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1.
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
2.
The crack growth behavior of several high temperature nickel-base alloys, under cyclic and static loading, is studied and
reviewed. In the oxide dispersion strengthened (ODS) MA 6000 and MA 754 alloys, the high temperature crack propagation exhibited
orientation dependence under cyclic as well as under static loading. The creep crack growth (CCG) behavior of cast nickel-base
IN-738 and IN-939* superalloys at 850 °C could be characterized by the stress intensity factor,K
1. In the case of the alloy IN-901 at 500 °C and 600 °C,K
1 was found to be the relevant parameter to characterize the creep crack growth behavior. The energy rate line integral,C*, may be the appropriate loading parameter to describe the creep crack growth behavior of the nickel-iron base IN-800H alloy
at 800 °C. The creep crack growth data of 1 Cr-Mo-V steel, with bainitic microstructure, at 550 °C could be correlated better
by C * than byK
1.
This paper is based on a presentation made in the symposium “Crack Propagation under Creep and Creep-Fatigue” presented at
the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee. 相似文献
3.
Elevated temperature tensile and creep-rupture tests were performed on INCONEL MA754 in longitudinal and transverse orientations
at temperatures from 700 °C to 1000°C. The transverse orientation was weaker and less ductile than the longitudinal orientation
due to a higher grain boundary density perpendicular to the applied stress axis. This effect was especially pronounced in
creep tests at 900 °C and 1000°C. Threshold creep behavior was observed for the longitudinal orientation, with stress exponents
ranging from 29 to 40. Stress exponents in the long transverse orientation ranged from 24 at 800 °C to 5 at 1000°C, indicating
a temperature-varying deformation mechanism. Creep ductility in the transverse orientation was extremely low, less than 1
pct for higher temperature, lower stress conditions. Failure in all transverse specimens was controlled by grain boundary
separation. Even in the relatively weak transverse direction, the strength of MA754 compares favorably with other alloys being
considered for advanced power plant applications. 相似文献
4.
Elevated temperature tensile and creep-rupture tests were performed on INCONEL MA754 in longitudinal and transverse orientations
at temperatures from 700°C to 1000°C. The transerve orientation was weaker and less ductile than the longitudinal orientation
due to a higher grain boundary density perpendicular to the applied stress axis. This effect was especially pronounced in
creep tests at 900°C and 1000°C. Threshold creep behavior was observed for the longitudinal orientation, with stress exponents
ranging from 29 to 40. Stress exponents in the long transverse orientation ranged from 24 at 800°C to 5 at 1000°C, indicating
a temperature-varying deformation mechanism. Creep ductility in the transverse orientation was extremely low, less than 1
pct for higher temperature, lower stress conditions. Failure in all transverse specimens was controlled by grain boundary
separation. Even in the relatively weak transverse direction, the strength of MA754 compares favorably with other alloys being
considered for advanced power plant applications. 相似文献
5.
6.
Fatigue crack propagation in aluminum- lithium alloy 2090: Part I. long crack behavior 总被引:1,自引:0,他引:1
K. T. Venkateswararao W. Yu R. O. Ritchie 《Metallurgical and Materials Transactions A》1988,19(3):549-561
A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercial plate of aluminum-lithium
alloy 2090-T8E41. In Part I, the crack growth and crack shielding behavior of long (≳5 mm) through-thickness cracks is examined
as a function of plate orientation and load ratio, and results compared to traditional high strength aluminum alloys. It is
shown that rates of fatigue crack extension in 2090 are, in general, significantly slower (at a given stress intensity range)
than in traditional alloys, although behavior is strongly anisotropic. Differences in growth rates of up to 4 orders of magnitude
are observed between the L-T, T-L, and T-S orientations, which show the best crack growth resistance, and the S-L, S-T, and
L + 45, which show the worst. Such behavior is attributed to the development of significant crack tip shielding (i.e., a reduction in local crack driving force), primarily resulting from the role of the crack path morphology in inducing crack
deflection and crack closure from the consequent asperity wedging. Whereas crack advance perpendicular to the rolling plane
(e.g., L-T,etc.) involves marked crack path deflection and branching, thereby promoting very high levels of shielding to cause the slowest
growth rates, fatigue fractures parallel to the rolling plane (e.g., S-L,etc.) occur by an intergranular, delamination-type separation, with much lower shielding levels to give the fastest growth rates.
The implications of such “extrinsic toughening” effects on the fracture and fatigue properties of aluminum-lithium alloys
are discussed in detail.
R. O. RITCHIE, Professor and Director, Center for Advanced Materials, Lawrence Berkeley Laboratory 相似文献
7.
Masuo Hagiwara Satoshi Emura Aya Araoka Seung Jin Yang Soo Woo Nam 《Metallurgical and Materials Transactions A》2004,35(7):2161-2170
The room-temperature tensile and high-cycle fatigue (HCF) behavior of orthorhombic Ti-22Al-27Nb alloy with varying lamellar
morphology was investigated. Varying lamellar morphology was produced by changing the cooling rate after annealing in the
single B2 phase region. A slower cooling rate of 0.003 K/s, for example, resulted in several large packets or colonies of
similarly aligned O-phase lamellae and a nearly continuous massive α
2 phase at the prior B2 grain boundaries, while a faster cooling rate of 0.1 K/s led to the refinement of colony sizes and
the O-phase lamellae. The interface of O-phase lamellae and B2 phases was semicoherent. Water quenching produced a very fine
tweed-like microstructure with a thin continuous O phase at the prior B2 grain boundaries. The 0.2 pct yield stress, tensile
strength, and HCF strength increased with increasing cooling rate. For example, the tensile strength and HCF strength at 107 cycles of 0.003 and 0.1 K/s-cooled were 774 and 450 MPa, and 945 and 620 MPa, respectively. Since the fatigue ratio, which
is the ratio of HCF strength at 107 cycles to tensile strength, did not show a constant value, but instead increased with increasing cooling rate, part of the
fatigue improvement was the result of improved resistance to fatigue associated with the microstructural refinement of the
lamellar morphology. Fatigue failure occurred by the subsurface initiation, and every initiation site was found to contain
a flat facet. Concurrent observation of the fatigue initiation facet and the underlying microstructure revealed that the fatigue
crack initiated in a shear mode across the colony, irrespective of colony size, indicating that the size of the initiation
facet corresponded to that of the colony. Therefore, the colony size is likely a major controlling factor in determining the
degree of fatigue improvement due to the microstructural refinement of lamellar morphology. For the water-quenched specimens,
fatigue crack initiation appeared to be associated with shear cracking along the boundary between the continuous grain boundary
O phase and the adjacent prior B2 grain. 相似文献
8.
Influence of texture on fatigue properties of Ti-6Al-4V 总被引:1,自引:0,他引:1
Tensile properties, high cycle fatigue strength, and fatigue crack propagation behavior were evaluated on highly textured
Ti-6Al-4V material to investigate the influence of a preferred crystallographic orientation on mechanical properties. Thermomechanical
treatments were used to develop three different textures: a basal, basal/transverse, and transverse type, all of which exhibited
the same homogeneously equiaxed microstructure. The Young’s modulus was found to vary between 107 and 126 GNm-2, and yield strength changed from 1055 to 1170 MNm-2. Ductility was only slightly affected by texture. High cycle fatigue and fatigue crack growth measurements were performed
in vacuum, laboratory air, and a 3.5 pct NaCl solution. It is shown that laboratory air can be regarded as a quite corrosive
environment. In vacuum the highest fatigue strength values were measured whenever loads were perpendicular to basal planes.
However, these conditions had the highest susceptibilities to air and 3.5 pct NaCl solution environments. Nearly no influence
of texture on fatigue crack propagation was found in vacuum, but in a corrosive environment crack growth parallel to (0002)-planes
was much faster than perpendicular to these planes. To explain the corrosive effect on the fatigue properties of the textured
material hydrogen is thought to play a key role. 相似文献
9.
Room temperature tension-tension fatigue tests were performed on two lamellar γ/γ′-δ alloys, one with 0 pct Cr and one with
6 pct Cr. The 6 pct Cr alloy was solidified at 3 cmJh while the 0 pct Cr alloy was solidified at 3 cm/h and 5.7 cm/h. Fatigue
testing was done on both alloys in the as-directionally solidified condition and on the 0 pct Cr alloy after heat treatment.
Increasing the growth speed of the 0 pct Cr alloy increased the fatigue life of the material at stresses above the 107 cycle fatigue limit. Partial solution treating and aging of the 0 pct Cr alloy,R = 3 cm/h, increased the fatigue life relative to the as-directionally solidified material at high stresses, to the same extent
as increasing the growth speed. Full solution treatment and aging of the 0 pct Cr alloy,R = 5.7 cm/ h, caused a reduction in the fatigue life relative to the as-directionally solidified material. Fatigue cracking
tended to be faceted in the 6 pct Cr alloy as opposed to the more ductile failure of the 0 pct Cr alloy. Microstructural perfection,
grain size and shape, interlamellar spacing, longitudinal cracking, and longitudinal and transverse ductility all are believed
to have influenced the fatigue resistance of the alloys. 相似文献
10.
D. C. Slavik C. P. Blankenship E. A. Starke R. P. Gangloff 《Metallurgical and Materials Transactions A》1993,24(8):1807-1817
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/(σys
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. 相似文献
11.
Vincent C. Nardone 《Metallurgical and Materials Transactions A》1992,23(2):563-572
Analysis of the tensile and fracture behavior of a composite system consisting of boron carbide particulate-reinforced NiAl
with continuous 304 stainless steel toughening regions was performed. The composite was fabricated by extrusion, with the
toughening regions extending along the length of the plate in the extrusion direction. Mechanical properties were determined
as a function of orientation. Tensile testing revealed that the composite modulus varied only slightly as a function of testing
direction, the strength was approximately 25 pct greater in the longitudinal relative to the transverse orientation, and the
transverse failure strain was only 0.3 pct compared to values in excess of 10 pct for longitudinal testing. Notched Charpy
impact testing indicated that the energy absorption values varied significantly as a function of specimen location and crack
growth direction, ranging from 2 to 40 Joules. In addition,K
IC values measured on subsize compact tension samples were found to range from 17 to 27 MPa ⋅ m1/2. It was also established that theK
max values determined from the maximum load measured during compact tension testing were similar to theK
Q values calculated from instrumented notched Charpy impact testing. Finally, the fatigue crack growth characteristics of the
composite were determined as a function of orientation. 相似文献
12.
The effect of extrusion-induced particle-orientation anisotropy on the mechanical behavior of metal-matrix composites (MMCs)
was examined. In this study, we have shown that this anisotropy has a significant influence on the tensile and fatigue behavior
SiC particle-reinforced Al alloy composites. The preferred orientation of SiC particles was observed parallel to the extrusion
axis, with the extent of orientation being highest for the lowest-volume-fraction composites. The composites exhibited higher
Young’s modulus and tensile strength along the longitudinal direction (parallel to the extrusion axis) than in the transverse
direction. The extent of anisotropic behavior increased with increasing volume fraction, because of the increasing influence
of the SiC reinforcement on the Young’s modulus and tensile properties. The preferred orientation also resulted in anisotropy
in the fatigue behavior of the composite material. The trends mirrored those observed in tension, with higher overall fatigue
strengths for both orientations and a higher anisotropy with increasing volume fraction of particles. The influence of particle-orientation
anisotropy and the resulting tensile and fatigue damage mechanisms is discussed. 相似文献
13.
Elevated temperature mechanical properties of the iron base oxide dispersion strengthened alloy ma 956 bar 总被引:1,自引:0,他引:1
J. Daniel Whittenberger 《Metallurgical and Materials Transactions A》1981,12(5):845-851
1144 to 1477 K elevated temperature tensile, stress rupture, and creep tests and residual room temperature tensile tests following
creep exposures were conducted on the iron-base oxide dispersion strengthened alloy MA 956, nominally Fe-20Cr-4.5Al-0.5Ti-0.5Y203. While the majority of the testing was in the longitudinal bar direction, a few tests in the long transverse bar direction
were also conducted. Under slow strain rate conditions in the longitudinal direction, MA 956 deforms via a crack nucleation
and growth mechanism eventually leading to sudden fracture. The longitudinal direction is stronger than the long transverse
direction. Small amounts (∼0.1 pct) of prior creep strain do not degrade subsequent room temperature tensile properties. 相似文献
14.
A new method is described for calculating the long fatigue life (>105 cycles) portion of the stress-life (S-N) fatigue curve for precipitation-hardened aluminum alloys. It is based upon a finite element model of the deformation of
a persistent slipband (PSB), and the only material parameter required is the ultimate tensile strength (UTS) of the alloy.
The stress dependence of the plastic strain at the tip of a PSB is shown to be very pronounced and to closely match that of
anS-N fatigue curve. Very good agreement is obtained for 6061-T6, 2014-T6, 2024-T4, and 7075-T6 aluminum, and the fatigue strength
(at 108 cycles) is calculated to be 26 pct of the tensile strength of each alloy, in agreement with experimental data. By contrast,
the plastic strain at a crack tip has a much weaker stress dependence. Thus, these calculations also confirm that the elongation
of a PSB, and not crack growth, is the rate-controlling process in high cycle fatigue. 相似文献
15.
A new method is described for calculating the long fatigue life (>105 cycles) portion of the stress-life (S-N) fatigue curve for precipitation-hardened aluminum alloys. It is based upon a finite element model of the deformation of
a persistent slipband (PSB), and the only material parameter required is the ultimate tensile strength (UTS) of the alloy.
The stress dependence of the plastic strain at the tip of a PSB is shown to be very pronounced and to closely match that of
anS-N fatigue curve. Very good agreement is obtained for 6061-T6, 2014-T6, 2024-T4, and 7075-T6 aluminum, and the fatigue strength
(at 108 cycles) is calculated to be 26 pct of the tensile strength of each alloy, in agreement with experimental data. By contrast,
the plastic strain at a crack tip has a much weaker stress dependence. Thus, these calculations also confirm that the elongation
of a PSB, and not crack growth, is the rate-controlling process in high cycle fatigue. 相似文献
16.
K- H. Chien T. Z. Kattamis F. R. Mollard 《Metallurgical and Materials Transactions B》1973,4(4):1069-1076
Microstructure and fatigue behavior were studied in sand-cast and end-chilled KO-1 high strength aluminum alloy. Secondary
dendrite arm spacing and volume percent microporosity increase with distance from the chill, whereas volume percent inter
dendritic nonequilibrium secondary phase decreases. Solution kinetics of the secondary phase depend on the dimensionless parameter
[Dt/L
2], where:D is the diffusivity of copper in this alloy at the solution temperature,L is half the dendrite arm spacing, andt the solutionizing time. The fatigue life of the alloy at room temperature was measured in reversed bending on unnotched specimens
at stress levels of 17,300 and 19,000 psi and was found to decrease with increasing distance from the chill. Unnotched solutionized
specimens exhibited a longer fatigue life than as-cast specimens, even though crack growth studies showed that cracks grew
more rapidly in the solutionized material. This would be attributed to a delay in crack initiation resulting from the decrease
in the amount of microporosity and the rounding of micropores during heat treatment. Micropores and inclusions acted as sources
of stress concentration for fatigue crack initiation. Cracks then usually grew transgranularly. 相似文献
17.
R. O. Ritchie F. A. McClintock H. Nayeb-Hashemi M. A. Ritter 《Metallurgical and Materials Transactions A》1982,13(1):101-110
To provide a basis for estimating fatigue life in large rotating generator shafts subjected to transient oscillations, a study
is made of fatigue crack propagation in Mode III (anti-plane shear) in torsionally-loaded spheroidized AISI4340 steel, and
results compared to analogous behavior in Mode I. Torsional S/N curves, determined on smooth bars containing surface defects,
showed results surprisingly close to expected unnotched Mode I data, with lifetime increasing from 104 cycles at nominal yield to 106 cycles at half yield. Fatigue crack growth rates in Mode III, measured on circumferentially-notched samples, were found to
be slower than in Mode I, although still power-law related to the alternating stress intensity(△K
III) for small-scale yielding. Mode III growth rates were only a small fraction (0.002 to 0.0005) of cyclic crack tip displacements(△CTD
III) per cycle, in contrast to Mode I where the fraction was much larger (0.1 to 0.01). A micromechanical model for Mode III
growth is proposed, where crack advance is considered to take place by a Mode II coalescence of cracks, initiated at inclusions
ahead of the main crack front. This mechanism is consistent with the crack increment being a small fraction of △CTDIII per cycle.
Formerly with Massachusetts Institute of Technology, Cambridge, MA
Formerly with M.I. T. 相似文献
18.
The fatigue crack growth behavior of MAR-M200 single crystals was examined at 982 °C. Using tubular specimens, fatigue crack
growth rates were determined as functions of crystallographic orientation and the stress state by varying the applied shear
stress range-to-normal stress range ratio. Neither crystallographic orientation nor stress state was found to have a significant
effect on crack growth rate when correlated with an effective ΔK which accounted for mixed-mode loading and elastic anisotropy. For both uniaxial and multiaxial fatigue, crack growth generally
occurred normal to the principal stress direction and in a direction along which ΔK
II vanished. Consequently, the effective ΔK was reduced to ΔKI and the rate of propagation was controlled by ΔK
I only. The through-thickness fatigue cracks were generally noncrystallographic with fracture surfaces exhibiting striations
in the [010], [011], and [111] crystals, but striation-covered ridges in the [211] specimen. These fracture modes are contrasted
to crystallographic cracking along slip bands observed at ambient temperature. The difference in cracking behavior at 25 and
982 °C is explained on the basis of the propensity for homogeneous, multiple slip at the crack tip at 982 °C. The overall
fracture mechanism is discussed in conjunction with Koss and Chan’s coplanar slip model. 相似文献
19.
J. O. Peters B. L. Boyce A. W. Thompson R. O. Ritchie O. Roder 《Metallurgical and Materials Transactions A》2000,31(6):1571-1583
The increasing incidence of military aircraft engine failures that can be traced to high-cycle fatigue (HCF) has prompted
a reassessment of the design methodologies for HCF-critical components, such as turbine blades and disks. Because of the high-frequency
vibratory loading involved, damagetolerant design methodologies based on a threshold for no crack growth offer a preferred
approach. As impact damage from ingested debris is a prime source of HCF-related failures, the current study is focused on
the role of such foreign-object damage (FOD) in influencing fatigue crack-growth thresholds and early crack growth of both
large and small cracks in a fan blade alloy, Ti-6Al-4V. FOD, which was simulated by the high-velocity (200 to 300 m/s) impact
of steel spheres on a flat surface, was found to reduce markedly the fatigue strength, primarily due to earlier crack initiation.
This is discussed in terms of four salient factors: (1) the stress concentration associated with the FOD indentation, (2)
the presence of small microcracks in the damaged zone, (3) the localized presence of tensile residual hoop stresses at the
base and rim of the indent sites, and (4) microstructural damage from FOD-induced plastic deformation. It was found that no
crack growth occurred from FOD impact sites in this alloy at ΔK values below ∼ 2.9 MPa √m, i.e., over 50 pct higher than the “closure-free”, worst-case threshold value of ΔK
TH
= 1.9 MPa √m, defined for large cracks in bimodal Ti-6Al-4V alloys at the highest possible load ratio. It is, therefore,
concluded that such worst-case, large fatigue crack thresholds can, thus, be used as a practical lower-bound to FOD-initiated
cracking in this alloy. 相似文献