The role of crystal misorientations during solid-state nucleation of ferrite in austenite

The role of grain and phase boundary misorientations during nucleation of ferrite in austenite has been investigated. Electron back-scatter diffraction (EBSD) was performed on a high-purity iron alloy with 20 wt.% Cr and 12 wt.% Ni with austenite and ferrite stable at room temperature in order to identify the crystallographic misorientation between austenite grains and between ferrite and austenite grains. It is observed that the specific orientation relationships between ferrite and austenite play a dominant role during solid-state nucleation of ferrite. Ferrite grains nucleate on grain faces independently of the misorientation between austenite grains, although random high-angle grain boundaries have a slightly higher efficiency. Different types of nucleation mechanisms are found to be active during ferrite formation at grain faces. A slight deformation of the austenite matrix was found to triple the number of ferrite nuclei during isothermal annealing.     

Densification and grain boundary nitrides in spark plasma sintered SAF 2205 -TiN composite

Densification and nature of α-ferrite γ-austenite interface associated with the spark plasma sintering (SPS) of SAF 2205 reinforced with TiN nanoparticles was investigated. The sintering data obtained during SPS was used to evaluate the densification mechanism. EBSD and TKD technique were used to characterize the SAF 2205 composite. The grain and grain boundary properties were evaluated using nanoindentation technique. Three densification stages relating to particles rearrangement, bulk deformation and rapid densification of the SAF 2205 composite was observed during SPS. The EBSD revealed a reverse ferrite to austenite phase transformation and a refined microstructure after SPS. TKD analysis of the grain boundary showed nanosized sintered-in nitride coexisting with ferrite and austenite upon sintering at α/γ interface but also at α/α and γ/γ interfaces. TKD-EDS compositional mapping revealed Ti-rich grain boundary. Nanoindentation experiments reveal that the TiN dominated grain boundaries exhibit lower penetration depths, an indication of higher hardness and modulus.     

基于温变形耦合逆相变细化铁素体制备

采用Gleeble-3500热模拟试验机进行650℃多道次温变形以研究某中碳钢奥氏体及铁素体晶粒的细化，并利用OM、TEM和EBSD等检测手段进行表征。结果表明，650℃下6道次的快速压缩变形(总应变为2.6)后有逆转变发生，所得奥氏体晶粒平均尺寸为3.4μm;再在10℃/s的速率冷却到600℃,可以形成平均晶粒尺寸为0.64μm的铁素体，且渗碳体呈粒状分布于基体之上。     

低碳钢形变强化相变的特征

介绍了低碳钢形变强化相变的基本概念及主要特征．系统的研究工作证实了变形显著地加速了低碳钢过冷奥氏体向铁素体的相变过程．形变强化相变是一个以形核为主导的过程直到相变完成以前，形核始终存在于新相与原奥氏体相界面的高应变区．由于几何空间与成分条件上受到一定的限制，长大及各向异性都不太明显，铁素体晶粒超细化．实验工作还证实了转变动力学呈现明显的3个阶段，它们分别与相变铁素体在原奥氏体晶界上的形核，在铁素体／奥氏体相界前沿高畸变区的形核，及被铁素体晶粒所包围的残存奥氏体上的相变形核等过程相对应。     

高温时效对2205双相不锈钢中σ相析出行为的影响

对2205双相不锈钢进行了750、800、850、900和950℃分别保温0.5、1、2h的时效处理,采用定量金相、SEM和EDS、化学萃取、XRD和电子背散射衍射(EBSD)等方法研究了2205双相不锈钢中σ相析出与时效时间、温度的变化规律。结果表明:2205双相不锈钢经不同时效工艺处理后的组织主要由奥氏体、铁素体、σ相组成,σ相一般在γ/α相界处或铁素体内析出;在相同时效温度下,随着时间的延长,σ相的析出量明显增多,而在850℃进行时效处理会使钢中σ相的析出量达到最高值。此外,采用EBSD方法有望对2205双相不锈钢中的σ相进行准确的定量分析。     

Q235碳素钢应变强化相变过程中的动力学问题

利用热模拟单向压缩实验,分析了Q235碳素钢应变强化相变过程中应变对铁素体晶粒数目及铁素体长大速率的影响,同时考察了铁素体转变动力学与应变速率、形变温度、奥氏体晶粒尺寸、纯净度的关系,并与无应变时进行比较,结果表明,应变使铁素体转变动力学发生一定的变化,应变作用下奥氏体晶界的形核率明显增加,但未[观察到应变明显提高铁素体长大速率的现象,应变作用下铁素体的长大速率被周围铁素体快速形成或铁素体的动态再结晶所抑制或掩盖,应变速率的提高主要使铁素体转变时间提前;形变温度越高,应变对铁素体转变的促进作用越明显;原始奥氏体晶粒尺寸不同,转变动力学曲线的斜率稍有不同,这主要由转变初期的形核益的多少引起。     

In-situ observation on the isothermal decomposition process of γ-Fe[N] by X-ray diffraction

The isothermal decomposition process of γ-Fe[N] (9.32 at.% N) prepared by a two-step gas nitriding method was investigated in-situ by X-ray diffraction (XRD). The XRD spectra showed that the α-Fe phase precipitated first from the parent austenite when the sample was aged at 225 °C, while the γ′-Fe₄N phase appeared half an hour later as extra fine particles. The quantity of these two phases increased quickly with the ageing time until the original austenite of γ-Fe[N] was completely consumed. Further investigation by scanning electronic microscopy (SEM) revealed that the α-Fe nucleated first at the primary grain boundaries and grew towards the grain interior with a lamellar bainitic-like structure, which was different to the homogeneous precipitates of γ′-Fe₄N and α-Fe inside the grains. A new transformation mechanism has thus been proposed for the isothermal decomposition of high nitrogen austenite on the grain boundaries (GBs).     

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.     

变形温度对含Nb双相钢显微组织的影响

通过单道次压缩及连续冷却实验,研究了变形温度(810-720℃)对具有超细原始奥氏体晶粒的含Nb双相钢显微组织的影响.实验结果表明:实验钢最终组织为铁索体加马氏体的双相组织.压缩过程中,实验钢应力-应变曲线上出现峰值,且峰值应力随变形温度的降低先增大后减小;随着变形温度的降低,铁索体的含量先增大再减小,但增减幅度不大,在最低变形温度(720℃)时,铁素体品粒尺寸降低到2.8 μm,弥散分布于铁素体晶界上的马氏体含量达到22.7%;随着变形温度的增加,铁索体晶粒硬度减小,最低可降至230 GPa;EBSD取向分析显示,随着变形温度的降低,组织中小角度晶界增多.     

中碳钢过冷奥氏体形变过程中碳的分布与扩散

利用热模拟压缩变形实验以及SEM、XRD和热磁法,研究了中碳钢过冷奥氏体变形时组织演变过程中碳原子的分布与扩散.结果表明,动态相变过程中碳的有效扩散系数与等温过程相比明显增大,相变完成时间显著缩短.在随后的片层状珠光体的球化过程中,相界以及形变过程中产生的高密度位错和空位等缺陷促进了间隙碳原子的扩散,使得球化动力学过程与等温退火相比显著缩短.渗碳体的溶解和铁素体中碳的过饱和现象得到证实,其中过饱和碳原子高度聚集在铁素体晶界和位错核心处,而不是均匀地分布在铁素体点阵的间隙位置.     

Fe-C-Mn-Si钢中奥氏体共格孪晶界对贝氏体铁素体变体选择的影响

将C含量(质量分数)分别为0.05%和0.4%的Fe-C-Mn-Si钢进行等温处理得到贝氏体组织,采用EBSD技术对奥氏体共格孪晶界上形成的贝氏体铁素体变体进行分析.结果表明,2种钢中的贝氏体铁素体与母相奥氏体均成近似K-S取向关系.奥氏体孪晶界两侧形成取向相同的变体对.此变体对形成后,孪晶界基本不再显现.晶体学分析表明,共格孪晶界两侧可能出现的变体对最多不超过3组,且这3组变体对的惯习面均与孪晶界平行,因此,贝氏体铁素体变体都将沿孪晶界生长.含C量为0.05%的Fe-C-Mn-Si钢中奥氏体孪晶界上只观察到一组贝氏体铁素体变体对的形成,这是因为C含量较低,贝氏体铁素体生长速度较快,消除了其它变体对的形核机会,先形核的变体对一旦形核就迅速覆盖整个孪晶面.而在含C量为0.4%的Fe C Mn-Si钢中,由于C含量较高,贝氏体铁素体生长速度较慢,3组变体对均有机会形核,因此,在孪晶界上可以观察到这3组变体对同时出现.     

Simulation of stress concentration in Mg alloys using the crystal plasticity finite element method

A crystal plasticity finite element method (CPFEM), considering both crystallographic slip and deformation twinning, was developed to simulate the spatial stress concentration in AZ31 Mg alloys during in-plane compression. A predominant twin reorientation (PTR) model was successfully implemented to capture grain reorientation due to deformation twinning in twin-dominated deformation. By using the direct mapping technique for electron backscatter diffraction (EBSD) data, CPFEM can capture the heterogeneity of stress concentration at the grain boundaries in AZ31 Mg alloys during in-plane compression. The model demonstrated that deformation twinning enhances the local stress concentration at the grain boundaries between untwinned and twinned grains.     

保温对低碳钢形变诱导相变组织的影响

通过热模拟实验，考察了不同变形温度和不同奥氏体晶粒尺寸等条件下保温对低碳钢形变后组织演变的影响。结果表明，在较低的温度下变形得到的铁素体在保温时更稳定，随温度升高，易发生铁素体向奥氏体的逆相变。细晶奥氏体转变后的铁素体在保温时长大缓慢，所得组织稳定，并且保温后的组织也更为均匀。     

两相区淬火对9Ni钢中逆转变奥氏体的影响

淬火+两相区淬火+回火(QIT)能显著的提高9Ni钢的低温韧性。利用扫描电镜(SEM)、透射电镜(TEM)、电子背散射衍射(EBSD)、X射线衍射(XRD)和电子探针(EPMA)对QIT处理的9Ni钢中逆转变奥氏体的含量、形貌、分布以及两相区淬火对逆转变奥氏体的影响进行了研究。结果表明,QIT处理的9Ni钢中逆转变奥氏体的含量约为10%,以块状和薄膜状形态分布在基体中;两相区淬火的9Ni钢中的大角度晶界增多,有利于逆转变奥氏体的形核;基体上某些区域的C、Mn和Ni元素含量较高,利于逆转变奥氏体长大和稳定化。     

Fe-C-Mn-Si钢中奥氏体共格孪晶界对贝氏体铁素体变体选择的影响

将C含量(质量分数)分别为0.05%和0.4%的Fe-C-Mn-Si钢进行等温处理得到贝氏体组织,采用EBSD技术对奥氏体共格孪晶界上形成的贝氏体铁素体变体进行分析.结果表明,2种钢中的贝氏体铁素体与母相奥氏体均成近似K-S取向关系.奥氏体孪晶界两侧形成取向相同的变体对.此变体对形成后,孪晶界基本不再显现.晶体学分析表明,共格孪晶界两侧可能出现的变体对最多不超过3组,且这3组变体对的惯习面均与孪晶界平行,因此,贝氏体铁素体变体都将沿孪晶界生长.含C量为0.05%的Fe-C-Mn-Si钢中奥氏体孪晶界上只观察到一组贝氏体铁素体变体对的形成,这是因为C含量较低,贝氏体铁素体生长速度较快,消除了其它变体对的形核机会,先形核的变体对一旦形核就迅速覆盖整个孪晶面.而在含C量为0.4%的Fe-C-Mn-Si钢中,由于C含量较高,贝氏体铁素体生长速度较慢,3组变体对均有机会形核,因此,在孪晶界上可以观察到这3组变体对同时出现.     

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 以CR600TRIP工业成品钢为研究对象,利用彩色金相、SEM、TEM和EBSD等技术手段,在详细考察TRIP钢的微观组织特别是残余奥氏体分布规律的基础上,进一步利用分阶段拉伸试验,对钢中残余奥氏体在应力作用的相变行为进行了考察分析。研究发现,钢中残余奥氏体的相变行为表现出显著的选择性：既有按其本身的稳定性由小晶粒到大晶粒逐步递进的总体趋势,更与应力应变的发展方向,以及所处晶界的曲率有着直接联系。     

变形温度对形变强化相变完成时临界应变量的影响

利用“形变强化相变”机制研究了低碳钢过冷奥氏体在740℃和780℃,10s-1变形时的变形温度对相变完成时临界应变量εc的影响。结果表明,变形温度对εc和组织演变的影响很大。在740℃和780℃变形时,εc分别为0.96和1.39,变形温度降低明显促进了相变。变形温度对εc的影响在组织演变上主要表现为铁素体形核地点的不同。740℃变形时,铁素体由奥氏体晶界形核过渡到以形变带形核为主,形核速率极高;780℃变形时,铁素体由奥氏体晶界形核过渡到在铁素体/奥氏体相界面前沿高畸变区快速形核。     

国产06Ni9DR 钢焊接热影响区组织和低温韧性

采用扫描电镜(SEM)、背散射电子衍射(EBSD)和焊接热模拟技术,研究了单次热循环不同峰值温度对国产06Ni9DR 钢焊接热影响区(HAZ)显微组织和低温冲击韧性的影响. 结果表明,06Ni9DR 钢HAZ的-196 ℃冲击吸收能量均低于母材,HAZ整体发生了脆化. 粗晶区脆化最为严重,原因是原始奥氏体晶粒粗大及其导致的有效大角度晶界较少,残余奥氏体量少且不稳定,以及较大的位错密度和粗大马氏体的存在. 晶界呈链状分布的大块逆转奥氏体和M-A组元的存在导致回火区脆化程度仅次于粗晶区. 细晶区和不完全脆化区的韧性低于母材,主要是因为淬火马氏体的存在和残余奥氏体的低温稳定性差.     

The effect of aluminium and phosphorus on the stability of individual austenite grains in TRIP steels

We have performed in situ synchrotron X-ray diffraction experiments to assess the influence of aluminium and phosphorus on the austenite stability in low-alloyed transformation-induced plasticity steels during the high-temperature bainitic holding and the subsequent martensitic transformation during cooling to temperatures between room temperature and 100 K. Although the addition of aluminium increases the chemical driving force for the formation of bainitic ferrite plates significantly, the phosphorus exerts a larger influence on the bainitic transformation kinetics. Consequently, the addition of phosphorus leads to a higher degree of carbon enrichment and a narrower grain volume distribution of the metastable austenite. The stability of the individual austenite grains with respect to their martensitic transformation depends on both the local carbon content and the grain volume for austenite grains smaller than 20 μm³. The presence of aluminium and phosphorus further stabilizes the austenite grains.     

9%Cr-1%Mo耐热钢焊缝金属连续冷却组织转变

利用Ｆｏｒｍａｓｔｏｒ－Ｄ全自动热膨胀记录仪测定了三种改进型９％Ｃｒ－１％Ｍｏ耐耐热钢焊缝金属的连续冷却组织转变相图,并用光学显微镜和透射是分析了各种冷却条件下的组织转变特点。结果表明,在很大的冷却范围内,三种焊缝金属的工体均只发生马氏体转变,只有当冷速足够慢时才发生先共析铁素体转变和共析转变。其中,含合金元素较少的２号焊缝金属具有最快的先共析铁素体形成冷速。研究发现,三种焊缝金属的奥氏体均形成板