热变形对中铬半钢热疲劳裂纹扩展动力学的影响

研究了热变形对中铬半钢热疲劳裂纹扩展动力学的影响。结果表明 :中铬半钢的热疲劳裂纹扩展符合 L =b Na(a<1)的关系。变形量小于 4 0 %时 ,随变形量的增加 ,裂纹扩展速率减小 ,其激活能随之增大。这些变化是由于碳化物形状的改变和粒状碳化物的析出所致。     

Effect of Environment on Fatigue Crack Wake Dislocation Structure in Al-Cu-Mg

Fatigue-induced dislocation structure was imaged at the crack surface using transmission electron microscopy (TEM) of focused ion beam (FIB)-prepared cross sections of naturally aged Al-4Cu-1.4Mg stressed at a constant stress intensity range (7?MPa??m) concurrent with either ultralow (~10^?8?Pa?s) or high-purity (50?Pa?s) water vapor exposure at 296?K (23?°C). A 200-to-600-nm-thick recovered-dislocation cell structure formed adjacent to the crack surface from planar slip bands in the plastic zone with the thickness of the cell structure and slip bands decreasing with increasing water vapor exposure. This result suggested lowered plastic strain accumulation in the moist environment relative to the vacuum. The previously reported fatigue crack surface crystallography is explained by the underlying dislocation substructure. For a vacuum, $ \left\{ { 1 1 1} \right\} $ facets dominate the crack path from localized slip band cracking without resolvable dislocation cells, but cell formation causes some off- $ \left\{ { 1 1 1} \right\} $ features. With water vapor present, the high level of hydrogen trapped within the developed dislocation structure could promote decohesion manifest as either low-index $ \left\{ { 100} \right\} $ or $ \left\{ { 1 10} \right\} $ facets, as well as high-index cracking through the fatigue-formed subgrain structure. These features and damage scenario provide a physical basis for modeling discontinuous environmental fatigue crack growth governed by both cyclic strain range and maximum tensile stress.     

Effect of Rare Earth Element on Formation and Propagation of Thermal Fatigue Crack in Low-Chromium Semi-Steel

The formation and growth of thermal fatigue crack in low chromium semi steel were investigated by means of optical microscope and scanning electron microscope, and the function of RE in low-chromium semi steel was analyzed. The results show that the thermal fatigue cracks are mainly generated at eutectic carbides, and the cracks not only grow and spread but also join each other. RE can improve the eutectic carbide‘s morphology, inhibit the generation and propagation of thermal fatigue cracks, and therefore promote the activation energy for the crack‘s propagation, which is especially more noticeable in case of the RE modification in combination with heat treatment. The mathematical model of the crack propagation is put forward.     

Molecular Dynamics Simulations of Dislocation Activity in Single-Crystal and Nanocrystalline Copper Doped with Antimony

Recent experimental and simulation results have indicated that high-temperature grain growth in nanocrystalline (NC) materials can be suppressed by introducing dopant atoms at the grain boundaries. However, the influence of grain boundary dopants on the mechanical behavior of stabilized NC materials is less clear. In this work, molecular dynamics (MD) simulations are used to study the impact of very low dopant concentrations (<1.0 at. pct Sb) on plastic deformation in single-crystal and NC Cu. A new interatomic potential for low Sb concentration Cu-Sb solid-solution alloys is used to model dopant/host and dopant/dopant interatomic interactions within the MD framework. In single-crystal models, the strained regions around the Sb atoms act as heterogeneous sources for partial dislocation nucleation; the stress associated with this process decreases with increasing Sb concentration. In NC models, MD simulations indicate that Sb dopants randomly dispersed at the grain boundaries cause an increase in the flow stress in NC Cu, implying that Sb atoms at the grain boundaries retard both grain boundary sliding and dislocation nucleation from grain boundary regions.     

钴磷镀层表面热疲劳裂纹的萌生及扩展机理

在自约束型热疲劳试验机上对化学镀钴磷合金镀层进行了热疲劳试验,采用光学显微镜、扫描电子显微镜研究了在热应力作用下镀层表面热疲劳裂纹萌生、扩展的方式和形态。结果表明:200次冷热循环后,V型缺口处发生塑性变形,并且随冷热循环的进行,热疲劳裂纹由缺口底部萌生并沿着循环方向扩展。重点分析了钴磷合金镀层表面热疲劳裂纹的萌生及扩展机理。     

Hydride-Phase Formation and its Influence on Fatigue Crack Propagation Behavior in a Zircaloy-4 Alloy

The hydride-phase formation and its influence on the fatigue behavior of a Zircaloy-4 alloy charged with hydrogen gas are investigated. First, the microstructure and fatigue crack propagation rate of the alloy in the as-received condition are studied. Second, the formation and homogeneous distribution of the delta zirconium hydride in the bulk and its effect on the fatigue crack propagation rate are presented. The results show that in the presence of hydrides, the zirconium alloy exhibits reduced toughness and enhanced crack growth rates. Finally, the influence of a preexisting fatigue crack in the specimen and the subsequent hydride formation are examined. The residual lattice strain profile around the fatigue crack tip is measured using neutron diffraction. It is observed that the combined effects of residual strains and hydride precipitation on the fatigue behavior are more severe leading to propagation of the crack under near threshold loading.     

淬火温度对60Si2Mn和GCr15钢疲劳裂纹扩展速率的影响

弹钢60Si2Mn和轴承钢GCr15正常淬火（870℃和840℃）及高温淬火（1200℃）的疲劳裂纹扩展速率试验结果表明,高温淬火使弹簧钢疲劳裂纹扩展速率下降,而使轴承钢疲劳裂纹扩展速率升高。     

航空用超高强度钢中夹杂物导致疲劳裂纹萌生与扩展的微观行为

采用扫描电镜原位观测的方法,跟踪观察了低周疲劳载荷下航空用超高强度钢MA250中夹杂物导致裂纹萌生与扩展的微观行为,得到了MA250钢低周疲劳裂纹萌生与扩展的基本特性,讨论了精确测定MA250钢疲劳裂纹扩展速率的方法.     

U71Mn和U75V钢轨钢疲劳短裂纹的扩展行为

在扫描电镜下对U71Mn和U75V钢轨钢的Ⅰ型低周疲劳加载和准静态加载下裂纹扩展的过程进行了观察.研究发现,从整体上看材料没有产生明显的宏观塑性变形,属准解理断裂.但从微观上看,裂纹的微观扩展路径曲折、不连续,常伴有分枝裂纹的形成.可观察到局部有韧窝、滑移线等典型的韧性断裂特征.尤其是U71Mn钢轨钢中还伴有局部的撕裂等更强烈的变形.钢轨钢U71Mn比U75V的抗疲劳裂纹扩展性能和韧性略优.     

Effect of Copper Alloying on Fatigue Life and Fatigue Crack Growth Behaviour of Ductile Cast Irons

Abstract

Specimens cut out and machined from Cu alloyed (about 1.6%) and unalloyed (less than 0.012%) ductile cast iron ingots were tested under cycling loading to reveal the effect of copper addition on fatigue performance. The influence of solidification rate and hence the microstructure were also studied by preparing sand cast and die cast ingots from each composition. Some high copper die cast specimens were annealed to spheroidize and homogenize the matrix’s pearlite structure. All experiments were carried out by subjecting CT type specimens to constant load amplitude and tensile to tensile sinusoidal load cycles. Electrical crack foils were employed to measure instantaneous crack sizes. Cu alloyed samples showed higher fatigue lives. Similarly, crack growth rate diagrams pointed out lower fatigue crack velocities and higher near threshold stress intensity factors in high Cu specimens. The spheroidization of pearlitic matrix resulted in an insignificant increase in fatigue crack growth rate and a 10% decrease in near threshold stress intensity value. The fatigue performance of sand cast pieces was better than die cast ones. It is concluded that the fatigue life of ductile cast irons can be improved by copper alloying.

Des éprouvettes coupées et usinées à partir de lingots de fonte ductile alliée au cuivre (approximativement 1.6%) et non alliée (moins de 0.012%) ont été évaluées sous effort cyclique afin de révéler l’effet de l’addition de cuivre sur le comportement de fatigue. On a également étudié l’influence du taux de solidification, et ainsi de la microstructure, en préparant des lingots coulés en sable et d’autres coulés en coquille, à partir de chaque composition. On a recuit quelques éprouvettes à haute teneur en cuivre, coulées en coquille, pour obtenir la sphéroïdisation et l’homogénéisation de la structure perlitique de la matrice. On a effectué toutes les expériences en exposant les éprouvettes de type C-T à des cycles avec charge d’amplitude constante, à charge sinusoïdale traction à traction. On a utilisé des films électriques à fissure pour mesurer la taille instantanée des fissures. Les échantillons alliés au cuivre ont montré une plus grande résistance à la fatigue. De façon similaire, des diagrammes de taux de croissance de la fissure indiquaient des vélocités plus basses de fissure due à la fatigue et des facteurs plus élevés d’intensité de contrainte près de la limite pour les éprouvettes à haute teneur en Cu. La sphéroïdisation de la matrice perlitique a résulté en une augmentation insignifiante du taux de croissance de fissure due à la fatigue et en une diminution de 10% de la valeur d’intensité de contrainte près de la limite. Le comportement de fatigue des pièces coulées en sable était meilleur que celui des pièces coulées en coquille. On conclut que la résistance à la fatigue des fontes ductiles peut être améliorée par alliage au cuivre.     

Effect of Structural State and Temperature on Crack Generation and Propagation Resistance in Pipe Steels

Metallurgist - Comparative research of microstructure and properties of different strength classes of plate steels produced by controlled rolling with accelerated cooling is performed. Standard...     

细晶强化和位错强化对中锰马氏体钢的强化作用

 研究了碳和锰含量对淬火中锰马氏体钢的位错密度、残余奥氏体含量、晶粒尺寸等组织结构以及室温力学性能的影响。借助于SEM、EBSD、TEM和XRD表征了材料的微观组织,探讨了马氏体钢的强化机制。结果表明:随着碳含量增加,淬火中锰钢的位错密度和残余奥氏体体积分数逐渐增加,板条束和板条块尺寸逐渐细化,大角晶界百分数逐渐增加,强度逐渐升高;增加锰含量能够提高马氏体钢的位错密度和抗拉强度。分析认为,位错强化和细晶强化是淬火中锰马氏体钢的主要强化机制。马氏体板条尺寸是马氏体抗拉强度的结构控制单元,而原奥氏体晶粒尺寸则是马氏体屈服强度的结构控制单元。     

1Cr13钢中相界面析出碳化物及其对裂纹扩展的影响

本文用薄晶体透射电镜法,探讨了1Cr13不锈钢等温分解时两种不同形态的相间析出碳化物—“相间沉定”和纤维状碳化物形成的条件,分析了它们的不同形态之间的联系;并利用扫描电镜同时观察冲击断口形貌及对应的倾斜面显微组织,发现了相界面析出碳化物对微裂纹传播的阻碍作用,最后用一次裂纹扩展作了验证,并与淬回火试样进行比较。     

Effects of Processing Residual Stresses on Fatigue Crack Growth Behavior of Structural Materials: Experimental Approaches and Microstructural Mechanisms

Fatigue crack growth mechanisms of long cracks through fields with low and high residual stresses were investigated for a common structural aluminum alloy, 6061-T61. Bulk processing residual stresses were introduced in the material by quenching during heat treatment. Compact tension (CT) specimens were fatigue crack growth (FCG) tested at varying stress ratios to capture the closure and K _max effects. The changes in fatigue crack growth mechanisms at the microstructural scale are correlated to closure, stress ratio, and plasticity, which are all dependent on residual stress. A dual-parameter ΔK–K _max approach, which includes corrections for crack closure and residual stresses, is used uniquely to connect fatigue crack growth mechanisms at the microstructural scale with changes in crack growth rates at various stress ratios for low- and high-residual-stress conditions. The methods and tools proposed in this study can be used to optimize existing materials and processes as well as to develop new materials and processes for FCG limited structural applications.     

加载方式与初生α相对钛合金裂纹扩展行为的影响

利用带载荷台的扫描电镜原位观察了近α型钛合金TA15中初生α相在单向拉伸和循环交变载荷下裂纹的扩展行为,结果表明:单向拉伸时,由于初生α相强度高于基体β转变组织,裂纹遇到初生α相时一般不能穿过而是绕过;而在交变载荷下,由于基体β转变组织产生循环硬化,其强度水平与初生α相相当,裂纹遇到初生α相时可直接贯穿,并沿与应力垂直的水平方向扩展.     

Effect of Environment on Fatigue and Creep Crack Growth in Inconel X-750 at Elevated Temperature

The fatigue crack growth rates (FCGR) of Inconel X-750 were measured in air and in vacuum at 25 °C and 650 °C as a function of test frequency. The wave shape was triangular and the frequency varied from 10 Hz to 0.01 Hz. The creep crack growth rates (CCGR) were also measured on single edge notch specimens at 650 °C in air and in purified argon. For a givenAK, the FCGR increases when temperature increases and frequency decreases. At low frequency the FCGR approach the creep crack growth rates. The mode of fracture changes from transgranular at 10 Hz to intergranular at 0.01 Hz. The effect of air environment is to accelerate the transition from transgranular to intergranular fracture modes with decreasing frequency. The role of oxidation in accelerating crack growth rate in fatigue and in creep is discussed in detail. F. GABRIELLI, was formerly a visiting scientist at Massachusetts Institute of Technology.     

Effect of Low Temperature on Fatigue Crack Formation and Microstructure-Scale Growth from Corrosion Damage in Al-Zn-Mg-Cu

The strong effect of cold temperature on the fatigue resistance of 7075-T651 is established. As temperature decreases from 296 K to 183 K (23 °C to ?90 °C), the formation life for cracking about pit and EXCO corrosion perimeters increases, microstructure scale crack growth rates decrease in the range from 20 to 500 μm beyond the corrosion topography, and long crack growth rates similarly decline. Fatigue crack surface features correlate with reduced hydrogen embrittlement with decreasing temperature fed by localized H produced during precorrosion for pit and EXCO-proximate cracks, as well as by crack tip H produced by water vapor reaction during stressing for all crack sizes. The importance of the former H source increases with decreasing temperature for cracks sized below 200 μm. Decreasing temperature to 223 K (?50 °C) eliminates the contribution of environmental H through interaction of reduced water vapor pressure in equilibrium with ice and reduced H diffusion. The Knudsen flow model and exposure parameter, $ P_{{{\text{H}}_{2} {\text{O}}}}/f $ , enables improved modeling of temperature dependent crack propagation, but does not fully describe low temperature fatigue behavior due to possible rate limitation by H diffusion. Further decreases in MSC da/dN to 183 K (?90 °C) are related to reduced mobility of the corrosion-precharged H which may associate with vacancies from dissolution. Crack formation, and growth rates correlate with either elastic stress intensity range or cyclic crack tip opening displacement, and are available to predict corrosion effects on airframe fatigue for the important low temperature regime.     

Effect of Hot Deformation on Formation and Growth of Thermal Fatigue Crack in Chromium Wear Resistant Cast Iron

Chromium wear resistant cast iron is widelyusedin engineering, mining and power industry forits high strength,hardness and wear resistance .Inproduction process , some wear-resistant parts ser-ving in alternative stress due to rapid heating andcooling rate ofteninduces thermal fatigue andresultsinfailure .The badthermal fatigue property of chro-mium wear resistant cast iron is due to eutectic car-bides which distribute as continuous net in matrix.Recent researches showthat hot deformation can …