马氏体相变晶体学概述(一)

阐明马氏体相变的惯习面、位向关系和亚结构等晶体学特征是马氏体相变研究的重要内容之一。金属材料马氏体相变的位向关系有K-S关系、N(或N-W)关系,G-T关系和Burgers关系等,相应的经典切变模型为K-S模型、N模型、G-T模型和Burgers模型。但这些模型均不能解释相变的表面浮凸和惯习面问题。以不变平面应变为基础的马氏体相变晶体学表象理论,即P1=RBP或P1P2=RB,能成功预测许多马氏体相变的晶体学特征。其不变线S的获得和求取是表象理论的关键,通过S的逆分解可以求解相变的惯习面和位向关系,结果与实验测定值相吻合。然而,应用上述理论求解板条马氏体和(225)f马氏体的晶体学特征的努力并不成功。近二三十年来,发展了晶体学理论的双点阵不变切变模型,F=RPS2S1。由此出发,能够说明板条马氏体和(225)f马氏体的晶体学特征。此外,简单介绍张文征等学者的一些马氏体相变晶体学方面的研究成果。     

ECAP变形下304L奥氏体不锈钢的形变诱导马氏体相变

研究了304L奥氏体不锈钢在严重塑性变形(等通道转角挤压,ECAP)下发生形变诱导马氏体转变的微观特征,包括形核特征、长大方式和相变晶体学,探讨了粗大晶粒和亚微米晶粒发生马氏体相变的异同和微观机理.结果表明:粗大奥氏体晶粒发生相变时,马氏体主要形核于微观剪切带(包括层错、变形孪晶和ε相等)的相互交割处,马氏体与奥氏体之间为K-S(Kurduumov-Sachs)关系,而不是西山(Nishiyama-Wassermann)关系;亚微米奥氏体晶粒发生相变时,马氏体则多在奥氏体晶界处形核,马氏体与奥氏体之间仍为K-S关系.     

300M超高强度钢板条马氏体的晶体学研究

电子衍射和晶体学测量证实,300M超高强度钢板条马氏体的惯习面为{335}_A型,与奥氏体的位向关系偏离K-S关系2.5°,为G—T关系,相邻板条间绕其公共面{110}_M法线相对旋转约60°,形成板条间不同的位向关系,包括近似{112}_M孪晶关系。     

20MnCr5钢中马氏体回火的EBSD研究

利用金相显微镜和扫描电镜中的电子背散射衍射(EBSD)技术研究了不同条件回火处理后20MnCr5钢中马氏体及其变体的取向关系变化情况.选择各试样取向成像图中原奥氏体晶粒区域做出相应的马氏体{110}a极图,并根据Kurdjumov-Sachs(K-S)关系和Nishiyama-Wassermann(N-W)关系,使用数值拟合方法分别对各极图进行拟合.结果表明,20MnCr5钢经淬火并低温回火后得到的马氏体取向关系主要为N-W关系,也有K-S关系存在.再经350℃或500℃保温处理后,钢中马氏体取向关系没有明显改变,但原奥氏体晶粒中马氏体变体数目有减少趋势.     

马氏体转变(九)

4.2晶体学表象理论[2,5,10,17,41] 美国的Wechsler,Lieberman和Read[43]于1953年以及澳大利亚的Bowles和Mackenzie[42]于1954年分别独立提出马氏体相变的表象理论.研究对象是马氏体相变后马氏体与母相之间存在的晶体学关系:晶体点阵类型、点阵参数与相变惯习面、位向关系之间的联系.根据该理论,由母相和产物相的点阵参数,晶体结构能够对转变发生的惯习面和位向关系以及形状应变进行预测.现在对之相应称为W-L-R理论和B-M理论.尽管它们都应用矩阵代数进行数学处理,却有明显不同.但理论的本质上具有一致性.1955年,Christian对这两种理论作了比较,认为它们实质上具有等同性.以后又出现Bul-lough和Bilby (1956)以及Bilby和Frank(1960)的分析方法.但Wayman提出其方法的基本原理与上述两个理论相同.     

中碳铁合金马氏体的一些特征

应用光学显微术及透射电子显微术和B-M表象晶体学理论对60和60Si2Mn钢中马氏体的形态、亚结构与晶体学特征进行了研究。中碳铁基马氏体的平均惯习面为{225}_f。铁基马氏体的微观惯习面为{111}_f。实测惯习面、取向关系及切变方向等与利用Bain应变及(100)[011]_f点阵不变切变系按B-M理论计算的结果一致。     

高锰TRIP/TWIP钢中ε马氏体X射线衍射分析

利用X射线衍射对高锰TRIP/TWIP钢中的马氏体相进行了晶体结构常数与全谱精修含量的分析,此高锰钢试样的ε马氏体由于锰的加入晶格参数变化较大,须先对其晶体学常数进行初步修正才能进行全谱拟合。全谱拟合分析得出此试样中的ε马氏体晶体学晶格参数为a=b=2.543×10-8cm,c=4.131×10-8cm。其六方ε马氏体含量为35.7%,从拟合图谱和Rwp值根据资料可以判断拟合取得成功,为高锰TRIP/TWIP钢中的相定量提供了一种快捷准确的分析方法。     

高钨高速钢中碳化物结构及界面特征

采用铸造法制备了含钨10%的高钨高速钢,利用XRD、SEM、HRTEM、EDAX研究了高钨高速钢中碳化物的类型、成分、结构及其界面晶体学特征。结果表明:高钨高速钢中的碳化物有两种,一种是凝固过程中析出的鱼骨状共晶碳化物,别一种为回火过程中析出的颗粒状二次碳化物,二者均为面立方晶体结构的M_6C。共晶M_6C的分子式可表达为(Fe_(0.55)W_(0.30)Cr_(0.08)Mo_(0.07))_6C,二次M_6C中的合金元素含量与之基本相同。共晶M_6C型碳化物与基体间存在晶体学关系,在铸态组织中,M_6C的(111)晶面平行于铁素体(110)晶面,即M_6C(111)//α-Fe(110),而高速钢回火后,基体组织转变成马氏体(M)与残留奥氏体,界面的晶体学位向关系变为:M_6C(111)//M(020)。二次M_6C型碳化物与马氏体界面为良好的冶金结合,M_6C的(133)晶面与马氏体的(101)晶面夹角约为140°。部分二次M_6C可以依附于共晶M_6C析出,二者的(202)晶面共格。     

马氏体相变晶体学的简易矢量分析方法

基于界面错配可以通过一组缺陷松弛的假设, 发展了一种简易矢量分析法来计算界面包含一组缺陷的系统的相变晶体学. 通过该方法可以方便地求解惯习面的取向. 本文运用该方法求解了fcc/bcc系统中经典马氏体表象理论对应的相变晶体学, 所有计算结果与文献报道完全一致. 并根据矢量分析方法求解了满足上述假设条件惯习面取向的一般表达式. 这一结果对系统地研究fcc/bcc中无理取向的惯习面提供了简便的分析工具.     

38Si2Mn2Mo钢等温下贝氏体的精细结构和晶体学特征

应用透射电镜研究了38Si2Mn2Mo实验用钢320℃等温处理后的下贝氏作组织的晶体学和精细结构．贝氏体含有中脊线、相变单元、残余奥氏体及ε碳化物,贝氏体及奥氏体内存在大量位错．电子衍射和晶体学测量表明下贝氏体惯习面为｛11,7,15｝f型,贝氏体与奥氏体的取向关系为G—T关系．相邻贝氏体板条绕公共面法线＜110＞相对旋转54．7°或60°,形成板条间的不同取向关系,包括近似的｛112｝b孪晶关系．下贝氏体的晶体学位向关系和惯习面分别与板条马氏体的相同或相近,且与由马氏体晶体学表象理论预测的基本一致,说明下贝氏体相交具有马氏体相变的特征．     

超高强度马氏体时效不锈钢中逆转变奥氏体的析出与长大行为

研究了一种Cr-Ni-Co-Mo系超高强度马氏体时效不锈钢中逆转变奥氏体的析出与长大行为。结果表明,经540℃×4 h时效后钢的强度达到峰值1940 MPa,并有薄膜状逆转变奥氏体沿马氏体板条界上非连续析出,使钢具有良好的综合力学性能。时效初期逆转变奥氏体在马氏体板条、板条界或原奥氏体晶界处形核,随着保温时间的延长,薄膜状的逆转变奥氏体通过与扩散有关的形核长大方式生长,最终形成块状,并与板条马氏体间保持着K-S或N-W关系。     

Bain对应和K-S模型的数学描述

利用Bain点阵对应对K－S模型的晶格改建过程进行了严密的数学描述，对于不同的马氏体正方度，提出了计算第一切变，第二切变及晶格调整的计算依据。并给出了普遍的计算公式。以Fe-1.4C和纯铁为例。计算了马氏体相变的点阵畸变。进一步分析表明。K－S模型实质是Bain模型的旋转。计算的结果与实测的取向关系相符合。     

Quantitative Analysis of the Crystallographic Orientation Relationship Between the Martensite and Austenite in Quenching–Partitioning–Tempering Steels

The orientation relationships(ORs)between the martensite and the retained austenite in low-and medium-carbon steels after quenching–partitioning–tempering process were studied in this work.The ORs in the studied steels are identified by selected-area electron diffraction(SAED)as either K–S or N–W ORs.Meanwhile,the ORs were also studied based on numerical fitting of electron backscatter diffraction data method suggested by Miyamoto.The simulated K–S and N–W ORs in the low-index directions generally do not well coincide with the experimental pole figure,which may be attributed to both the orientation spread from the ideal variant orientations and high symmetry of the low-index directions.However,the simulated results coincide well with experimental pole figures in the high-index directions{123}_(bcc).A modified method with simplicity based on Miyamoto’s work was proposed.The results indicate that the ORs determined by modified method are similar to those determined by Miyamoto’method,that is,the OR is near K–S OR for the low-carbon Q–P–T steel,and with the increase of carbon content,the OR is closer to N–W OR in medium-carbon Q–P–T steel.     

铁基合金马氏体的切变过程

根据铁合金中马氏体相变的Ｋ－Ｓ位向关系，提出了马氏体的形核长大相变位错模型，并推导出马氏体切变过程的数学表达式。这些表达式弥补了马氏体转变晶体学唯象理论（ＰＴＭＣ）的缺陷，对钢中马氏体亚结构（孪晶和位错）的形成提出了新的晶体学几何解释。     

低合金高强度钢焊缝金属中针状铁素体的微观组织

利用光学显微镜,扫描电子显微镜及EBSD(electron backscattering diffraction)分析技术对800MPa级低碳微合金高强度钢的焊缝金属进行了分析.结果表明,焊缝金属组织主要为针状铁素体和贝氏体.针状铁素体以夹杂物为核心形核,该夹杂物主要是以Al2O3为核心形成的钛氧化物.针状铁素体以夹杂物为核心多维形核呈放射状生长,仅某些方向的针状铁素体晶核迅速长大,生长方向存在取向择优.EBSD分析同样表明针状铁素体晶粒存在取向择优.观察到由同一夹杂物生长,沿同一直线方向背向生长的针状铁素体取向相同,沿不同方向生长的针状铁素体取向不同,说明其沿原奥氏体惯习面生长,并非与夹杂物共格.     

Mapping the parent austenite orientation reconstructed from the orientation of martensite by EBSD and its application to ausformed martensite

A new method is developed for reconstruction of the local orientation of the parent austenite based on the orientation of lath martensite measured by electron backscattered diffraction. The local orientation of austenite was obtained by least squares fitting as the difference between the experimental data and the predicted martensite orientation was minimal, assuming the specific orientation relationship (OR) between martensite and the parent austenite. First, the average OR between austenite and lath martensite was precisely determined and it was shown that both close packed planes and directions between martensite and the parent austenite deviated by more than 1° in low carbon martensite. The quality of the reconstructed austenite orientation map depended strongly on the OR used for the calculation. When Kurdjumov–Sachs (K–S) or Nishiyama–Wasserman (N–W) ORs were used the austenite orientation was frequently mis-indexed as a twin orientation with respect to the true orientation because of the mirror symmetry of (0 1 1)_α stacking in the K–S or N–W ORs. In contrast, the frequency of mis-indexing was significantly reduced by using the measured OR, where the close packed planes and directions were not parallel. The deformation structure in austenite was successfully reconstructed by applying the proposed method to ausformed martensite in low carbon steel.     

Maraging behavior in an Fe-19.5Ni-5Mn alloy I: Precipitation characteristics

The aging behavior of an Fe-19.5Ni-5Mn alloy has been studied in detail. A substantial maraging-hardening response was obtained upon aging at between 300–550°C, and it displayed classical hardening behavior. The pronounced hardness was attributed to strain hardening caused by coherent, fine spherical precipitates. The activation energy for precipitation, calculated from microhardness data, was 41 kcal/gmole. Ordered fct θ-NiMn precipitates were identified with two different shapes, depending on the aging temperature. Higher aging temperatures resulted in disk-shaped precipitates, while rod precipitates appeared at lower temperatures. Twin-related Widmanstätten austenite grains appeared in a lenticular shape that were coupled together by a twin boundary. Their orientation relationship with the parent martensite was found to be of the Kurdjumov-Sachs (K-S) type.     

Maraging behavior of an Fe-19.5Ni-5Mn alloy II: Evolution of reverse-transformed austenite during overaging

The evolution of reverse-transformed austenite in a maraging Fe-19.5Ni-5Mn alloy was examined using transmission electron microscopy. The types of reversed austenite observed were determined by the aging temperature and time. It was found that matrix austenite appeared first, followed by lath-like austenite and then by recrystallized austenite, the latter after prolonged aging at higher temperatures. Each evolved at different preferential sites. The first two types of reversed austenite contained dense dislocations that probably contributed to the subsequent recrystallization. The lath austenite was twin-related, and its orientation relationship with the residual martensite films obeyed either Nishiyama (N) or Kurdjumov-Sachs (K-S) relations, depending on the aging temperature. Each individual austenite lath nucleated independently with a habit plane close to {111}_f, and constituted a lamellar structure with the residual martensite. Its formation possibly involved a shear mechanism accompanied by element redistribution. The matrix austenite and the lath-like austenite eventually recrystallized to form fine grains of austenite. Part of the recrystallized austenite transformed to martensite after cooling so that the resultant microstructure was a mixture of a few newly formed martensite laths and oval/spherical ferrite particles distributed in the recrystallized austenite matrix. The recrystallized austenite and ferrite particles, rather than the lamellar structure of lath-like austenite, are thought to be the equilibrium phases at 550°C.     

BUTTERFLY MARTENSITE NUCLEATION IN AN Fe-Ni-V-C ALLOY

The butterfly martensite nuclei in an Fe-Ni-V-C alloy were observed under TEM,they con-sist of enveloping dislocations with Burger's vector(1/2)[(?)]_(fcc) and arrays of dislocationswith Burger's vector(1/2)[(?)10]_(fcc) piled up within the former.The enveloping dislocationsand the arrays of dislocations provide the required shear when the planes((?)11)_(fcc)move witheach other to alter their stacking order and change into the planes(110)_(bcc) respectively.Thenecessary adjustment of the spacings is provided by the dislocations in martensite.Thecrystallographic relationship after double-shear is K-S one.