变形高温合金缺口试样静蠕变和循环蠕变断口分析

本文对GH33,GH37和GH49三种变形高温合金的缺口静蠕变和缺口循环蠕变的断样断口进行了观察和分析。结果表明,GH33合金的缺口循环蠕变断口与缺口静蠕变断口相比,除存在编变空洞之外,还存在大量的滑移线和疲劳条纹,二次裂纹明显增多,沿晶界链状碳化物明显长大并连接成裂纹,而原来在缺口静蠕变下的三叉点裂纹消失。GH37合金的断口上存在大量的蠕变空洞。与缺口静蠕变相比,缺口循环的蠕变空洞明显长大,晶界氧化加剧,二次裂纹增加,但疲劳条纹明显减少。GH49合金在缺口循环蠕变时的断口,蠕变空洞和晶界氧化也远比缺口静蠕变时的严重得多,其二次裂纹也严重得多,但除在个别区域发现少量的滑移线外,未见疲劳条纹。本文还就所观察到的结果对循环加载大大降低缺口试样蠕变断裂寿命的实质进行了讨论。     

低磷GH169和常规GH169合金力学性能的对比研究

对国内生产的低磷GH169[s（P）＜0．001％]及常规GH169[w（P）0.008％～0．013％]变形合金锻件的力学性能进行了对比研究。结果表明，室温拉伸和高温拉伸强度未显现很大差异，然而低磷GH169合金反而显示出高温塑性特别是650℃／686MPa持久塑性的明显降低，且低磷合金在蠕变和疲劳／蠕变交互作用下的裂纹扩展速率皆高于常规合金，低磷合金的595℃／895MPa缺口周期持久寿命亦有降低的趋势。俄歇能谱分析表明，磷在晶界发生偏聚，透射电镜观察，发现低磷合金中的σ相的片状形式沿晶界析出，而磷含量提高使σ相的胞状形式沿晶界析出。     

P92钢蠕变-疲劳交互作用下的裂纹扩展行为EI北大核心CSCD

在630℃下,对P92钢进行应力控制下的蠕变-疲劳交互作用实验,研究P92钢高温蠕变-疲劳交互作用下的裂纹扩展行为,并结合断口形貌分析蠕变-疲劳裂纹扩展的机理以及a-N曲线的转折点含义。结果表明:P92钢在蠕变-疲劳交互作用下的断裂属于蠕变韧性断裂,应该用(C_t)_(avg)作为裂纹扩展的断裂参量;P92钢在蠕变-疲劳交互条件下,试样的断口主要表现为蠕变孔洞以及微裂纹。此外,发现a-lg(N_i/N_f)曲线以及(da-dN)-N曲线中的拐点,分别对应蠕变-疲劳裂纹萌生区向扩展区转变周次以及扩展区向瞬断区转变的周次。     

喷射成形GH738合金的疲劳裂纹扩展行为

进行了不同温度、频率和应力比条件下喷射成形GH738合金紧凑拉伸(CT)试样的疲劳裂纹扩展试验,分析了相应条件下的疲劳裂纹扩展速率及其对疲劳裂纹扩展行为的影响规律。结果表明:随着温度的升高,裂纹扩展速率略有加快;加载频率降低,疲劳裂纹扩展加速;裂纹扩展速率da/d N随应力比R的增大而增大。疲劳断口呈现多裂纹源特征,裂纹稳定扩展为疲劳条带机制。     

镍基单晶合金紧凑拉伸试样高温断裂特性的实验研究

采用紧凑拉伸（CT）试样对三种晶体取向[001]，[011]，[111]的镍基单晶合金DD3在950，850℃和760℃下的拉伸以及950℃时的蠕变和疲劳性能进行了实验研究。应用光学显微镜（OM）和扫描电子显微镜（SEM）分别对单晶体裂纹扩展路径和断口表面进行了观察和分析。拉伸实验结果表明：单晶体裂纹扩展路径沿着特定的晶体学矢量方向扩展而在试样自由表明呈现Z字型外观，其扩展方向及形状取决于晶体取向。温度对试样的断裂形式影响较为显著，760℃时的断裂特征为剪切型脆性断裂，断裂平面为沿滑移面的光滑斜断口；950℃下试样断裂逐渐转变为微孔聚集型韧性断裂，断口由粗糙的剪切唇和纤维区构成。蠕变和疲劳实验结果表明：镍基单晶具有明显的蠕变和疲劳性能各向异性，蠕变寿命以[011]，[111]，[001]顺序依次减小，疲劳寿命以[111]，[011]，[001]顺序依次减小；蠕变及疲劳裂纹扩展路径均与加载方向垂直，断裂表面均为光滑的平断口。试样断口细观分析显示，蠕变试样断口由杯锥状韧窝组成，而疲劳试样断口则由疲劳裂纹组成。疲劳损伤比蠕变损伤更有利于裂纹的扩展。     

GH4169合金焊接接头高温疲劳裂纹扩展性能试验研究

对GH4169合金焊接接头在650℃下的疲劳裂纹扩展性能进行了测试和分析.结果表明,疲劳裂纹扩展速率、初始裂纹尺寸、裂纹容限和温度对焊接接头的疲劳裂纹扩展寿命都有影响.母材及焊缝的疲劳裂纹扩展速率和裂纹容限均有较大的差异.     

用直流电压降法研究高温下GH3230合金的疲劳裂纹扩展行为

采用直流电压降法对GH3230合金进行了高温下的疲劳裂纹扩展试验,分析了温度及应力强度因子对GH3230合金疲劳裂纹扩展速率的影响,并利用扫描电子显微镜对断口进行分析。结果表明：在相同的应力强度因子下,随着温度的升高,合金的裂纹扩展速率增大;温度从750℃升高到850℃时,裂纹扩展速率明显增大,从850℃升高到950℃时,小应力强度因子下的裂纹扩展速率相差不大,随着应力强度因子的增大和温度的升高,裂纹扩展速率的差距增大;观察断口表面可知,在裂纹扩展区和瞬断区,断口表面呈现典型的疲劳辉纹和韧窝特征,随着温度的升高,断口表面的氧化物颗粒增多,裂纹扩展区的疲劳辉纹不明显。     

镍基高温合金蠕变-疲劳-环境交互作用研究

为了研究在高温下常见的疲劳裂纹扩展速率在低频下显著增加的现象而进行了本研究工作。对两种不同热处理状态的lnconel718合金,在空气和氦_7环境中于650℃进行了蠕变试验和0.01、0.1与1.0Hz三种频率下的疲劳试验。蠕变裂纹扩展速率在空气中比在氦中快50～100倍。在氦中的疲劳试验表明,裂纹扩展速率对频率几乎不敏感;但在空气中的试验表明,在较低频率下,裂纹扩展速率显著增加。这些结果说明,无论在蠕变试验还是在疲劳试验中,空气环境都起了主要的作用。氧扩散到晶界中好象是加速空气中裂纹扩展的原因。过时效热处理降低了裂纹扩展速率。     

Ti-6Al-4V合金包覆叠轧薄板的轧制工艺对力学性能的影响

主要研究了包覆叠轧加工及热处理工艺对Ti-6Al-4V合金室温拉伸及疲劳性能的影响规律,用金相显微镜和扫描电镜观察分析了疲劳裂纹的扩展路径及断口形貌.结果表明,在交叉轧制状态下,Ti-6Al-4V合金板材的织构较弱,且该状态下的疲劳性能最好,疲劳裂纹尖端有塑性钝化;而在较低轧制温度和单向轧制状态下,合金板材具有强烈的织构,疲劳裂纹呈快速扩展.     

DD3单晶合金高温低周疲劳行为研究

DD3 Ni基单晶高温合金是我国自行研制的第一个高性能低成本单晶合金,并且已在某型号发动机上进行试车考验。为使DD3单晶叶片安全可靠地使用,本文研究了在疲劳蠕变交互作用下合金的时间相关疲劳行为,并采用应变能区分法对DD3合金的时间相关疲劳进行了寿命预测。结果表明,该方法对DD3合金具有良好的寿命预测能力。最后,对760℃低周疲劳与蠕变-疲劳交互作用下的合金断口及显微组织特征做了对比与讨论。     

INFLUENCE OF CREEP-FATIGUE INTERACTION ON HIGH TEMPERATURE FATIGUE CRACK GROWTH BEHAVIOR of Ti-1100

Abstract— A series of crack growth experiments has been preformed on the near alpha titanium alloy, Ti-1100, to determine the mechanism of the creep-fatigue interaction. Based on pure creep crack growth results, the increase in the creep-fatigue crack growth rate is not amenable to separate contributions of creep crack growth and fatigue crack growth.
A mechanism has been proposed to account for the increase in creep-fatigue crack growth rate that is based on the planar slip of titanium alloys which results in the formation of dislocation pileups at the prior beta grain boundaries and leads to intergranular fracture. This mechanism has been validated through crack growth experiments preformed on a Ti-1100 that has been microstructurally modified through the precipitation of internal slip barriers. These show that the intergranular fracture and increase in crack growth rate are absent.     

Damage model of creep-fatigue interaction

The interaction between creep and fatigue has been studied theoretically by considering a macroscopic crack, interacting with continuously distributed microdamage. This damage is a measure of an assumed deterioration of the material. A Dugdale crack model is used with most deformation and damage concentrated to narrow regions ahead of the crack tips. As studied previously, in the pure creep case, the method predicts the creep rupture curve, well known from creep rupture tests. In the pure fatigue case, the method predicts Paris' law for fatigue crack growth with an exponent approximately equal to four and a finite fatigue lifetime. In the creep-fatigue interaction case, studied here for different material parameters and external load levels, the method always predicts an interaction stronger than the linear creep-fatigue interaction.     

FATIGUE CRACK GROWTH BEHAVIOUR OF A ZINC-ALUMINIUM ALLOY AT HIGH HOMOLOGOUS TEMPERATURES

Abstract— The fatigue crack growth behaviour and crack closure response of a zinc base die casting alloy at high homologous temperature were studied. The crack growth rate was both frequency and temperature dependent. The frequency dependence of crack growth rate, which has been commonly attributed to creep-fatigue interaction, can be rationalized by the crack closure phenomenon. The temperature dependence is contrary to that observed in other materials and cannot be simply explained in terms of the interaction between creep and fatigue damage. The effect of a single tensile overload on the crack growth behaviour at high homologous temperatures has also been investigated.     

CONTINUOUS SEM OBSERVATIONS OF CREEP-FATIGUE DAMAGE PROCESSES

In order to examine the relation between damage evolution and changes in microstructure, e.g. from creep cavities, surface micro-cracks and dislocation structures at high temperature, strain controlled creep-fatigue tests were performed and interrupted at several damage levels on Types 304 and 316 stainless steels. The creep-fatigue tests on Type 304 stainless steel at a low strain level were conducted in a high-temperature fatigue testing machine combined with a scanning electron microscope, and the micro-crack initiation and growth behaviour were continuously observed to clarify the damage extension mechanism. It was found that even though many cavities were initiated and grew on the internal grain boundaries of the specimens during the strain-controlled tests, the failure life was governed by the propagation of surface cracks. On the other hand, micro-cracks of about the order of one grain size were initiated mainly along grain boundaries normal to the loading axis under low stress creep-fatigue, and the crack propagation rate of the micro-cracks was slow and random due to the nature of the microstructures. The micro-cracks gradually opened in the loading direction with increasing number of cycles and coalescence contributed to growth.     

An assessment of three creep–fatigue life prediction methods for nickel‐based superalloy GH4049

High‐temperature low‐cycle fatigue tests with and without a 10‐s strain hold period in a cycle were performed on a nickel base superalloy GH4049 under a fully reversed axial total strain control mode. Three creep–fatigue life prediction methods are chosen to analyse the experimental data. These methods are the linear damage summation method (LDS), the strain range partitioning method (SRP) and the strain energy partitioning method (SEP). Their ability to predict creep‐fatigue lives of GH4049 at 700, 800 and 850 °C has been evaluated. It is found that the SEP method shows an advantage over the SRP method for all the tests under consideration. At 850 °C, the LDS and SEP methods give a more satisfactory prediction for creep–fatigue lives. At the temperatures of 700 and 800 °C, the SRP and SEP methods can correlate the life data better than the LDS method. In addition, the differences in predictive ability of these methods have also been analysed. The scanning electron microscopy (SEM) examination of fracture surfaces reveals that under creep–fatigue test conditions crack initiation mode is transgranular, while crack propagation mode is either intergranular plus transgranular or entirely intergranular, dependent on test temperature.     

Fatigue crack initiation and propagation in several nickel-base superalloys at 650°C

The modes of crack initiation and propagation of several nickel-base superalloys have been examined after fatigue and creep-fatigue testing at 650°C. In fatigue, crack initiation was transgranular and frequently associated with porosity or inclusions in the higher strength alloys. These defects were usually located at the surface, except for tests at low strain ranges where larger, internal defects often initiated failure. Although fatigue crack initiation was transgranular, in those alloys with grain sizes of less than 15 μm, fatigue crack growth quickly became intergranular. This transition was environmentally assisted and did not occur for subsurface cracks until the crack broke through to the atmosphere. In the creep-fatigue cycle, which included a 900 s tensile dwell, crack initiation and propagation wer e both intergranular in all alloys.     

Thermomechanical fatigue—damage mechanisms and mechanism-based life prediction methods

An existing extensive database on the isothermal and thermomechanical fatigue behaviour of high-temperature titanium alloy EVII 834 and dispersoid-strengthened aluminum alloy X8019 in SiC particle-reinforced as well as unreinv conditions was used to evaluate both the adaptability of fracture mechanics approaches to TMF and the resulting predictive capabilities of determining material life by crack propagation consideration. Selection of the correct microstructural concepts was emphasised and these concepts were, then adjusted by using data from independent experiments in order to avoid any sort of fitting. It is shown that the cyclic /-integral (δJ_eff concept) is suitable to predict the cyclic lifetime for conditions where the total crack propagation rate is approximately identical to pure fatigue crack growth velocity. In the case that crack propagation is strongly affected by creep, the creep-fatigue damage parameter δ_CF introduced by Riedel can be successfully applied. If environmental effects are very pronounced, the accelerating influence of corrosion on fatigue crack propagation can no longer implicitly be taken into account in the fatigue crack growth law. Instead, a linear combination of the crack growth rate contributions from plain fatigue (determined in vacuum) and from environmental attack is assumed and found to yield a satisfactory prediction, if the relevant corrosion process is taken into account.     

Book Reviews

‘Early’ creep-fatigue crack growth rates have been measured in complex-cycle large single edge notched bend feature-specimen tests on a 1¼ CrMoV turbine casting steel at 550°C. Crack propagation rates initially accelerate with increasing distance below the stress concentration to a peak value. The depth at which this maximum occurs depends on the notch geometry and the magnitude of any superimposed primary loading. ‘Early’ creep-fatigue crack growth rates are dependent on crack size, notch root strain range and any creep damage accumulated due to primary and secondary loading.     

CRACK GROWTH RATES DURING CREEP, FATIGUE AND CREEP-FATIGUE IN AN AUSTENITIC FEATURE WELD SPECIMEN

Abstract— Static creep crack growth tests and displacement controlled fatigue and creep-fatigue crack growth tests have been performed on austenitic feature weld specimens at 650°C. The creep-fatigue tests incorporated hold times of up to 96 h. During these tests, crack growth appeared to comprise cyclic and dwell components. Cyclic crack growth components were characterised by the fracture mechanics parameter K whilst creep crack growth contributions were correlated with C *. In order to determine K and C * for the non-standard feature weld specimen, elastic and elastic-plastic creep finite element analyses were conducted. Good correspondence is shown between the feature weld data and comparable data from compact tension specimen tests on similar materials. Equations obtained from the compact tension specimen results, which describe total crack growth rates as the sum of the cyclic and dwell contributions, are shown to adequately describe the features test results also. Furthermore, it is demonstrated that a reference stress approach can be used to estimate C * for the features specimens.