共查询到20条相似文献,搜索用时 31 毫秒
1.
Deformation structure and subsurface fatigue crack generation in austenitic steels at low temperature 总被引:2,自引:0,他引:2
In order to progress in the understanding of fatigue crack generation for high-strength alloys, the subsurface fatigue crack
initiation sites were characterized and the deformation structure was investigated for the solution-treated 24Cr-15Ni-4Mn-0.3N
and 32Mn-7Cr-0.1N austenitic steels. High-cycle fatigue tests of those steels were carried out at 4, 77, and 293 K. Subsurface
crack initiation was detected in the lower-peak stress and/or in the longer-life range at the three temperatures. The subsurface
crack initiation sites were intergranularly formed. The localized deformation and/or strain concentration by dislocation arrays
of the (111)–〈110〉 system assisted intergranular cracking due to incompatibility at grain boundaries. Dislocation movements
were restricted to their slip planes. Even at the lower stress level, dislocations had generated in more than one slip system
and piled up to a grain boundary. The peak cyclic stress was lowered with the increasing size of the subsurface crack initiation
site. The dependence of the subsurface crack size on the peak cyclic stress was discussed. 相似文献
2.
Denny A. Jones 《Metallurgical and Materials Transactions A》1985,16(6):1133-1141
A mechanism of stress corrosion cracking (SCC) is outlined in which anodic dissolution at film rupture sites relieves strain
hardening and reduces the fracture stress at the crack tip. Experimental evidence is cited to suggest that relief of strain
hardening occurs by interaction of subsurface dislocations with divacancies generated by the anodic dissolution. A transgranular
crack propagates by accumulation of divacancies on prismatic planes which then separate by cleavage under plane strain conditions
at the crack tip. At appropriate metallurgical and chemical conditions, anodic dissolution and/or divacancy migration may
be enhanced at grain boundaries, leading to an intergranular failure mode. Evidence is also available to indicate that cyclic
loading relieves strain hardening. Relief of strain hardening by combined cyclic loading and corrosion accounts for the higher
incidence of corrosion fatigue cracking (CFC) without the requirement of any critical dissolved species. Data on fatigue of
stainless steel at elevated temperature in both vacuum and air provide additional support for the proposed mechanism. 相似文献
3.
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. 相似文献
4.
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. 相似文献
5.
Fatigue Strength and Crack Initiation Mechanism of Very-High-Cycle Fatigue for Low Alloy Steels 总被引:2,自引:0,他引:2
Youshi Hong Aiguo Zhao Guian Qian Chengen Zhou 《Metallurgical and Materials Transactions A》2012,43(8):2753-2762
The fatigue strength and crack initiation mechanisms of very-high-cycle fatigue (VHCF) for two low alloy steels were investigated. Rotary bending tests at 52.5?Hz with hour-glass type specimens were carried out to obtain the fatigue propensity of the test steels, for which the failure occurred up to the VHCF regime of 108 cycles with the S-N curves of stepwise tendency. Fractography observations show that the crack initiation of VHCF is at subsurface inclusion with ??fish-eye?? pattern. The fish-eye is of equiaxed shape and tends to tangent the specimen surface. The size of the fish-eye becomes large with the increasing depth of related inclusion from the surface. The fish-eye crack grows faster outward to the specimen surface than inward. The values of the stress intensity factor (K I ) at different regions of fracture surface were calculated, indicating that the K I value of fish-eye crack is close to the value of relevant fatigue threshold (??K th ). A new parameter was proposed to interpret the competition mechanism of fatigue crack initiation at the specimen surface or at the subsurface. The simulation results indicate that large inclusion size, small grain size, and high strength of material will promote fatigue crack initiation at the specimen subsurface, which are in agreement with experimental observations. 相似文献
6.
Bending fatigue studies of low carbon steel after gaseous nitrocarburizing have shown that the fatigue strength is increased
by about 140 pct relative to annealed material. X-ray diffraction and electron microscopy studies have demonstrated that fine
scale precipitation occurs in the outermost 400 μ region of the diffusion zone and, beyond this, both nitrogen and carbon
essentially remain in solid solution. Substantial cyclic hardening has been observed during the fatigue of nitrocarburized
material. The higher the applied stress the greater the depth of hardening and at high applied stresses the depth of hardening
extends beyond the diffusion zone into the core of the material. Fading of surface residual stresses during fatigue is observed,
however subsurface compressive stresses are present throughout the life of the material. At low applied stresses subsurface
plastic deformation occurs within the nitrocarburized material. Under long life fatigue conditions the amount of bulk plastic
deformation required to cause fracture of the nitrocarburized material is significantly greater than for annealed material,
but for short lives, the reverse is observed. 相似文献
7.
8.
In this investigation, the effect of a frequency variation between 2 cpm and 6 × 104 cpm on the 1400°F fatigue properties of wrought Udimet 700 was determined at a constant stress range of 85 ksi. It was found
that a peak existed in the cyclic life vs frequency curve such that a) an increase in frequency from 2 to 600 cpm increased
the fatigue life 100 times and b) an increase in frequency from 600 to 6 × 104 cpm reduced the fatigue life sevenfold. The peak in the cyclic life vs frequency curve is the result of two competing processes:
1) there is a reduction in the effects of creep and oxidation with increased frequency that tends to increase the life and
2) there is an increase in the heterogeneity of deformation with increased frequency that tends to reduce the life. At low
frequencies, crack initiation occurred at surface-connected grain boundaries. Crack propagation was initially intergranular
and then proceeded noncrystallographically normal to the stress axis (Stage II mode). Crack initiation at high frequencies
occurred at subsurface brittle phases located at grain boundaries or at the intersection of coherent annealing twin boundaries.
Crack propagation was entirely transgranular, proceeding initially along twin boundaries or slip bands (Stage I mode) and
then changing to the Stage II mode. The statistical nature of the fracture process, the significance of subsurface crack initiation,
and the relation of these results to existing high temperature fatigue models are discussed. 相似文献
9.
The influence of texture on the deformation behavior and monotonic and cyclic properties of two recrystallized P/MX7091-T651
plates was investigated. Thermal mechanical treatments were used to produced two different textures which varied in intensity
by a factor of four. The two plates had similar grain and precipitate structures. The deformation behavior and mechanical
properties were correlated with grain orientation and grain boundary misorientation. Differences in fracture surface roughness
and crack deflection frequencies were observed for the two texture variants during fatigue crack propagation studies. Deformation
behavior, crack closure, and crack deflection affected the fatigue crack growth rates. A small but measurable improvement
in tensile strength, fatigue strength, and fatigue crack growth resistance was obtained in the sharply textured material when
compared with the weakly textured counterpart. 相似文献
10.
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. 相似文献
11.
12.
R. D. Carter E. W. Lee E. A. Starke C. J. Beevers 《Metallurgical and Materials Transactions A》1984,15(3):555-563
The effects of slip character and grain size on the intrinsic material and extrinsic closure contributions to fatigue crack
growth resistance have been studied for a 7475 aluminum alloy. The alloy was tested in the underaged and overaged conditions
with grain sizes of 18 μm and 80 μm. The fracture surface exhibited increased irregularity and planar facet formation with
increased grain size, underaging, and tests in vacuum. These changes were accompanied by an increased resistance to fatigue
crack growth. In air the 18 μm grain size overaged material exhibited relatively poor resistance to fatigue crack growth compared
with other microstructural variants, and this was associated with a lower stress intensity for closure. All materials exhibited
a marked improvement in fatigue crack growth resistance when tested in vacuum, with the most significant difference being
˜1000× at a ΔK of 10 MPa m1/2 for the 80 μm grain size underaged alloy. This improvement could not be accounted for by either an increase in closure or
increased crack deflection and is most likely due to increased slip reversibility in the vacuum environment. The intrinsic
resistance of the alloy to fatigue crack growth was microstructurally dependent in vacuum, with large grains and planar slip
providing the better fatigue performance. 相似文献
13.
J. L. Walley E. J. Lavernia J. C. Gibeling 《Metallurgical and Materials Transactions A》2009,40(11):2622-2630
The cyclic deformation behavior of cryomilled (CM) AA5083 alloys was compared to that of conventional AA5083-H131. The materials studied were a 100 pct CM alloy with a Gaussian grain size average of 315 nm and an alloy created by mixing 85 pct CM powder with 15 pct unmilled powder before consolidation to fabricate a plate with a bimodal grain size distribution with peak averages at 240 nm and 1.8 μm. Although the ultra-fine-grain (UFG) alloys exhibited considerably higher tensile strengths than those of the conventional material, the results from plastic-strain-controlled low-cycle fatigue tests demonstrate that all three materials exhibit identical fatigue lives across a range of plastic strain amplitudes. The CM materials exhibited softening during the first cycle, similar to other alloys produced by conventional powder metallurgy, followed by continual hardening to saturation before failure. The results reported in this study show that fatigue deformation in the CM material is accompanied by slight grain growth, pinning of dislocations at the grain boundaries, and grain rotation to produce macroscopic slip bands that localize strain, creating a single dominant fatigue crack. In contrast, the conventional alloy exhibits a cell structure and more diffuse fatigue damage accumulation. 相似文献
14.
The effect of thermomechanical processing on fatigue crack propagation (FCP) is examined for 70/30 brass and 305 stainless
steel. It is found that grain size and cold work induced changes in yield strength, ductility, and preferred orientation have
a minor effect on FCP. Rather, cyclically stabilized properties of material in the crack tip plastic zone are believed to
control the FCP process. Although mechanical processing fails to significantly alter the rate of FCP, it is apparently responsible
for the unique fracture path observed in specimens oriented at an angle(A) to the rolling direction. Deviation of the crack path out of the plane of maximum net section stress is believed to be associated
with mechanical fibering andJor crystallographic texturing effects. The complex fracture mode transition observed in cold
worked 70/30 brass also is associated with the deformation texture of the starting material. For the cold-worked 305 stainless
steel, striation spacings are correlated with the stress intensity range for specimens tested in the longitudinal, transverse,
and “angle” orientations. Comparison of these data with corresponding macroscopic data indicate that an approximately one-to-one
correspondence exists between macroscopic and microscopic fatigue crack growth rates over the range investigated. 相似文献
15.
This article proposes a simple and fast method of induction heating and quenching to produce surface gradient structure for S38C steel, and its mechanical behavior and strengthening mechanism are revealed. The variation of the gradient structure from surface to interior is characterized by electron backscatter diffraction, and the tensile behavior of the gradient structure at different depths is acknowledged by the small-scale tensile tests. The gradient structure is tempered martensite microstructure, which significantly improves the hardness and tensile strength of surface and subsurface regions. Accordingly, with the strengthening of the gradient structure, the general tensile strength and fatigue behavior of the S38C steel are increased close to those of high-strength steel. Moreover, the fatigue crack initiation mechanism of the gradient structure is studied by energy dispersive spectroscopy, transmission Kikuchi diffraction, and transmission electron microscope characterization on the crack initiation regions. It reveals that the fatigue failure of the gradient structure can be due to stress concentration on the surface and around subsurface inclusions, and the crack initiation modes present surface crack initiation and internal crack initiation, respectively. 相似文献
16.
D. Eylon F. H. Froes D. G. Heggie P. A. Blenkinsop R. W. Gardiner 《Metallurgical and Materials Transactions A》1983,14(12):2497-2505
A wide range of microstructures was generated using various thermomechanical processing sequences in Ti-6A1-4V Rotating Electrode
Process (REP) powder compacts of low contaminant content. Low cycle fatigue results were found to be superior to those in
higher contaminant compacts tested in a previous program. All microstructural groups showed fatigue strengths equivalent to
those found in wrought alloy, with the beta-annealed condition being lowest as expected. Alpha + beta work and solution treatment
resulted in an excellent fatigue strength of 875 MN/m2 (127 ksi) at 105 cycles; 85 pct of the UTS. In the five conditions tested, the fatigue strength increased with increasing tensile strength,
decreasing grain size, and increasing volume fraction of low aspect ratio primary alpha. Most crack initiation sites were
observed at the specimen surface. Only alpha + beta worked and solution-treated material exhibited subsurface initiations,
none of which was associated with any defect or with a lower fatigue life. Although compacts contained some tungsten particles,
in no case were they associated with crack initiation sites, indicating that they were innocuous in the conditions evaluated. 相似文献
17.
The high cycle fatigue (HCF) and cyclic crack growth rate (CCGR) properties of the dispersion strengthened ODS-alloy MA 6000
were investigated with smooth bars and with fracture mechanics samples at 850 °C. The material was very coarse grained with
the grains elongated in the rolling direction. Fatigue crack initiation and crack propagation were studied parallel and perpendicular
to the rolling direction and a pronounced influence of orientation was found. The fatigue limit of sam-ples cut parallel to
the grain elongation direction (p-samples) was almost a factor of 2 higher than the one of samples cut transverse to the elongation
direction (t-samples). Inclusions were found to be responsible for crack initiation. For p-samples a reasonable agreement
between particle size, fatigue limit, and crack growth behavior was found. For t-type samples such an agreement also exists
provided differences in the crack growth behavior of short cracks and long cracks are taken into consideration. The low fatigue
strength of t-samples could be linked with low Young's modulus in this direction. The crack propagation rate of long cracks
is lower in t-samples than in p-samples due to crack branching along the grain boundaries. HCF-strength of MA 6000 is high
compared to conventional cast alloys mainly because of reduced size of crack nucleation sites and higher fatigue threshold
stress intensity range ΔKth, as a result of higher Young's modulus. 相似文献
18.
The effect of intermediate thermomechanical treatments on the fatigue properties of a 7050 aluminum alloy 总被引:2,自引:0,他引:2
A study was undertaken to determine the effect of microstructures produced by different ingot processing techniques on the
fatigue properties of a 7050 aluminum alloy. The different microstructures investigated were produced by hot-rolling to simulate
commercial processing (CP) methods or intermediate thermomechanical treatments (ITMT). Characterization of the microstructures
revealed that the CP 7050 material was partially recrystallized (<50 pct) due to the use of hot-rolling as the final deformation
step. The ITMT materials were examined in the as-recrystallized (AR) condition or in AR + hot rolled condition (AR + HR).
Results of the investigation showed thattotal fatigue life, both low and high cycle, were not greatly affected by the grain structures of the experimental materials. However,
metallographic studies indicated that crack initiation is probably more difficult in the fine-grained AR material. The results
of fatigue crack growth tests showed that higher crack growth rates observed at low ΔK values for ITMT {dy7050} were most likely due to the detrimental effects of undissolved Al2CuMg particles. These particles, which also contribute to low fracture toughness and higher crack growth rates at high ΔK levels, are formed during a furnace-cooling step in the ITMT processing schedule. 相似文献
19.
The influence of microstructure and texture on the monotonic and cyclic properties of X7091-T651 was investigated. The various
structures were developed from conventional ingot metallurgy (I/M), powder metallurgy (P/M) and intermediate thermal mechanical
treatments (ITMT). Powder metallurgy produced a finer grain structure and particle distribution than I/M. Intermediate thermomechanical
treatment produced a recrystallized, coarse grain structure with a weak texture, compared to the unrecrystallized grain structure
and sharp texture obtained with conventional processing (CP). All materials had comparable monotonic properties. The resistance
to fatigue crack initiation (FCI) increased with both a reduction in grain size and a finer particle distribution. Smaller
grain sizes and finer particle distributions reduced the degree of cyclic strain localization. The CP-P/M alloy had the poorest
ductility and FCI resistance of all the materials, although the slip was fairly homogeneous. This may be due to the presence
of oxides at the grain boundaries and a sharp texture. The threshold stress intensity, ΔKth, and the fatigue crack growth rate (FCGR) roughly follow a grain size dependence with the resistance of fatigue crack propagation
(FCP) increasing with increasing grain size. It appears that large grains allow more reversible slip and reduce the amount
of accumulated plastic strain within the reverse plastic zone. It is also believed that a greater degree of fatigue crack
closure, which may be associated with large grains and a rough FCP surface, results in a lower FCGR in the lowΔK region. The intermediate thermomechanical treatment of P/M X7091 produced the optimum microstructure giving the best combination
of mechanical properties. The important features include a small recrystallized grain structure, a fine particle distribution,
a weak texture, and a low concentration of oxides at grain boundaries.
Formerly Director, Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA. 相似文献
20.
Robert E. Spencer Steven K. Deweese David K. Matlock David L. Olson 《Metallurgical and Materials Transactions A》1980,11(10):1758-1761
The potential use of liquid lithium, in conjunction with several of the advanced energy generation systems currently under
study, has led to many recent investigations of the effects of liquid lithium on the engineering properties of various materials.12 Particular emphasis has been placed on understanding the corrosion processes in liquid lithium which lead to material degradation.
The mode of corrosion attack has been shown to be controlled by temperature,3 nitrogen content in the liquid lithium,3 alloy
composition,4 and heat treatment.5 For example, an increase in nitrogen content in the liquid lithium accelerates both weight
loss and grain boundary penetration rates in 304 L stainless steel,3 and alters the relative rates of both processes to promote
grain boundary penetration at high nitrogen concentrations. The grain boundary penetration rate in pure Armco iron has been
shown to be enhanced with the application of either a tensile or compressive stress.6 Furthermore, lithium penetrated grain
boundaries have been shown to lack mechanical integrity.1 Thus for proper design of materials to contain liquid lithium, the
interaction of stress with the various corrosion processes must be understood. In this note, the results of a study of the
fatigue crack propagation characteristics of 2 1/4 Cr-1 Mo steel in liquid lithium are presented.
Formerly Graduate Research Assistant at Colorado School of Mines 相似文献