A fractographic study of Stage I fatigue cracking in a nickel-base superalloy single crystal

An electron fractrographic study was made of opposing crystallographic (Stage I) fatigue fracture surfaces of specimens of a single crystal nickel-base superalloy, low carbon MAR-M200, that were tested at room temperature. In regions near the crack initiation site, features on both surfaces are the same. In regions at some distance from the initiation site significant differences in the type and extent of microfeatures were observed. For a crack propagating in an upward direction, irregular markings were observed on the upper fracture surface and regularly spaced slip offsets and slip band cracks were observed on the lower surface. These observations are explained by a consideration of the elastic stress field and the resultant glide forces on all possible slip systems surrounding a crack growing at an angle to the principal stress direction. Additionally the results are used to support a previously proposed model for Stage I fatigue crack propagation.     

无镍高氮不锈钢弯曲疲劳裂纹萌生与扩展行为

 采用四点弯曲疲劳试验研究了不同应力水平下无镍高氮不锈钢的疲劳行为,并对材料疲劳裂纹的微观形貌、萌生位置及扩展路径进行了分析。结果表明,试验钢疲劳为多裂纹起裂,随着应力水平的升高,裂纹总长度逐渐增加,当应力水平接近材料屈服极限时,裂纹长度趋于稳定;裂纹大多数在滑移带处萌生,裂纹在扩展过程中产生了扭曲、偏移和分叉现象;裂纹在晶内主要沿单滑移带或多滑移带交替扩展,穿过晶界或孪晶界时大多发生了偏转。     

The Small Fatigue Crack Growth Behavior of an AM60 Magnesium Alloy

The effects of thermomechanical processing and subsequent heat treatment on the small fatigue crack growth (FCG) behavior of an AM60 (Mg-6.29Al-0.28Mn wt pct) alloy were evaluated. The effects of mechanical loading parameters, such as maximum stress and load-ratio, on the small FCG behavior were also determined. Maximum stress did not appear to affect the crack propagation rate of small cracks in the stress and crack size ranges considered. Materials with different microstructures and yield stresses, introduced by different processing conditions, showed similar crack growth rates at equivalent stress intensity factor ranges. The effect of load ratio on small crack growth rates was recorded. Fracture surface characterization suggested that the fatigue crack propagation mechanism was a mixture of transgranular and intergranular cracking. Porosity and other material defects played respective important roles in determining the fatigue crack initiation and propagation behavior.     

Fatigue crack initiation and propagation in a quenched and tempered niobium bearing HSLA steel

The fatigue crack initiation and propagation behavior of a niobium bearing HSLA steel heat treated to give two tempered martensitic microstructures presumably with and without fine niobium carbides has been studied by light microscopy, electron microscopy, and strain gage measurements of plastic zone deformation. The high cycle, stress controlled fatigue life of the steel in both heat treated conditions was quite similar with the steel presumably containing the fine niobium carbides having slightly better resistance at low stress amplitudes. This slightly better high cycle resistance is associated with better resistance to fatigue crack initiation for this heat treatment. The fatigue crack propagation behavior of the steel was the opposite. The steel presumably containing the fine niobium carbides exhibited a much faster fatigue crack growth rate than that without them. The difference in growth rates is explained in terms of the plastic work expended during the propagation of the fatigue crack.     

Fatigue crack initiation and propagation behavior of high cobalt molybdenum stainless bearing steel

The crack initiation and propagation behavior of high cobalt molybdenum stainless bearing steel was studied by rotating bending fatigue test with smooth cylindrical specimens and notched specimens (theoretical stress concentration factor Kt=3). The fatigue limit and S- N curve of bearing steel were measured by up- and- down method and group method, respectively. The fractures of the specimens were observed by scanning electron microscopy. The results show that the cracking type of the smooth specimens is single source initiation. The crack source is surface defects and subsurface inclusion. The surface defects are surface roughness, persistent slip band and machining dent, while the subsurface inclusion is Al2O3- CaO- MgO- SiO2 composite inclusion. The fatigue limit of notched specimens is significantly decreased. The cracking type of the notched specimens is multi- source initiation. The notch sensitivity factor qf of bearing steel is 1. 18. The fatigue failure of the smooth specimens is transferred from the surface roughness with high stress amplitude to the persistent slip bands, the machining dents and the inclusions with low stress amplitude. The fatigue crack initiation life accounts for more than 94. 1% of the whole fatigue life.     

The effect of microstructure on the fatigue crack propagation behavior of an Al-Zn-Mg-Cu alloy

The effect of slip distribution on the fatigue crack propagation behavior in vacuum of a high purity Al-5.9Zn-2.6Mg-l.7Cu alloy in various age-hardened conditions has been investigated. The crack propagation resistance was observed to be significantly higher for underaged microstructures containing shearable precipitates in comparison to overaged conditions with nonshearable precipitates. The improved crack propagation resistance is attributed in part to an increased amount of reversed slip in the plastic zone at the crack tip due to a higher degree of planar slip for conditions with shearable precipitates. The observed increase in fatigue crack propagation resistance with decreasing precipitate size for microstructures containing a constant volume fraction of shearable precipitates cannot be explained on the basis of such slip reversibility alone. The variation in ductility for the different microstructures has also to be taken into account. It was found that the enhanced crack propagation resistance can be correlated to the increased ductility with decreasing precipitate size. This explanation was supported by the experimental observation that microstructures containing different volume fractions and sizes of shearable precipitates but exhibiting the same ductility showed approximately the same resistance against fatigue crack propagation. formerly with German Aerospace Research Establishment (DFVLR), Cologne, Germany. formerly with Ruhr-University, Bochum, Germany.     

Fatigue behavior in the potentiostatic passive corrosion regime of the iron-base superalloy A-286

Surface crack initiation and propagation behavior of the iron-base superalloy A-286 were tested using smooth hourglass specimens under passive corrosion conditions. In the very underaged (VUA) condition, cracks were initiated at slip steps (stage I) and later propagated in a stage II mode, resulting in a minimum or dip in crack growth rate. For the highly overaged condition (HOA), cracks initiated at inclusions, and only stage II was observed; therefore, anomalies in crack growth behavior were not observed. A planar slip distribution, observed in VUA, was associated with a reduced crack growth rate. In general, the VUA microstructure was superior to HOA, as it exhibited an impressive combination of excellent mechanical properties, decreased susceptibility to corrosion, good resistance against crack initiation, and low crack growth rate, under corrosion fatigue conditions. Possible reasons for such behavior are discussed. Formerly Postdoctoral Associate, Carnegie Mellon University Formerly Professor, Carnegie Mellon University     

The influence of temperature and cyclic frequency on the fatigue fracture of cube oriented nickel-base superalloy single crystals

Carbon-free single crystals of Mar-M200 were tested in pulsating tension, stress-controlled fatigue at temperatures and frequencies ranging from 1033 to 1255°K and 0.033 to 1058 Hz, respectively. The axis of loading was parallel to [001], the natural growth direction for directionally-solidified nickel-base alloys. Except for the lowest frequency at the higher temperatures where creep damage was extensive, crack initiation occurred at subsurface microporosity. Cracks initiated and propagated in the Stage I mode (crystallographic cracking on the {111} slip planes) at the lower temperatures and higher frequencies, whereas Stage (perpendicular to the principal stress axis) crack initiation and propagation was found at the higher temperatures and lower frequencies. Often a transition from Stage II to Stage I crack propagation was observed. It was established that Stage I cracking occurred under conditions of heterogeneous, planar slip and Stage II cracking under conditions of homogeneous, wavy slip. A thermally activated recovery process with an activation energy of 368 KJ/mole (88 Kcal/mole) determined the instantaneous slip character,i.e., wavy or planar, at the crack tip. In addition, it was found that an optimum frequency existed for maximizing fatigue life. At frequencies below the optimum, creep damage was detrimental, while at frequencies greater than the optimum, intense, planar slip was detrimental. The optimum frequency increased with increasing temperature.     

The influence of temperature and cyclic frequency on the fatigue fracture of cube oriented nickel-base superalloy single crystals

Carbon-free single crystals of Mar-M200 were tested in pulsating tension, stress-controlled fatigue at temperatures and frequencies ranging from 1033 to 1255°K and 0.033 to 1058 Hz, respectively. The axis of loading was parallel to [001], the natural growth direction for directionally-solidified nickel-base alloys. Except for the lowest frequency at the higher temperatures where creep damage was extensive, crack initiation occurred at subsurface microporosity. Cracks initiated and propagated in the Stage I mode (crystallographic cracking on the {111} slip planes) at the lower temperatures and higher frequencies, whereas Stage (perpendicular to the principal stress axis) crack initiation and propagation was found at the higher temperatures and lower frequencies. Often a transition from Stage II to Stage I crack propagation was observed. It was established that Stage I cracking occurred under conditions of heterogeneous, planar slip and Stage II cracking under conditions of homogeneous, wavy slip. A thermally activated recovery process with an activation energy of 368 KJ/mole (88 Kcal/mole) determined the instantaneous slip character,i.e., wavy or planar, at the crack tip. In addition, it was found that an optimum frequency existed for maximizing fatigue life. At frequencies below the optimum, creep damage was detrimental, while at frequencies greater than the optimum, intense, planar slip was detrimental. The optimum frequency increased with increasing temperature.     

Crystallography of Fatigue Crack Initiation and Growth in Fully Lamellar Ti-6Al-4V

Fatigue crack initiation in titanium alloys is typically accompanied by the formation of planar, faceted features on the fracture surface. In the present study, quantitative tilt fractography, electron backscatter diffraction (EBSD), and the focused ion beam (FIB) have been used to provide a direct link between facet topography and the underlying microstructure, including the crystallographic orientation. In contrast to previous studies, which have focused mainly on the α-phase crystal orientation and the spatial orientation of the facets, the present analysis concentrates on the features that lie in the plane of the facet and how they relate to the underlying constituent phases and their crystallographic orientations. In addition, due to the anisotropic deformation behavior of the three basal slip systems, the orientation of the β phase as it relates to facet crystallography was investigated for the first time. The implication of the β-phase orientation on fatigue crack initiation was discussed in terms of its effect on slip behavior in lamellar microstructures. The effect of the local crystallographic orientation on fatigue crack initiation was also investigated by studying cracks that initiated naturally in the earliest stages of growth, which were revealed by FIB milling. The results indicate that boundaries that are crystallographically suited for slip transfer tend to initiate fatigue cracks. Several observations on the effect of the crystallographic orientation on the propagation of long fatigue cracks were also reported.     

Crack initiation and near-threshold surface fatigue crack propagation behavior of the iron-base superalloy A-286

The fatigue behavior of the iron-base superalloy A-286 was studied at room temperature in air for three aging conditions: underaged, peak aged, and overaged. A fatigue strength at 10⁷ cycles of about 200 MPa, independent of aging condition, was measured for an applied load ratio ofR =0.1. Surface crack initiation and propagation were measured using hourglass specimens. Surface cracks were invariably initiated in slip bands orientated between 45 and 55 deg to the load axis, and an average ratio of crack depth to crack length of about 0.45 for these semi-elliptical cracks was measured. These earliest observable short surface cracks grew at an accelerated propagation rate in the near-threshold regime but were retarded in a transition stage, resulting in a minimum in crack growth rate. This behavior was correlated to the interaction of the crack with specific microstructure features. Following this minimum, the crack growth accelerated again with increasing ΔK and appeared to converge with the crack growth behavior expected for long through cracks. The crack propagation rate at fixed ΔK was lowest in underaged, compared to peak aged and overaged microstructures. The minimum and trends in crack growth rate appeared to depend on the development of roughness-induced closure. M. A. DAEUBLER, formerly with Carnegie Mellon University     

Intrinsic fatigue crack growth

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al₃Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK _max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σ_ys E)^1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.     

Fatigue behavior of A356-T6 aluminum cast alloys. Part I. Effect of casting defects

The influence of casting defects on the room temperature fatigue performance of a Sr-modified A356-T6 casting alloy has been studied using un-notched polished cylindrical specimens. The numbers of cycles to failure of materials with various secondary arm spacings (SDAS) were investigated as a function of stress amplitude, stress ratio, and casting defect size. To produce pore-free samples, HIP-ed and Densal™ treatments were applied prior to the T6 heat treatment. It was observed that casting defects have a detrimental effect on fatigue life by shortening not only the crack propagation period, but also the initiation period. Castings with defects show at least an order of magnitude lower fatigue life compared to defect-free ones. The decrease in fatigue life is directly correlated to the increase of defect size. HIP-ed alloys show much longer fatigue lives compared to non-HIP-ed ones. There seems to exist a critical defect size for fatigue crack initiation, below which fatigue crack initiates from other competing initiators such as eutectic particles and slip bands. A fracture mechanics approach has been used to determine the number of cycles necessary to propagate a fatigue crack from a casting defect to final failure. Fatigue life of castings containing defects can be quantitatively predicted using the size of the defects. Moreover, the fatigue fracture behavior of aluminum castings is well described by Weibull statistics. Crack originating from different defects (such as porosity and oxide films) can be readily identified from the Weibull modulus and the characteristic fatigue life. Compared with oxide films, porosity is more detrimental to fatigue life.     

Influence of microstructure on fatigue behavior and surface fatigue crack growth of fully pearlitic steels

This work examined the influence of microstructure on the surface fatigue crack propagation behavior of pearlitic steels. In addition to endurance limit or S(stress amplitude)-N(life) tests, measurements of crack initiation and growth rates of surface cracks were conducted on hourglass specimens at 10 Hz and with aR ratio of 0.1. The microstructures of the two steels used in this work were characterized as to prior austenite grain size and pearlite spacing. The endurance tests showed that the fatigue strength was inversely proportional to yield strength. In crack growth, cracks favorably oriented to the load axis were nucleated (stage I) with a crack length of about one grain diameter. Those cracks grew at low ΔK values, with a relatively high propagation rate which decreased as the crack became longer. After passing a minimum, the crack growth rate increased again as cracks entered stage II. Many of the cracks stopped growing in the transition stage between stages I and II. Microstructure influenced crack propagation rate; the rate was faster for microstructures with coarse lamellar spacing than for microstructures with fine lamellar spacing, although changing the prior austenite grain size from 30 to 130 jμm had no significant influence on crack growth rate. The best combination of resistance to crack initiation and growth of short cracks was exhibited by microstructures with both a fine prior austenite grain size and a fine lamellar spacing. Formerly with Carnegie Mellon University     

The effect of microstructure on the mode of monotonie fracture,fatigue crack initiation,and stage i crack growth in an Fe-21 Pct Cr-11 Pct Ni heat resisting steel

This paper describes a study carried out at room temperature on an Fe-21 pct Cr-11 pct Ni heat resisting alloy under tensile and fatigue deformation. Specific microstructures were developed by heat treating the as-received alloy at different temperatures and times. The surface condition of all specimens displayed surface grain boundary oxidation to a maximum depth of 0.16 mm. In addition, the microstructure of specimens in one batch (B) contained intergranular chromium carbides. The major conclusions drawn from this study are that different microstructures respond differently to monotonie and cyclic modes of deformation. In particular, the embrittling effect of intergranular chromium carbides observed during the monotonie mode of deformation was different from that found when deformation was cyclic. During cyclic deformation these chromium carbides assisted in reducing the damaging effects of the surface grain boundary oxidation. Also during cyclic deformation, the overall fatigue life was found to depend on the mode of both fatigue crack initiation and Stage I crack growth. Fatigue life was reduced when crack initiation and Stage I crack growth were intergranular while it was enhanced when crack initiation occurred at slip bands and subsequent Stage I crack growth was transgranular. It was observed that surface grain boundary oxidation is a most deleterious micro-structural feature especially under fatigue loading but, if this feature is unavoidable then the presence of intergranular chromium carbides is considered to be highly beneficial in increasing the overall fatigue resistance of the material. Formerly a Postgraduate Student, School of Materials Science and Engineering, University of New South Wales, Kensington, New South Wales 2033.     

Intrinsic stage i crack growth of directionally solidified Ni-Base superalloys during low-cycle fatigue at elevated temperature

Strain-controlled low-cycle fatigue tests of cylindrical smooth specimens of two kinds of directionally solidified Ni-base superalloys, RENé 80+Hf and CM 247LC, were carried out at a temperature of 873 K, and the successive process from the crack initiation to small crack propagation was investigated by employing a replication technique. Both materials exhibited typical features of stage I fatigue fracture; that is, the fracture occurred on the crystallographic 111 planes, the most important slip planes in face-centered cubic (fcc) materials. It was found that the rate of stage I crack growth, when not influenced by a nearby grain boundary, proportionally increased with the crack length. However, as the crack tip neared a grain boundary, the rate rapidly decreased. It was also shown that the crack growth rate fell when the crack deflection occurred due to secondary slip. Comparison was also made between the stage I crack growth rate and the long crack growth rate in polycrystalline Ni-base superalloys.     

Q345R钢中微缺陷的裂纹萌生与扩展

 对压力容器与压力管道用钢Q345R在低周疲劳下微孔（[?]40～200 μm）的裂纹萌生与扩展规律进行了研究。研究表明小裂纹的萌生主要机理为滑移带启裂,并且由剪应力起主导作用。微缺陷的尺寸、应力幅等因素对疲劳寿命均有影响显著,当应力幅值较低时,微孔的尺寸对疲劳寿命有明显影响。当应力幅值水平较高时,小孔直径对疲劳寿命的影响则不敏感。微观缺陷尺寸存在临界值,当缺陷尺寸大于临界值时,疲劳寿命下降很快。在同一应力幅水平下,裂纹萌生寿命与疲劳总寿命的比值[（Nt/Nf）]与微孔尺寸没有关系,本试验的低周疲劳下约为10%～25%。     

Monotonic and fatigue deformation of Ni-W directionally solidified eutectic

Unlike many eutectic composites, the Ni-W eutectic exhibits extensive ductility by slip. Furthermore, its properties may be greatly varied by proper heat treatments. Here results of studies of deformation in both monotonic and fatigue loading are reported. During monotonie deformation the fiber /matrix interface acts as a source of dislocations at low strains and an obstacle to matrix slip at higher strains. Deforming the quenched-plus-aged eutectic causes planar matrix slip, with the result that matrix slip bands create stress concentrations in the fibers at low strains. The aged eutectic reaches generally higher stress levels for comparable strains than does the as-quenched eutectic, and the failure strains decrease with increasing aging times. For the composites tested in fatigue, the aged eutectic has better high-stress fatigue resistance than the as-quenched material, but for low-stress, high-cycle fatigue their cycles to failure are nearly the same. However, both crack initiation and crack propagation are different in the two conditions, so the coincidence in high-cycle fatigue is probably fortuitous. The effect of matrix strength on composite performance is not simple, since changes in strength may be accompanied by alterations in slip modes and failure processes.     

Slow crystallographic fatigue crack growth in a nickel-base alloy

Fatigue crack propagation rates at very low cyclic stress intensity levels (1 to 3 MNm^-372) have been measured in cube-oriented, planar slip nickel-base superalloy monocrystals using a high frequency (20 kHz) resonant fatigue testing technique. It is found that crack propagation is entirely along the crystallographic slip planes and the crack growth rate does not drop off into a threshold behavior but follows a power law with a power law exponent close to 4, which is similar to the functional dependency observed at higher cyclic stress intensity levels in similar superalloys. The observed behaviors are discussed with respect to a new theory on threshold and the effects of strong crystallographic constraints on crack propagation behavior.