Grain boundary engineering for enhancing intergranular damage resistance of ferritic/martensitic steel P92

Ferritic/martensitic(F/M)steel is widely used as a structural material in thermal and nuclear power plants.However,it is susceptible to intergranular damage,which is a critical issue,under service conditions.In this study,to improve the resistance to intergranular damage of F/M steel,a thermomechanical process(TMP)was employed to achieve a grain boundary engineering(GBE)microstructure in F/M steel P92.The TMP,including cold-rolling thickness reduction of 6%,9%,and 12%,followed by austenitization at 1323 K for 40 min and tempering at 1053 K for 45 min,was applied to the as-received(AR)P92 steel.The prior austenite grain(PAG)size,prior austenite grain boundary character distribution(GBCD),and connectivity of prior austenite grain boundaries(PAGBs)were investigated.Compared to the AR specimen,the PAG size did not change significantly.The fraction of coincident site lattice boundaries(CSLBs,3≤Σ≤29)and Σ3^(n) boundaries along PAGBs decreased with increasing reduction ratio because the recrystallization fraction increased with increasing reduction ratio.The PAGB connectivity of the 6%deformed specimen slightly deteriorated compared with that of the AR specimen.Moreover,potentiodynamic polarization studies revealed that the intergranular damage resistance of the studied steel could be improved by increasing the fraction of CSLBs along the PAGBs,indicating that the TMP,which involves low deformation,could enhance the intergranular damage resistance.

GRAIN BOUNDARY CHARACTER DISTRIBUTION, TEXTURE AND SUPERPLASTIC DEFORMATION BEHAVIOR OF AN Al-Li ALLOY

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Effects of initial grain size and strain on grain boundary engineering of high-nitrogen CrMn austenitic stainless steel

18 Mn18 Cr0.5 N steel with an initial grain size of 28–177 μm was processed by 2.5%–20% cold rolling and annealing at 1000°C for 24 h,and the grain boundary character distribution was examined via electron backscatter diffraction.Low strain(2.5%) favored the formation of low-Σ boundaries.At this strain,the fraction of low-Σ boundaries was insensitive to the initial grain size.However,specimens with fine initial grains showed decreasing grain size after grain boundary engineering processing.The fraction of low-Σ boundaries and the(Σ9 + Σ27)/Σ3 value decreased with increasing strain; furthermore,the specimens with fine initial grain size were sensitive to the strain.Finally,the effects of the initial grain size and strain on the grain boundary engineering were discussed in detail.

Effects of cold-rolling reduction on the recrystallization texture and grain boundary of ferritic stainless steel

The study investigates the effects of cold-rolling reduction on the recrystallization-annealed Nb ＋ Ti stabilized ultra-pure ferritic stainless steel with 21% Cr in regards to its microstructure evolution, grain size,recrystallization texture, and grain boundary characteristic distribution and disorientation angle. The research employed the electron back scattered diffraction technique and its results have shown that the average grain size was reduced and the {111 / 〈 112 〉 component was strengthened,which rotated towards {5541 〈225 〉 and {4451 〈384 〉 ,with an increasing cold-rolling reduction. The number fraction of the low-angle grain boundary and the coincidence site lattice （CSL） boundary ,which was mainly made up of ∑3,∑7∑11 and ∑13b, also increased.

Role of grain boundary character on Bi segregation-induced embrittlement in ultrahigh-purity copper

Bismuth(Bi),as an impurity element in copper and copper-based alloys,usually has a strong tendency of grain boundary(GB)segregation,which depends on the GB characters.However,the influence of such a segregation on the properties of ultrahigh-purity copper has been rarely reported and the exact structural arrangements of Bi atoms at different GBs remain largely unclear.In this study,we investigated the influ-ence of trace amounts of Bi(50-300 wt ppm)on the ductility of an ultrahigh-purity copper(99.99999%)in the range of room temperature to 900°C.The tensile results show that the addition of Bi seriously damages the ductility of the ultrahigh-purity copper at temperatures of 450-900°C,which is due to the GB segregation of Bi.On this basis,such a segregation behavior at different types of GBs,including high and low angle GBs(HAGBs/LAGBs),and twin boundaries(TBs),via the scanning electron microscope-electron backscattered diffraction(SEM-EBSD)and aberration-corrected scanning transmission electron microscope(AC-STEM)investigations,combined with the first-principles calculations were systematically studied.The atomistic characterizations demonstrate an anisotropic Bi segregation,where severe enrich-ment of Bi atoms typically occurs at the HAGBs,while the absence of Bi adsorption prevails at LAGBs or TBs.In particular,the segregated Bi at random HAGBs exhibited the directional bilayer adsorption,while the special symmetrical7 HAGB presented a unique Bi-rich cluster superstructure.Our findings pro-vide a comprehensive experimental and computational understanding on the atomic-scale segregation of impurities in metallic materials.

Optimization of Grain Boundary Character Distribution in Fe-18Cr-18Mn-0.63N High-Nitrogen Austenitic Stainless Steel

Grain boundary engineering（GBE） is a practice of improving resistance to grain boundary failure of the material through increasing the proportion of low Σ coincidence site lattice（CSL） grain boundaries（special grain boundaries） in the grain boundary character distribution（GBCD）. The GBCD in a cold rolled and annealed Fe-18Cr-18Mn-0.63N high-nitrogen austenitic stainless steel was analyzed by electron back scatter difraction（EBSD）. The results show that the optimization process of GBE in the conventional austenitic stainless steel cannot be well applied to this high-nitrogen austenitic stainless steel. The percentage of low ΣCSL grain boundaries could increase from 47.3% for the solid solution treated high-nitrogen austenitic stainless steel specimen to 82.0% for the specimen after 5% cold rolling reduction and then annealing at 1423 K for 10 min.These special boundaries of high proportion efectively interrupt the connectivity of conventional high angle grain boundary network and thus achieve the GBCD optimization for the high-nitrogen austenitic stainless steel.

不同类型冲击电流对ZnO压敏电阻老化的影响研究

为了比较不同类型冲击电流对氧化锌(ZnO)压敏电阻直流老化的影响,将相同类型的ZnO压敏电阻样品分别开展2 ms方波(峰值为2 kA)耐受试验和4/10μs冲击电流(峰值为100 kA)试验。实验结果表明,在1~3轮(3~9次)的冲击耐受下,2 ms方波对ZnO压敏电阻的老化特性能起到一定优化作用,而4/10μs大电流冲击则使得ZnO压敏电阻老化特性持续劣化。利用扫描电子显微镜(SEM)、光谱仪、数字源表研究其微观结构以及电气性能参数变化。结合实验结果分析原因,4/10μs冲击电流对ZnO压敏电阻的损伤程度大于2 ms方波冲击电流的损伤程度,更容易引起ZnO压敏电阻的老化,在1~3轮(3~9次)2 ms方波耐受下,可以改善压敏电阻老化特性,其原因在于冲击电流产生热效应使构成晶界势垒亚稳定成分填隙锌离子在晶界发生反应而降低其浓度,得到比冲击前更稳定的晶界结,从而提高了其老化特性。

Microtexture Evolution and Grain Boundary Character Distribution of Interstitial-Free Steels With Moderate Levels of Cold Rolling Reductions

High-strength interstitial-free steel sheets have very good deep drawability when processed to have { 111 } recrystallization texture. The microtexture evolution and grain boundary character distribution of interstitial-free steels as a function of moderate levels of cold rolling reductions were investigated by the metallographic microscopy and electron backscatter diffraction technique. The results showed that there was a close relationship between micro- texture and grain boundary character distribution for interstitial-free steel, especially the distribution and features of some specific types of coincident-site lattice boundaries. In addition, a-fiber texture was weakened to vanish while 7- fiber texture strengthened gradually as cold rolling reduction was increased from 20% to 75 % for cold rolled and an- nealed samples. Accordingly, increasing the rolling reduction from 20 % to 750% would lead to a significant increase in the proportion of ∑3 boundaries. Also, it was found that the microtexture of 20% cold rolled sample would induce a high frequency of ∑11 grain boundaries, but the microtexture of 75% cold rolled sample would produce more ∑7 and ∑17 grain boundaries. It was suggested that texture played a significant role in the formation of grain boundary character distribution.

Improving Intergranular Stress Corrosion Cracking Resistance in a Fe–18Cr–17Mn–2Mo–0.85N Austenitic Stainless Steel Through Grain Boundary Character Distribution Optimization

The grain boundary character distribution(GBCD) optimization and its effect on the intergranular stress corrosion cracking(IGSCC) resistance in a cold-rolled and subsequently annealed Fe-18 Cr-17 Mn-2 Mo-0.85 N high-nitrogen nickel-free austenitic stainless steel were systematically explored.The results show that stacking faults and planar slip bands appearing at the right amount of deformation(lower than 10%) are beneficial cold-rolled microstructures to the GBCD optimization.The proportion of special boundaries gradually increases in the subsequent stages of recrystallization and grain growth,accompanying with the growth of twin-related domain in the experimental steel.In this way,the fraction of low ∑ coincidence site lattice(CSL) boundaries can reach as high as 82.85% for the specimen cold-rolled by 5% and then annealed at 1423 K for 72 h.After GBCD optimization,low ∑ CSL boundaries and the special triple junctions(J2,J3) of high proportion can greatly hinder the nitride precipitation along grain boundaries and enhance the capability for intergranular crack arrest,thus improving the IGSCC resistance of the experimental steel.

退火处理对拉拔态B10白铜管材组织和腐蚀行为的影响

对取自工程试制的大口径B10白铜管坯进行晶界工程处理:经21%冷扩径拉拔变形后进行成品退火,随后空冷。采用EBSD、SEM、EDS、XPS和失重法研究不同退火温度对拉拔态B10白铜管材晶界特征分布和腐蚀行为的影响。研究结果表明:高温再结晶退火有助于优化拉拔态B10白铜管材的晶界特征分布和提高耐蚀性能。B10白铜管冷拔变形21%,在720℃退火10 min时,低ΣCSL晶界的比例达到73.25%,形成了大尺寸的“互有Σ3n取向关系”的晶粒团簇,具有最优的晶界特征分布。海水浸泡腐蚀52 d后,B10白铜表面主要形成保护性的Cu_(2)O膜;基体的腐蚀形式随退火温度的升高由剥蚀转变为晶间腐蚀。其中在720℃退火10 min的B10白铜管材具有最高比例的低ΣCSL晶界、最大的晶粒团簇平均尺寸和较少的接近<111>取向的晶粒,同时腐蚀表面易形成较厚且愈致密的Cu_(2)O膜,表现出较佳的耐蚀性能,其浸泡腐蚀速率相比于拉拔态B10白铜管材降低了24.33%。

晶界界面匹配表征方法及其在晶界工程中的应用

晶界界面匹配是指相邻两个晶粒终止于其晶界平面处的晶体学晶面之间的匹配。晶界界面匹配决定着晶界的位错结构、自由体积、原子键合状态和小面化行为等,是晶界结构的本征参量。本文重点介绍了基于电子背散射衍射和体视学的晶界界面匹配表征方法,主要包括晶界重构和晶界提取、晶界过滤、五参数晶界面取向分布统计以及晶界界面匹配定性和定量表征等,总结了近年来晶界界面匹配表征方法在高层错能面心立方金属、体心立方金属以及六方WC/Co硬质合金和Si3N4先进陶瓷材料晶界工程研究中的应用,强调了晶界界面匹配表征方法的普遍适用性及其在材料的基于晶界结构与特性的创新性组织设计和性能优化研究领域的潜在价值。

基于晶界特征分布优化的FCC高熵合金耐蚀性提升

采用电子背散射衍射、动电位极化曲线和电化学交流阻抗谱技术,针对FCC结构单相CoCrFeMnNi高熵合金,研究10%~40%冷轧退火合金的晶界特征分布(GBCD)及其在0.5 mol·L-1H2SO4溶液中的电化学行为。结果表明:10%冷轧退火使特殊晶界比例提升到初始状态(45%)的1.6倍(71.4%),组织形成大尺寸晶粒团簇,即实现了GBCD优化,而20%~40%冷轧退火导致特殊晶界比例显著下降。10%冷轧退火对该合金GBCD的优化,降低其自腐蚀电流密度至3.990×10-6A·cm-2、提高膜层电阻至4.666×104Ω·cm2以及增大电荷传递电阻至3 770Ω·cm2,使该合金表现出优异的耐蚀性。揭示了钝化膜致密性是控制该合金电化学反应的主导因素,提升特殊晶界比例可改善钝化膜致密性,促使膜层电阻急剧增大。

EBSD分析技术及其在钢铁材料研究中的应用

电子背散射衍射(EBSD)是一种较新的,用来分析多晶材料显微结构和组织的扫描电子显微镜技术。简述了EBSD技术的发展历程、基本原理、硬件装置和试验分析方法。并结合实例说明EBSD技术在钢铁材料中取向、晶界研究等方面的应用。

形变及热处理对白铜B10合金晶界特征分布的影响

研究晶界工程处理过程中的冷轧变形量和再结晶退火对白铜B10合金晶界特征分布的影响,采用电子背散射衍射(EBSD)技术表征分析晶界网络的变化。结果表明:白铜B10合金经冷轧7%后在800℃退火10 min可使低ΣCSL(Coincidence site lattice,Σ≤29)晶界比例提高到75%以上,同时形成尺寸较大的"互有Σ3n取向关系晶粒的团簇"显微组织。当变形量小于7%时,经800℃退火后没有完全再结晶;当变形量大于7%时,低ΣCSL晶界比例和平均晶粒团簇的尺寸随冷轧变形量的增加而下降。

不同温度轧制Pb-Ca-Sn-Al合金高温退火后的晶界特征分布

采用电子背散射衍射(EBSD)技术研究了不同温度轧制的Pb-0.09Ca-1.5Sn-0.026Al(质量分数,%)合金经后续270℃/10 min退火后的晶界特征分布(GBCD),结果表明:经室温和液氮(-196℃)轧制的样品中出现了比较多的∑3,∑9和∑27特殊晶界,∑3品界比例超过了60%,(Σ9+∑27)晶界比例超过了12%,并且特殊晶界有效地阻断了一般大角度晶界的网络连通性,表明合金的GBCD破优化;在100℃轧制的样品中特殊晶界比例较少,Σ3品界比例在30%左右,(∑9+∑27)晶界比例在3%左右,特殊晶界不能有效地阻断一般大角度晶界的网络连通性,合金的GBCD没有被优化,通过基于EBSD的单一截面迹线法对共格和非共格Σ3品界进行了区分测定,显示被优化的晶界特征分布中,非共格Σ3品界是主要部分。

IF钢织构与晶界特征分布的研究

对IF钢板的罩式退火(BA)和连续退火(CA)工艺进行了模拟实验,并运用EBSD技术对两种IF钢板的织构、晶界特征分布及其与二次加工脆性之间的关系进行了研究.结果表明,两种不同退火工艺的IF钢板在织构和晶界特征分布上存在很大的差异:(1)连续退火IF钢板呈现强烈的{111}再结晶织构,它由{111}(110)和{111}(112)两类取向晶粒组成;罩式退火IF钢板的{111}再结晶织构相对较弱,且主要由{111}(110)组成;(2)连续退火IF钢板中含有较多的∑3,∑13,∑9和∑11重位晶界,而罩式退火的IF钢板中只含有∑3和∑13重位晶界;(3)连续退火IF钢板中低能晶界与高能随机晶界均匀分布,而罩式退火IF钢板中低能晶界成团集中分布,高能晶界分布在团簇周围构成粗大的网状,这是IF钢板产生二次加工脆性的重要原因.

镍基690合金时效过程中晶界碳化物的形貌演化

用SEM研究了含有高比例低∑重位点阵(CSL)晶界的镍基690合金在715℃时效过程中不同类型晶界上碳化物的形貌演化.结果表明:低∑CSL晶界与一般大角晶界上碳化物的形貌演化特征有明显区别.晶界上的碳化物粒子尺寸随∑值降低而减小,共格∑3晶界上的小尺寸碳化物在长时间时效后无明显变化;非共格∑3晶界和∑9晶界附近都观察到板条状碳化物,并随时效时间延长明显长大,非共格∑3晶界两侧都存在碳化物板条,而∑9晶界上的碳化物板条只在晶界一侧生长;∑27晶界与一般大角晶界处的碳化物形貌相似,在晶界附近未观察到板条状碳化物.

冷轧变形Pb-Ca-Sn-Al合金在回复和再结晶过程中的晶界特征分布

采用电子背散射衍射(EBSD)技术研究了冷轧变形Pb-0．1％Ca-1．5％Sn-0．026％Al合金在回复和再结晶过程中的晶界特征分布(GBCD)．结果表明:在回复过程中,合金内形成了比例接近50％的平直∑3晶界,这类晶界不处在由一般大角度晶界构成的晶界网络上,不能使合金的GBCD得到优化;相反,在再结晶过程中,除了生成比例超过50％的∑3晶界外, 还出现了较多的∑9和∑27等低∑重位点阵晶界(CSL),并且这些晶界和相当多的弯曲的∑3晶界均处在由一般大角度晶界构成的晶界网络上,可以使合金的GBCD得到优化．进一步的分析指出:回复过程中所形成的平直的∑3晶界是共格孪晶界, 它们能量很低,很难迁移;在再结晶过程中,除了生成不可迁移的共格的∑3孪晶界外,还可形成大量可迁移的弯曲的非共格∑3 晶界,这类晶界的迁移和彼此会合可形成∑9和∑27等∑3n(n为正的整数)晶界,这是合金GBCD得到优化的根源．

高温退火过程中铅合金晶界特征分布的演化

应用电子背散射衍射(EBSD)和取向成像显微(OIM)技术,对经过不同形变及热处理的铅合金样品晶界特征分布(GBCD)进行了分析,研究结果表明：进行合适的冷轧后在高温(0．9T_m)短时间退火,可将铅合金的低∑(∑≤29)重位点阵(CSL)晶界比例提高到70％以上,回复过程中形成∑1晶界的同时,也出现了∑3晶界,∑3晶界在再结晶的初期过程中得到发展,这是提高低∑CSL晶界比例的主要原因,在低∑CSL晶界比例较高时,从OIM中很容易找到由三条CSL晶界构成的界角,该界角的三个晶粒之间存在特定的取向关系。

形变及热处理对690合金晶界特征分布的影响

应用电子背散射衍射(EBSD)和取向成像显微技术(OIM)研究了材料初始状态、冷轧压下量和1100℃退火对690合金晶界特征分布(GBCD)的影响。低层错能面心立方金属镍基690合金,冷轧5%后在1100℃退火5min可使低ΣCSL(coincidence site lattice,Σ≤29)晶界比例提高到70%以上(Palumbo-Aust标准),同时形成大尺寸的晶粒团簇。低ΣCSL晶界比例和这种晶粒团簇的尺寸随冷轧压下量的增加而下降。初始状态的固溶或时效对690合金在1100℃再结晶退火后的晶界特征分布无明显影响。