Microstructure-Sensitive Computational Estimates of Driving Forces for Surface Versus Subsurface Fatigue Crack Formation in Duplex Ti-6Al-4V and Al 7075-T6

Statistical realizations of three-dimensional digital microstructures with different crystallographic orientation distributions, grain shapes, and grain size distributions are subjected to uniaxial cyclic straining to compare the cases of bulk and free surface on driving forces for fatigue crack formation. Crystal plasticity finite element simulations are conducted using both fully periodic (more representative of the bulk) and traction-free (i.e., free surface) boundary conditions for duplex Ti-6Al-4V and rolled Al 7075-T6. Following elastic–plastic shakedown, mesoscale volume-averaged fatigue indicator parameters (FIPs) are computed within fatigue damage process zones of grains and are fit to known extreme value distributions (EVDs). Owing to differences in crystallographic slip symmetry, FIPs for fcc Al 7075-T6 statistically occur much closer to the traction-free surface than for hcp Ti-6Al-4V. Additionally, surface versus bulk EVDs of FIPs vary differently for the two material systems, indicating a coupled role of microstructure and surface proximity that had not been previously elucidated.

     

High-cycle fatigue of nickel-base superalloy René 104 (ME3): Interaction of microstructurally small cracks with grain boundaries of known character

High-cycle fatigue (HCF), involving the premature initiation and/or rapid propagation of small cracks to failure due to high-frequency cyclic loading, has been identified as one of the leading causes of turbine engine failures in aircraft. In this work, we consider the feasibility of using grain-boundary engineering to improve the HCF properties of a polycrystalline nickel-base superalloy, René 104 (also known as ME3), through systematic modification of the grain-boundary distribution. In particular, we investigate the growth of microstructurally small fatigue cracks at ambient temperature in microstructures with varying proportions of “special” vs. “random” boundaries, as defined by coincident-site lattice theory. Specifically, we examine the interaction of propagating small (∼10–900 μm) surface cracks with grain boundaries of known character, with respect both to any deflection in crack trajectory that occurs at or near the boundary, and more importantly to any local changes in crack-growth rates. In addition, finite-element calculations are performed to evaluate the effective driving force and plastic-zone profile for such small-crack propagation, incorporating information from both the local microstructure (from electron backscattering diffraction scans) and the surface crack-path profile.     

AZ91系列压铸镁合金高周疲劳断口形貌分析

AZ91D压铸镁合金中加入富Ce稀土后,组织得到明显细化,压铸过程中产生的气孔变小,分布更加均匀,能够减少疲劳裂纹源的产生。AZ91D合金拉伸断口表现为既有解理台阶又存在细小撕裂棱及韧窝的混合断裂特征,加入1?后,合金断口的韧性断裂特征增加,有利于提高合金的室温力学性能。AZ91D合金疲劳裂纹源萌生于合金内部的气孔和夹杂处,疲劳裂纹沿晶界扩展,加入稀土后,疲劳裂纹扩展区有疲劳辉纹出现,疲劳断裂呈现出准解理和韧窝断裂的混合特征。     

表层组织状态对6005A铝合金MIG焊接头液化裂纹及疲劳性能的影响

采用扫描电镜、透射电镜、电子背散射衍射和高周疲劳试验,研究了表层组织状态对轨道交通用6005A铝合金MIG焊接头液化裂纹及疲劳性能的影响.?结果表明,粗晶组织晶界附近第二相粗大,导致热影响区晶界液膜厚度达到8?～?10?μm,使得液化晶界抵抗拉应力的能力降低,从而对液化裂纹缺陷更敏感.?因表层粗晶组织形成的液化裂纹成为...     

Effect of stress ratio on long life fatigue behavior of Ti-Al alloy under flexural loading

A new ultrasonic three-point bending fatigue test device was introduced to investigate fatigue life ranging up to 10^10 cycles and associated fr＇dcture behavior of Ti-Al alloy. Tests were performed at a frequency of 20 kHz with stress ratio R=0.5 and R=0.7 at ambient temperature in air. Three groups of specimens with different surface roughness were applied to investigate the effect of surface roughness on fatigue life. Furthermore, optical microscopy（OM） and scanning electron microscopy（SEM） were used for microstructure characteristic and fracture surface analysis. The S-N curves obtained show that fatigue failure occurs in the range of 10^5-10^10 cycles, and the asymptote of S-N curve inclines slightly in very high cycle regime, but is not horizontal for R=0.5. Fatigue limit appears after 10s cycles for R=0.7. Surface roughness （the maximum roughness is no more than 3 μm） has no influence on the fatigue properties in the high cycle regime. A detailed investigation on fatigue fracture surface shows that the Ti-Al alloy studied here is a binary alloy in the microstructure composed of α2-Ti3Al and γ-Ti-Al with fully lamellar microstructure. Fractography shows that fatigue failures are mostly initiated on the surface of specimens, also, in very high cycle regime, subsurface fatigue crack initiation can be found. Interlamellar fatigue crack initiation is predominant in the Ti-Al alloy with fully lamellar structure. Fatigue crack growth is mainly in transgranular mode.     

Predicting fatigue crack initiation through image-based micromechanical modeling

The influence of individual grain orientation on early fatigue crack initiation in a four-point bend fatigue test was investigated numerically and experimentally. The 99.99% aluminium test sample was subjected to high cycle fatigue (HCF) and the top surface microstructure within the inner span of the sample was characterized using electron-beam backscattering diffraction (EBSD). Applying a finite-element submodelling approach, the microstructure was digitally reconstructed and refined studies carried out in regions where fatigue damage was observed. The constitutive behaviour of aluminium was described by a crystal plasticity model which considers the evolution of dislocations and accumulation of edge dislocation dipoles. Using an energy-based approach to quantify fatigue damage, the model correctly predicts regions in grains where early fatigue crack initiation was observed. The tendency for fatigue cracks to initiate in these grains appears to be strongly linked to the orientations of the grains relative to the direction of loading – grains less favourably aligned with respect to the loading direction appear more susceptible to fatigue crack initiation. The limitations of this modelling approach are also highlighted and discussed, as some grains predicted to initiate cracks did not show any visible signs of fatigue cracking in the same locations during testing.     

显微组织对TB17钛合金高周疲劳性能的影响

对比研究了TB17钛合金3种典型组织形态(双态组织、网篮组织和片层组织)对其高周疲劳性能的影响,并分析了高周疲劳断口形貌。结果表明：双态组织特征的TB17钛合金具有最高的强塑性匹配水平,但其疲劳寿命与应力呈双线性关系,疲劳性能并不稳定;网篮组织的强塑性稍差,但具有最高的疲劳强度和疲劳比;片层组织的疲劳强度比网篮组织略低,但其疲劳比和拉伸塑性最差。高周疲劳加载应力处于低应力状态时,疲劳裂纹倾向于试样内部、单源萌生,而处于高应力状态时,疲劳裂纹倾向于试样表面、多源萌生。网篮组织存在更多的二次裂纹,且疲劳条带更为清晰密集,裂纹扩展路径更曲折,在扩展时消耗的能量更多。     

基于FIP的焊趾短裂纹行为仿真

短裂纹的成核与早期扩展是疲劳裂纹演变过程的重要组成部分,对疲劳裂纹的演变行为和寿命预测具有重要意义. 针对焊趾局部的微观结构短裂纹,以微观敏感的疲劳指标参量为主要驱动力,基于泰森多边形方法构建了焊趾区域的晶粒模型,模拟了微观结构短裂纹的早期演变过程. 通过与试验结果对比,证实了微观敏感的疲劳指标参量及其计算模型的合理性和有效性. 仿真结果表明,疲劳寿命同时受晶粒所在位置、晶粒尺寸和晶粒取向等因素的影响,晶粒取向的随机性对裂纹宏观尺寸的分散性具有重要作用.     

金属表层梯度微结构对力学性能和裂纹扩展的影响

表层梯度结构广泛应用于工程材料,经表面强化技术处理后,零件表层结构呈梯度分布,可以改善零件的服役能力.综述了金属表层梯度微结构对力学性能和裂纹扩展的影响,从微观结构和力学性能的角度,结合非均匀材料中裂纹的扩展行为,考虑了微结构和力学性能之间的相互影响规律,分析了梯度结构中影响裂纹扩展的因素.经表面强化后,材料表面晶粒细化,晶粒尺寸沿纵深方向呈现梯度分布,改变裂纹萌生位置,同时对裂纹扩展起到阻碍,使零件疲劳寿命延长.梯度结构中金相组织由表层单一组织到芯部多相组织的转变,引起表层硬度和残余压应力向纵深方向逐渐减小,表面压应力可以减弱外力的作用,从而影响疲劳寿命.然而,梯度结构中微观组织和力学性能互相关联,像晶粒和位错能改变残余应力的分布,也会对裂纹扩展路径产生影响,如何定量表征他们的影响还有待进一步研究.     

A reaction-layer mechanism for the delayed failure of micron-scale polycrystalline silicon structural films subjected to high-cycle fatigue loading

A study has been made to discern the mechanisms for the delayed failure of 2-μm thick structural films of n⁺-type, polycrystalline silicon under high-cycle fatigue loading conditions. Such polycrystalline silicon films are used in small-scale structural applications including microelectromechanical systems (MEMS) and are known to display ‘metal-like’ stress-life (S/N) fatigue behavior in room temperature air environments. Previously, fatigue lives in excess of 10¹¹ cycles have been observed at high frequency (~40 kHz), fully-reversed stress amplitudes as low as half the fracture strength using a surface micromachined, resonant-loaded, fatigue characterization structure. In this work the accumulation of fatigue-induced oxidation and cracking of the native SiO₂ of the polycrystalline silicon was established using transmission electron and infrared microscopy and correlated with experimentally observed changes in specimen compliance using numerical models. These results were used to establish that the mechanism of the apparent fatigue failure of thin-film silicon involves sequential oxidation and environmentally-assisted crack growth solely within the native SiO₂ layer. This ‘reaction-layer fatigue’ mechanism is only significant in thin films where the critical crack size for catastrophic failure can be reached by a crack growing within the oxide layer. It is shown that the susceptibility of thin-film silicon to such failures can be suppressed by the use of alkene-based monolayer coatings that prevent the formation of the native oxide.     

Damage mechanism of a nodular cast iron under the very high cycle fatigue regime

Fatigue behaviour in the very high cycle regime (VHCF) of 10¹⁰ cycles were investigated with a cast iron (GS51) under ultrasonic fatigue test system (20 kHz) in ambient air at room temperature with a stress ratio R = −1.

The influence of frequency was examined by comparing similar data generated on conventional servo hydraulic test systems. An advanced, high-speed, and high-sensitivity infrared imaging system was used to measure the temperature changes during ultrasonic fatigue test at various load levels caused by internal damping due to a very high frequency cycling. The temperature field on the surface specimen was determined by using a non-destructive measurement technique called infrared pyrometer. An infrared camera made up of a matrix of 320 × 240 detectors was used.

The S-N curves obtained show that fatigue failure occurred beyond 10⁹ cycles, fatigue limit does not exist for the cast iron and there is no evidence of frequency effect on the test results. A detailed study on fatigue specimens subjected to ultrasonic frequency shows that the temperature evolution of the cast iron specimen is very evident, the temperature increased just at the beginning of the test, the temperature increased depending on the maximum stress amplitude. Under the current test conditions, the high cycle fatigue (VHCF) behaviour of the cast iron exhibited a typical fatigue crack growth process, that is, fatigue initiation takes place always at the surface graphite or subsurface void; the distinctive stable fatigue crack growth zone can be found around the fatigue crack initiation site, the change of fatigue initiation site from surface to subsurface is associated with the complex effects of applied maximum stress level, surface condition.

Under lower stress amplitude and high cycle condition, surface graphite fatigue initiation is predominantly depended on cyclic stress amplitude; subsurface void fatigue initiation is determined by maximum cyclic stress.

In the process of small crack propagation, the temperature in local plastic zone increase very sharply. The temperature field of ultrasonic fatigue specimen can be changed with the cooling condition; internal heating can accelerate surface fatigue crack initiation and propagation.

Fatigue properties in VHCF regime were studied for cast iron (GS51) at 20 kHz frequency and for the first time, crack initiation and propagation stages were analyzed using a high-sensitivity infrared camera. The new Paris's model for fish eye formation in the gigacycle fatigue were also confirmed by this study.     

激光冲击强化ZCuAl10Fe3Mn2合金的热疲劳裂纹生长行为

为了提高高温构件的热疲劳性能、减少表面裂纹,研究激光冲击对ZCuAl10Fe3Mn2合金硬度、表面形貌、残余应力和热疲劳性能的影响.采用扫描电子显微镜(SEM)和能谱仪(EDS)分析合金的显微组织和裂纹形貌.结果表明:在4 J脉冲能量下,激光冲击能显著改善合金的热疲劳性能.在热应力和交变应力的作用下,试样缺口附近组织氧...     

Failure mode and fatigue mechanism of laser-remelted plasma-sprayed Ni alloy coatings in rolling contact

The failure mode and rolling contact fatigue failure mechanism of the laser-melted plasma-sprayed coatings were investigated in this paper. The coating was deposited by using a high-efficiency plasma spraying system and remelted by using a CO₂ laser in a continuous mode. Thirteen rolling contact tests were performed to obtain the statistical result at an identical condition. Experimental results showed that spalling is the main failure mode of the remelted coating in rolling contact. Prior to the formation of the spall, the surface main crack and ring-type crack have been generated. However, only the main crack might not directly cause the formation of the spall. The joining of the ring-type cracks and subsurface branched cracks was directly responsible for the spall formation.     

Fatigue of Austempered Ductile Iron with Two Strength Grades in Very High Cycle Regime

In this study, Austempered ductile irons (ADIs) with two different strength grades were produced and the fatigue properties were measured at 10⁹ cycles. The results show that the S-N curves give a typical step-wise shape and there is no fatigue limit in the very high cycle fatigue regime. The two grades ADI have the similar fracture behaviors and fatigue failure can initiate from defects at specimen surface and subsurface zone. On the fracture surfaces of some specimens, the ‘granular-bright-facet’ area with rich carbon distribution is observed in the vicinity of the defect. The microstructure affects the crack behaviors at the early propagation stage. The ADI with upper and lower bainite shows higher fatigue strength compared with the ADI with coarse upper bainite.     

STUDY ON FATIGUE DAMAGE BELOW THE FATIGUE LIMIT AND THE COAXING EFFECTS

ＳＴＵＤＹＯＮＦＡＴＩＧＵＥＤＡＭＡＧＥＢＥＬＯＷＴＨＥＦＡＴＩＧＵＥＬＩＭＩＴＡＮＤＴＨＥＣＯＡＸＩＮＧＥＦＦＥＣＴＳＷＵＺｈｉｘｕｅ;ＬＵＷｅｎｇｅａｎｄＸＵＨａｏ１）（ＦｕｓｈｕｎＰｅｔｒｏｌｅｕｍｉｎｓｔｉｔｕｔｅ,Ｆｕｓｈｕｎ１１３００１,...     

纳米晶Ni疲劳行为的实验研究

系统研究了纳米晶Ni与粗晶Ni的疲劳行为. 通过疲劳实验获得了这2种材料的疲劳应力--寿命曲线, 并采用AFM对纳米晶Ni样品表面进行观察以研究其裂纹萌生的微观机制, 利用纳米压痕仪对疲劳实验前后样品的力学性能和显微组织变化进行了研究. 结果表明, 纳米晶Ni具有比粗晶Ni更高的疲劳极限. AFM观察表明,纳米晶疲劳后样品表面出现平均尺寸为73 nm的胞状起伏, 疲劳后样品的晶粒尺寸未发生明显改变. 压痕硬度结果表明, 疲劳过程材料的力学性能也未发生明显变化.     

7050铝合金搅拌摩擦焊接头超高周疲劳性能

采用超高周疲劳试验系统研究7050-T7451铝合金搅拌摩擦焊接头的超高周疲劳性能. 试验结果表明,焊接接头在107周次以上仍然会发生疲劳失效,S-N曲线在108周次左右出现转折点,呈折线型下降;通过SEM对超高周疲劳断口形貌进行观察发现,当应力范围较高时,试件的疲劳裂纹往往在表面萌生,随着应力范围的降低,裂纹有亚表面和内部萌生的倾向;裂纹萌生位置取决于表面起裂和内部起裂相互竞争的结果;试件的断裂位置多为焊接接头的热力影响区和热影响区,EBSD和接头硬度的分析结果表明断裂位置与接头组织不均匀密切相关.     

A size-dependent crystal plasticity finite-element model for creep and load shedding in polycrystalline titanium alloys

A rate-dependent anisotropic elastic-crystal plasticity based finite-element (FE) model with size-dependent yield strength is developed for polycrystalline Ti-6242. The initial slip system deformation resistances in the crystal plasticity relations are expressed as Hall–Petch type relations, where the grain size, lath size and colony size are chosen as characteristic lengths depending on the nature of slip. The FE model incorporates accurate phase volume fractions and orientation distributions that are statistically equivalent to those observed in orientation imaging microscopy (OIM) maps of the microstructure. The model is validated with experimental results on constant strain rate and creep tests. A relationship between the macroscopic flow stress and grain size and lath size for two-phase Ti-6242 is proposed. The effect of grain morphology on creep-induced load shedding and localization is discussed.     

铝合金应变诱导晶粒异常长大行为的晶体塑性-相场法耦合模拟（英文）

提出一种预测变形铝合金在退火过程中应变诱导晶粒异常长大行为的介观耦合建模方法。基于晶体塑性有限元法(CPFE)计算塑性变形中的位错密度和变形储能分布;以储能和局部界面曲率为晶界迁移驱动力,建立基于连续场法的改进相场(PF)模型,并采用插值法将CPFE计算结果向PF模型进行映射,实现CPFE-PF的耦合模拟。采用理想双晶组织模拟初步校准PF模型,然后将该CPFE-PF耦合模型应用于变形AA3102铝合金退火过程的多晶组织演化模拟,进一步验证模型有效性。结果表明,退火时变形基体中的低储能形核点优先长大;而当变形量在临界塑性应变附近时,由于结晶形核数量的有限性和储能分布的不均匀性,将产生晶粒异常长大现象。     

Effects of temperature, slip amplitude, contact pressure on fretting fatigue behavior of Ti811 alloys at elevated temperatures

Effects of the temperature,slip amplitude,and contact pressure on fretting fatigue(FF) behavior of the Ti811 titanium alloy were investigated using a high frequency fatigue machine and a home-made high temperature apparatus.The fretting fatigue failure mechanism was studied by observing the fretting surface morphology features.The results show that the sensitivity to fretting fatigue is high at both 350 and 500 °C.The higher the temperature,the more sensitive to the fretting fatigue failure is.Creep is an i...