Influence of the alloy grain size on the oxidation behaviour of PM2000 alloy

The influence of the alloy grain size on the oxidation behaviour in air between 900 and 1400 °C of oxide dispersion strengthened PM2000 alloy was studied. The grain size of the alloy affects the morphology, the composition of the alumina scale as well as the size, composition and volume fraction of nodules formed on the alumina scale, but has only a small influence in the kinetics. In the fine-grained PM2000, grain boundaries act as rapid pathways for aluminium diffusion, leading to the establishment of a purer alumina scale than that formed on coarse-grained material. On the coarse-grained alloy, there is enough time for the nucleation of oxides other than alumina. These oxides, impeding alumina grain growth, can lead to the formation of a fine-grained alumina layer. The outward cation and inward oxygen diffusion in this fine-grained alumina layer may contribute to both the slight increase in mass-gain and the increased tendency to form nodules at the outer surface.     

氧化时间对低合金弹簧钢奥氏体晶粒度测定的影响

氧化法可用于钢中晶粒度的测定.研究控制氧化时间对弹簧钢50CrV奥氏体晶粒度的影响.结果发现,氧化过程大致分为3个阶段:第一阶段,过剩铁素体相不断转变为奥氏体;第二阶段,基体与氧化膜之间的阴阳离子再次加快扩散,下层金属基体继续氧化脱碳,部分之前试样表面的奥氏体晶界被替代;第三阶段,一、二阶段的奥氏体晶粒逐渐长大,新旧氧...     

Effect of grain size on high-temperature oxidation behavior of Cu-80Ni alloy

1　INTRODUCTIONTheoxidationofunalloyedcopperandnickelhasalreadybeenstudiedindetail.Whilethehigh tem peratureoxidationofCu Nialloys ,alsostudiedanumberoftimestodate[13] ,isanexampleofarela tivelysimpleclassofscalingofbinaryalloysbyasin gleoxidant,becausethemetalsformacontinuousse riesofsolidsolutions ,whiletheiroxides ,CuO ,Cu2 OandNiO ,exhibitsmallmutualsolubilitiesandshowsignificantdifferencesinthethermodynamicstabilityandparabolicgrowthrates .Thus ,copper richalloysformexternalscales…     

晶粒度对Inconel 690合金微动磨损行为的影响

为预防及减缓微动损伤对核反应堆蒸汽发生器传热管的危害,深入研究了晶粒尺寸对Inconel 690合金微动磨损行为的影响。采用微动磨损试验方法对Inconel 690合金的微动磨损特性展开研究,并利用光学显微镜(OM)、维氏硬度计、扫描电镜(SEM)、能谱仪(EDS)和激光共焦扫描显微镜(LSCM)等对不同固溶温度下材料的微观组织结构、硬度和磨痕特征进行观察和分析。结果表明,随着固溶温度的升高,Inconel 690合金晶粒尺寸增大,硬度降低;在完全滑移区,摩擦因数随晶粒度和硬度的变化很小,其值均约为0.48;当Inconel 690合金平均晶粒尺寸为112μm,且SS304与Inconel 690合金硬度比为260∶176.4时,Inconel 690合金磨损体积最少;不同晶粒度和硬度下Inconel 690合金的微动磨损机制主要为剥层磨损、磨粒磨损和粘着磨损。     

Influence of grain size and microstructure on oxidation rates in titanium alloy Ti-6Al-4V under superplastic forming conditions

Ti-6Al-4V (Ti-6-4) sheets of two different grain sizes were exposed to time and temperature conditions representative of superplastic forming (SPF). The influence of SPF conditions on oxidation rates was evaluated in terms of weight gain, α-case depth, and microhardness profile. Differences in the response are related to the difference in grain size between the two lots of Ti-6-4. Fine grain Ti-6Al-4V exhibits faster oxygen diffusion in all three areas examined in this study, weight gain, α-case thickness, and increased microhardness depth. The differences were found to be significant relative to diffusion analysis and processing during manufacturing. Results from this work support reduced temperature SPF using fine grain material and the accompanying benefits in manufacturing superplastic parts. This paper was presented at the International Symposium on Superplasticity and Superplastic Forming sponsored by the Manufacturing Critical Sector at the ASM International AeroMat 2004 Conference and Exposition, June 8–9, 2004, in Seattle, WA. The symposium was organized by Daniel G. Sanders, The Boeing Company.     

Effect of grain size on high-temperature stress relaxation behavior of fine-grained TC4 titanium alloy

In order to analyze the effect of grain size on stress relaxation (SR) mechanism, the SR tests of TC4 alloy with three kinds of grain size were performed in a temperature range of 650–750 °C. A modified cubic delay function was used to establish SR model for each grain size. A simplified algorithm was proposed for calculating the deformation activation energy based on classical Arrhenius equation. The grain size distribution and variation were observed by microstructural methods. The experimental results indicate that smaller grains are earlier to reach the relaxation limit at the same temperature due to lower initial stress and faster relaxation rate. The SR limit at 650 °C reduces with decreasing grain size. While the effect of grain size on SR limit is not evident at 700 and 750 °C since the relaxation is fully completed. With the increase of grain size, the deformation activation energy is improved and SR mechanism at 700 °C changes from grain rotation and grain boundary sliding to dislocation movement and dynamic recovery.     

铀铌合金铸造晶粒度的计算机模拟与实验研究

利用ProCast和CalcoSoft-3D软件中的CA-FE模块,采用元胞自动机-有限元方法(Cellular automatonfinite element),对U-Nb合金以50℃/min的铸造冷却速度铸造的d107mm/d77mm×80mm管形铸件作为算例,对该铸件的微观组织和晶粒度进行模拟计算.结果表明:U-Nb合金的凝固微观组织的晶粒度变化规律为,在铸件的同一高度上,内层晶粒半径＜外层晶粒半径＜中间层晶粒半径;按从上到下的顺序晶粒半径逐渐减小.用金相方法实测了U-Nb合金铸件在各个高度位置之中间层的凝固微观组织晶粒半径,实测值小于模拟计算值近1倍,但是二者晶粒度的变化趋势是一致的,模拟结果达到了定性或半定量的水平.     

钛合金TIG焊接头晶粒尺寸非线性梯度特征

以显微组织与晶粒尺寸统计分析为基础,研究了TC4钛合金不同TIG焊工艺的焊接接头显微组织及其晶粒尺寸变化,引入了晶粒梯度特征值概念,以表征钛合金焊接接头显微组织的非均匀性.结果表明,TC4钛合金TIG焊接头显微组织的晶粒尺寸从焊缝中心经热影响区至母材呈现非线性变化,晶粒尺寸变化率(晶粒梯度)在柱状晶区(CZ)和粗晶区(CGZ)交界处出现最大值,称之为晶粒梯度特征值;随着TIG焊热输入增大,晶粒梯度特征值逐渐减小,但相对位置不发生改变.晶粒尺寸梯度特征值能够反映钛合金焊接接头组织的不均匀性,这对于探索钛合金焊接接头的服役性能具有重要的意义.     

Synergistic influence of alloy grain size and Si content on the oxidation behavior of 9Cr−1Mo steel

The synergistic influence of prior-austenite grain size and silicon content of 9Cr–1Mo steel on the resistance to scale spallation has been studied in air at 773 K (for 500 hr) and 973 K (12 hr). Two steels, irrespective of their grain size and Si content, did not show spallation during oxidation at 773 K. Spallation occurred at 973 K, and fine-grain steels exhibited less spallation resistance than coarse-grain ones (in low-as well as high-Si steels). Among the four possible combinations of grain size ans Si content, the steel with low Si and fine grains showed least resistance to spallation, while the steel with high Si and coarse grains showed the best resistance. Spallation was found to initiate in the areas adjoining the oxide ridges formed at the alloy grain boundaries. Oxide scales at the ridges and within the grains were analyzed by scanning electron microscopy (SEM/EDX) and secondary-ion mass spectrometry (SIMS). These analyses suggest depletion of silicon from the areas adjoining grain boundaries, resulting in thicker scaling that triggers spallation in such areas. For similar grain-size materials, the necessary thickness for spallation was attained earlier with low-Si steel rather than in high-Si steel.     

镍基合金大锻件成形过程流变行为与晶粒尺寸预测

基于IN718合金热态成形过程中流动应力软化特征,采用动态再结晶软化本构模型来描述锻造过程中变形-传热-微观组织演变的相互作用。借助DEFORM-3D二次开发,将本构模型和微观演变模型进行耦合,模拟预测了IN718合金在多道次锻造过程中流变行为与晶粒尺寸演变,为大锻件锻造热力参数的合理制定与控制提供了依据。     

基于BP神经网络的镁合金晶粒尺寸及流变应力模型

通过等温压缩试验研究了热变形条件对AZ61B镁合金再结晶晶粒尺寸及其流变应力的影响,并采用人工神经网络方法分别建立了动态再结晶晶粒尺寸及流变应力的模型。利用反向传播算法对网络进行试探性训练研究,得到了最佳参数。结果表明,所建立的网络模型具有优良的性能,能精确预测AZ61B合金热变形条件下的再结晶晶粒尺寸及流变应力。     

热处理对Al-Zn-Mg-Cu合金第二相粒子分布和晶粒尺寸的影响

通过对均匀化改锻后的坯料进行了固溶+过时效和直接过时效的中间热处理,分析了新型Al-Zn-Mg-Cu高强铝合金经过不同中间热处理后微观组织中第二相粒子的大小和分布特征,以及第二相粒子分布特征对随后热变形和退火工序过程中晶粒尺寸演变的影响。结果表明,锻坯经过400 ℃×12 h直接过时效中间处理后,组织中第二相粒子的尺寸呈现“双峰”分布特征;把具有该组织特征的试样加热到420 ℃的始锻温度,进行50%的压缩变形,再经400 ℃×1 h退火处理后,得到比较均匀细小的晶粒组织。     

High-temperature oxidation behavior of iridium-based alumina-forming ternary intermetallics

The oxidation behavior of iridium-based intermetallics in the ternary iridium-aluminum-silicon system has been investigated. Additions of up to 20 at. % silicon to iridium-aluminum binary alloys reduce the aluminum concentration necessary for the formation of a protective alumina scale from greater than 55 at.% in the absence of silicon to as low as 20 at.% when 20 at.% silicon is present. Ternary iridium-aluminum-silicon alloys with 30–50 at.% aluminum and 8–10 at.% silicon exhibit parabolic oxidation behavior, and the rate constants are in good agreement with those previously determined for binary iridium-aluminum alloys. The oxidation rate of ternary iridium-aluminum-silicon alloys with 10 at.% silicon and 30–40 at.% aluminum deviates from the parabolic-oxidation behavior for alumina scale growth in the later oxidation stage, presumably due to the formation of a silica inner layer. Thermal cycling of the Ir-50 Al-8Si and Ir-60-Al samples does not result in scale spallation due to the coefficient of thermal expansion match between the alumina scale and intermetallic substrate.     

Oxidation of Fe-C alloys at 500°C

Fe-C alloys containing 0.1, 0.5, and 1.0% C were oxidized in 1 atm O₂ at 500°C. Two specimen preparations were used: annealed followed by slow cooling to form coarse pearlite plus proeutectoid ferrite or cementite; and cold-worked by abrading after annealing. The cold-worked alloys oxidize more rapidly. Annealed pearlite oxidizes faster than annealed ferrite. The differences in oxidation rate are caused by differences in the Fe₃O₄ grain size, that is, by the number of oxide grain boundaries available to act as easy diffusion paths for the outward diffusion of Fe through the Fe₃O₄. The oxidation rate constant is 10 times larger for fine-grained poly crystalline oxide than for oxide in which the Fe₃O₄ is monocrystalline.     

The effect of thermal cycling on the subsequent oxidation behavior of an aligned Co-based TaC eutectic alloy

In situ aligned composites are envisioned for applications in environments involving simultaneous exposure to the effects of thermal cycling and oxidizing atmospheres. Therefore, to examine the extent of any synergism between these two effects, the oxidation behavior at 800°C and 1000°C of the aligned eutectic alloy TaC-Co50B-3W was examined in both the as-solidified state and after exposure to thermal cycling effects. The results presented here indicate that thermal cycling strongly influences the oxidation behavior of this alloy.     

基于Matlab的300M钢奥氏体晶粒的长大规律

研究了300 M钢在不同加热温度(850～1180℃)和保温时间(5～120 min)下的奥氏体晶粒长大规律。绘制了300 M钢奥氏体晶粒尺寸在不同加热温度和保温时间下的等值线图;利用Sellars晶粒长大模型,构建了300 M钢的奥氏体晶粒长大数学模型。结果表明,300 M钢在高温加热时具有较好的抗晶粒粗化能力,在1050℃左右开始粗化。奥氏体晶粒尺寸等值线图可定性和定量预测奥氏体晶粒长大规律;奥氏体晶粒长大数学模型可用两个数学公式来描述,即当加热温度为850℃≤T≤1050℃时,d6.14=texp(68.97-64945.88/T);当加热温度为1050℃≤T≤1180℃时,d7.39=texp(134.56-144504.52/T)。     

Fe-Cr-Al电热合金的高温氧化行为

对一种Fe-Cr-Al系电热合金0Cr25Al5进行了不同温度(950、1050、1150 ℃)的氧化质量增加试验。结果表明,合金试样在950 ℃下属于完全抗氧化级,在1050 ℃下属于抗氧化级,在1150 ℃下保温40 h以上时,由于Cr₂O₃的氧化挥发导致氧化膜出现空洞,大大降低了合金的抗氧化性能。XRD物相分析表明合金表面氧化膜主要由TiO₂、Al₂O₃和FeCr₂O₄组成。     

Effect of grain size reduction on high temperature oxidation of binary two-phase alloys

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Influence of ball‐milling on the oxidation behavior of Ag–Zn alloy powders

The gas atomized Ag–15.2 at% Zn alloy powders were treated using ball‐milling. The effect of ball‐milling on surface morphology, microstructure, and oxidation behavior of the alloy powders was investigated. The results showed that ball‐milling treatment enhanced the oxidation rate of Ag–Zn alloy powders due to the increase of dislocation density and the decrease of the diffusion distance of oxygen during internal oxidation. The fully oxidized time of Ag–Zn alloy powders decreases with the ball‐milling time. In addition, the ball‐milling treatment not only changes the oxidation mechanism but also improves microstructure and properties of the Ag–ZnO materials fabricated by internal oxidation.