULAR BAINITE AND GRANULAR STRUCTURE

本文研究了低碳合金钢奥氏体在连续冷却或等温转变过程中形成的组织形态和转变机理。结果表明在中温贝氏体区得到“粒状贝氏体”组织,在先共析转变区得到“粒状组织”。两者都有“铁素体基体+小岛”形貌。但前者有表面浮凸现象,铁素体呈长条状,与母相维持K-S关系,惯习面为{111}_γ,小岛也多星长条状平行排列。后者无表面浮凸,铁素体呈无规则形状,与母相无严格位向关系,小岛亦呈无规则排列。然而这两种组织都是由扩散型相变机制形成的。最后,提出了形成这两种组织的相变模型。     

Application of interrupted cooling experiments to study the mechanism of bainitic ferrite formation in steels

New interrupted cooling experiments have been designed to study the kinetics of bainitic ferrite formation starting from a mixture of austenite and bainitic ferrite. It is found that the kinetics of bainitic ferrite formation during the cooling stage is determined by the isothermal holding time. The formation rate of bainitic ferrite at the beginning of the cooling decreases with increasing prior isothermal holding time. An unexpected stagnant stage during the cooling stage appears when the isothermal holding time increases to a critical point. There are two reasons for the occurrence of the stagnant stage: (i) a solute spike in front of the interface; and (ii) kinetic transition. A so-called Gibbs energy balance approach, in which the dissipation of Gibbs energy due to diffusion inside the interface and interface friction is assumed to be equal to the available chemical driving force, is applied to theoretically explain the stagnant stage. A kinetics transition from a fast growth mode without diffusion of Mn and Si inside the austenite–bainitic ferrite interfaces to a slow growth mode with diffusion inside the interface is predicted. The stagnant stage is caused by the transition to a slow growth mode. The Gibbs energy balance approach describes the experimental observations very well.     

低碳钢中晶界铁素体/原奥氏体界面对贝氏体转变的影响

采用电子背散射衍射 (EBSD) 研究了低碳Fe--C--Mn--Si钢中晶界铁素体/原奥氏体界面对贝氏体形核的影响. 通过两阶段等温热 处理, 获得了晶界铁素体和贝氏体的混合组织. 结合金相观察和取向测量, 发现晶界铁素体与贝氏铁素体之间的界面分为两种, 一种界面不清晰, 一种界面清晰. 分析表明, 在晶界铁素体/贝氏体界面不清晰一侧, 晶界铁素体与原奥氏体保持取向关系, 贝氏体在这类界面形 核生长, 且取向与晶界铁素体保持一致; 在晶界铁素体/贝氏体界面清晰一侧, 晶界铁素体与原奥氏体无取向关系, 且贝氏体与晶界铁素体之间取向差较大.     

IN SITU OBSERVATION OF LOWER BAINITIC CARBIDES PRECIPITATION

The lower bainitic carbides precipitation in 40CrMnSiMoV steel previously austempered at310℃ was observed in situ by employing a high temperature stage of an ultra high voltageTEM(JEM-1000).The typical lower bamitic carbides were found to have precipitated fromwithin the carbide-free bainitic ferrite after tempering for certain period at a temperaturehigher than that of the isothermal transformation.This revealed that the lower bainitic ferriteis supersaturated with carbon to some extent.The carbide may also precipitate from austenite,but they have no typical morphological features of lower bainitic carbide.     

微合金高强钢焊接热影响区中粒状贝氏体微观结构的实验研究

运用电子显微技术研究了ＱＴ型微合金钢模拟焊接热影响区粒状贝氏体的微观结构。结果显示,在金相截面上呈长条形的Ｍ－Ａ组元的空间形态为薄板状;而在金相截面上呈不规则多边形的Ｍ－Ａ组元的空间形态为近似于等轴的块状。Ｍ－Ａ组元与铁素体之间的界面可进一步划分为马氏体／铁素体界和残余奥氏体／铁素体界。在具有块状Ｍ－Ａ组元的粒状贝氏体团中,贝氏体与铁素体之间有高角晶界相隔,并分析和讨论了这些现象与理解断裂和韧性的     

贝氏体相变简介

钢、有色合金和一些陶瓷材料中都存在贝氏体相变。贝氏体钢正成为有益的工程材料。总结评述切变学派和扩散学派作者们以形貌、动力学或晶体学对贝氏体相变机制所持的论点。钢中贝氏体相变以过饱和铁素体开始形成之说迄未得到支持。一些实验已发现替代（置换）型合金元素在相界面上的偏聚，并以此所呈现的拖曳效应说明相变的不完全现象，切变学者以切变机制来解释这个现象，但此现象不是钢中贝氏体相变的普遍情况。贝氏体形成时呈现帐篷形浮突，不具不变平面应变特征；有时马氏体相变晶体学表象理论能近似地应用于贝氏体相变晶体学，但不能以此来判定其相变机制为扩散或切变。溶质拖曳效应以及高分辨电镜对相界面结构实验、热力学研究、磁场和应力场对贝氏体相变影响以及一些预相变现象都确证贝氏体相变籍扩散机制进行。本文作者定义贝氏体为：在Ms温度以上，经扩散相变的产物，多呈片状，形成时会在自由表面上呈现帐篷形浮突。提出贝氏体相变机制进一步研究和应用的展望。     

Fe－Ni合金贝氏体长大原位观察

利用透射电镜观察了Ｆｅ—３０ｗｔ—％Ｎｉ合金贝氏体铁素体／奥氏体界面结构，并利用透射电镜高温样品台对贝氏体铁素体台阶长大进行了原位动态观测。结果表明，铁素体呈台阶长大形态，在铁素体／奥氏体宽面上存在Ｂｕｒｇｅｒｓ矢是为［１１１］ｂ型的刃型错配位错，铁素体台阶的增厚速率与按Ｚｅｎｅｒ—Ｈｉｌｌｅｒｔ方程计算结果相符。为关于贝氏体生长台阶长大机制提供实验证据。     

THERMODYNAMIC STUDY OF BAINITIC TRANSFORMATION IN Fe-C ALLOYS

The growth of bainitic ferrite is a couple process of the shear transformation of Fe atoms withthe diffusion of C atoms.The diffusion growth of the ferrite is impossible when the tempera-ture is below 400℃,and the paraequilibrium shear growth may occur at the temperature be-low 550℃.This paraequilibrium shear growth model explains the overlap of the bainitescreated from two growth mechanisms at 400—550℃.     

Mechanism of Stress Relief Cracking in a Granular Bainitic Steel

Coarse-grained heat-affected zone(CGHAZ) of a low alloyed,granular bainitic steel T24 was simulated in a Gleeble apparatus.The stress relief of the CGHAZ was analyzed by annealing the samples.The morphology and behavior of the microstructure near the grain boundaries during stress relief were investigated by means of focused ion beam,in situ tensile testing,transmission electron microscopy,scanning electron microscopy and electron back-scatter diffraction.It was observed that there were large martensite/austenite islands distributed along the grain boundaries of CGHAZ.During stress relief at elevated temperature,the retained austenite at the grain boundaries decomposed into M_3C carbides and a ferrite forming softening zone.Together with the stress relief,piled up of dislocations occurred within the ferrite in the area adjacent to the ferrite/M_3C interface,which resulted in high level of stress accumulation and caused crack initiation along the grain boundaries.These results indicate that the stress relief induced the grain boundary crack is controlled by other mechanisms rather than the creep-like grain boundary sliding.     

Effect of copper additions on the isothermal bainitic transformation in hypereutectoid copper and copper-nickel steels

During the isothermal bainitic transformation in hypereutectoid steels alloyed with copper or copper and nickel, it was found that at all the transformation temperatures studied, the formation of equally copper supersaturated bainitic ferrite and cementite always occurred. This observation implies the formation of bainitic ferrite and cementite from the parent austenite phase without redistribution of the alloying elements, since the solubility of copper in cementite is negligible and very low in bainitic ferrite. By carefully designing suitable tempering treatments, it is possible to produce copper precipitation not only within tempered bainitic ferrite in both types of steel using low tempering temperatures, but also within the tempered bainitic cementite of the copper steel at higher tempering temperatures. The interpretation of these experimental data strongly supports the theory that bainite formation is promoted through a shear controlled type of mechanism.     

低碳微合金钢中针状铁素体的形成与控制

低碳微合金钢连续冷却时都会产生多种类型的中温转变组织,低冷速下主要得到粒状贝氏体组织,高冷速下主要得到板条贝氏体组织．连续冷却时,在较高中温转变温度范围可形成针状铁素体,其转变受冷却速度和过冷温度影响．通过控冷可以在低碳微合金钢中得到贝氏体和针状铁素体多相组织,利用针状铁素体能改善高强度低碳微合金钢的综合力学性能．     

Atomic scale observations of bainite transformation in a high carbon high silicon steel

A fine-scale bainitic microstructure with high strength and high toughness has been achieved by transforming austenite at 200 °C. X-ray diffraction analysis showed the carbon concentration of these bainitic ferrite plates to be higher than expected from para-equilibrium. Atom-probe tomography revealed that a substantial quantity of carbon was trapped at dislocations in the vicinity of the ferrite–austenite interface. These results suggest that the carbon trapping at dislocations prevents the decarburization of super-saturated bainitic ferrite and therefore alters the carbide precipitation sequence during low-temperature bainite formation.     

CARBIDE FORMATION IN AUSTEMPERED DUCTILE IRON ALLOYED WITH NICKEL AND COPPER

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