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1.
Hydrogen-induced slow crack growth in Ti-6Al-6V-2Sn 总被引:1,自引:0,他引:1
The effect of hydrogen and temperature on threshold stress intensity and crack growth kinetics was studied in Ti-6Al-6V-2Sn
containing 38 ppm hydrogen. A slight decrease in threshold values occurred as temperature decreased from 300 K while they
increased significantly above 300 K. For a given test temperature, crack growth rates exhibited an exponential dependence
on stress intensity over a major portion of growth. At 300 K the rates reached a maximum. Slow crack growth occurred predominately
by cleavage ofα grains which has been associated with hydride formation. The stress intensity required for hydride formation at a crack tip
can be determined from hydrogen concentration and solubility considerations under stress. As these values differed from observed
thresholds, a strong influence of microstructure was suggested and subsequently revealed by crack front examination. Quantification
of this effect with a modified Dugdale-Barenblatt model relates the effective stress intensity at the crack tip to the applied
stress intensity. Microstructure was also found to exert a strong influence on slow crack growth behavior when examined in
terms of the effective stress intensity,K
eff. From Arrhenius plots of crack growth rates for variousK
eff, activation energies of 27.0 to 32.8 kJ/mol were obtained and related to the diffusion of hydrogen through theβ phase. The increase in crack growth rates with increasing temperatures up to 300 K is attributed to the temperature dependence
of hydrogen diffusion. The decrease in crack growth rates above 300 K is related to a hydride nucleation problem. 相似文献
2.
A critical evaluation of the stress-corrosion cracking mechanism in high-strength aluminum alloys 总被引:3,自引:0,他引:3
Seong-Min Lee Su-Il Pyun Young-Gab Chun 《Metallurgical and Materials Transactions A》1991,22(10):2407-2414
Attempts have been made to elucidate the mechanism of stress-corrosion cracking (SCC) in high-strength Al-Zn-Mg and Al-Li-Zr
alloys exposed to aqueous environments by considering the temperature dependence of SCC susceptibility based upon the anodic
dissolution and hydrogen embrittlement models. A quantitative correlation which involves the change of threshold stress intensity,K
ISCC, with temperature on the basis of anodic dissolution has been developed with the aid of linear elastic fracture mechanics.
From the derived correlation, it is concluded that the threshold stress intensity decreases as the test temperature increases.
This suggestion is inconsistent with that predicted on the basis of hydrogen embrittlement. It is experimentally observed
from the Al-Zn-Mg and Al-Li-Zr alloys that the threshold stress intensity,K,ISCC, decreases and the crack propagation rate,da/dt, over the stress intensity increases with increasing test temperature. From considering the change in SCC susceptibility
with temperature, it is suggested that a gradual transition in the mechanism for the stress-corrosion crack propagation occurs
from anodic dissolution in stage I, where the crack propagation rate increases sharply with stress intensity, to hydrogen
embrittlement in stage II, where the crack propagation rate is independent of stress intensity. 相似文献
3.
Modern precipitation-hardened ultra-high-strength AERMET 100 steel (Fe-Co-Ni-Cr-Mo-C) is susceptible to severe transgranular
hydrogen environment–assisted cracking (HEAC) in neutral 3.5 pct NaCl solution. The threshold stress intensity for HEAC, K
TH
, is reduced to as low as 10 pct of K
IC
, and the stage II subcritical crack growth rate, da/dt
II, is up to 0.5 μm/s. Low K
TH
and high da/dt
II are produced at potentials substantially cathodic, as well as mildly anodic, to free corrosion. However, a range exists at
slightly cathodic potentials (–0.625 to –0.700 VSCE), where the crack growth rate is greatly reduced, consistent with reduced crack-tip acidification and low cathodic overpotential
for limited H uptake. Short crack size (250 to 1000 μm) does not promote unexpectedly severe HEAC. High-purity AERMET 100 is susceptible to HEAC because martensite boundary trapping
and high crack-tip stresses strongly enhance H segregation to sites that form a transgranular crack path. The stage II da/dt is H diffusion rate limited for all potentials examined. A semiquantitative model predicts the applied potential dependence
of da/dt
II using reasonable input parameters, particularly crack-tip H uptake reverse calculated from measured K
TH
and a realistic critical distance. Modeling challenges remain.
This article is based on a presentation given in the symposium entitled “Deformation and Fracture from Nano to Macro: A Symposium
Honoring W.W. Gerberich’s 70th Birthday,” which occurred during the TMS Annual Meeting, March 12–16, 2006 in San Antonio,
Texas and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS.
相似文献
Richard P. Gangloff (Ferman W. Perry Professor, and Chair)Email: |
4.
Precracked specimens of Ti-8Al-lMo-lV were tested in salt-water, and the subcritical crack velocity was measured as a function
of applied stress-intensity. The minimum applied stress intensity which produces subcritical cracking,K
Iscc, varies inversely with the average chloride-ion concentration of the environment, butK
Iscc does not vary significantly with temperature. Plots of subcritical crack velocity,a, against applied stress intensity,K
I, reveal two types of cracking behavior. At low stress intensitiesa is approximately proportional toK
I (Stage I crack propagation). At high stress intensities,a is approximately constant, independent ofK
I (Stage II crack propagation). The apparent activation energy for subcritical crack propagation is 5600 cal-(g-mole)-1, anda ∞ [Cl-]1/6. It is suggested that the instantaneous subcritical crack velocity is limited by the rate of production or by the diffusion
of some embrittling species.
Formerly with the Metal Science Group, Battelle Columbus Laboratories, Columbus, Ohio. 相似文献
5.
The kinetics of sustained-load subcritical crack growth for 18 Ni maraging steels in high purity hydrogen are examined using
crack-tip stress intensity,K, as a measure of crack driving force. Crack growth rate as a function of stress intensity exhibited a clearly definedK-independent stage (Stage II). Crack growth rates in an 18 Ni (250) maraging steel are examined for temperatures from -60°C
to 100°C. A critical temperature was observed above which crack growth rates became diminishingly small. At lower temperatures
the activation energy for Stage II crack growth was found to be 16.7 ± 3.3 kJ/mole. Temperature and hydrogen partial pressure
are shown to interact in a complex manner to determine the apparentK
th
and the crack growth behavior. Comparison of results on ‘250’ and ‘300’ grades of 18 Ni maraging steel indicate a significant
influence of alloy composition and/or strength level on the crack growth behavior. These phenomenological observations are
discussed in terms of possible underlying controlling processes.
Formerly a Graduate Student and Research Assistant.
Based on a thesis submitted in partial fulfillment of requirements for the M.S. degree in Metallurgy and Materials Science,
Lehigh University, Bethlehem, PA. 相似文献
6.
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
7.
J. D. Boyd 《Metallurgical and Materials Transactions B》1973,4(4):1037-1045
Optical and electron metallographic studies of stress-corrosion cracks in Ti-8Al-lMo-lV have verified that the principal crack
extension mechanism is cleavage of theα grains. There are two distinct crack morphologies which correspond to the two regimes of subcritical crack velocity. At low
stress intensities(a ∞ K
I) the microscopic crack front consists of small cleavage facets approximately 1 to 4α grain diameters in size, and ligaments of material which fracture by ductile rupture and corrosion. At high stress intensities
(a ≅ constant), the crack front consists of large cleavage “fingers”, 20 to 50α grain diameters in length, separated by regions which fracture by a combination of cleavage (on a much smaller scale), ductile
rupture, and corrosion. The transition from Stage I to Stage II crack propagation apparently occurs when the strain-energy
release rate is sufficient to support two crack branches,i.e., KI≥ √2K
Iscc. Thereafter, the diameter of the plastic zone at the crack tip remains constant, suggesting that the effective stress intensity
at the tip of each branch is also invariant. The slip within the plastic zone is markedly nonhomogeneous, and trenches are
often observed along the slip steps.
Formerly with the Metal Science Group, Battelle Columbus Laboratories, Columbus, Ohio. 相似文献
8.
The time and temperature dependence of wetting Cu and Ag substrates by molten 60Sn40Pb drops was determined using several
fluxes. Four stages were identified. In Stage I, the solder melts and develops into a spherical cap, which spreads rapidly
to the quasi-equilibrium stage II, wherein the contact angle θ changes only slightly with time. This is followed by stage
III, where6 decreases with a time exponent of ≈0.2, leading ultimately to stage IV, where θ again changes only little with time. An intermetallic
compound was observed at the liquid/solid inter-face throughout stages II to IV. The flux influenced the magnitude of θ and
the small time dependence in stage II. For a nonactivated rosin flux, the temperature dependence of θII yielded an apparent activation energy QII
a = 2 to 3 kcal/mole for all substrates, including pretinned Cu. It is speculated that the driving force for the decrease in
θ during stage III may result from a decrease in the free energy of the system by diffusion, with a corresponding change in
one or more of the interfacial tensions. Pretinning Cu, which formed the intermetallic compound Cu3Sn on the surface, had
a significant effect on the time dependence of θ, the effect in stage II being relatively greater than in stage III. 相似文献
9.
C. M. Austin N. S. Stoloff D. J. Duquette 《Metallurgical and Materials Transactions A》1977,8(10):1621-1627
Fatigue crack propagation rates were determined for directionally solidified Co-10Ni-10Cr-14Ta-1.0C (CoTaC) at room temperature
in laboratory air. Single edge crack specimens, 0.25 cm thick, tested in tension-tension at a stress ratio of less than 0.1
produced a relationship between crack growth rates,da/dN, and stress intensity range,AK, as follows:da/dN = 8 × HF △K(m and MN/m).
A stress ratio ofR = 0.5 increasedda/dN by a factor of six. A prestrain sufficient to break fibers into 5 to 10 μm long segments had no effect upon subsequent crack
growth rate. Compact tension specimens, tested with the stress axis normal to the fiber axis, exhibited more rapid cracking
for equivalent △K and a steeper slope, obeying the relationshipda/dN = 1.2 × lO △K.
Fractographic examination showed Stage I cracking for △K less than 10 MN/m, mixed Stage I and Stage II cracking for 10 MN/m
<AK < 20 MN/m and only Stage II cracking for larger △K. The extent of fiber failure was measured and found to be proportional
toK
max. The plastic zone size was shown to be three times greater at the surface than at the interior. 相似文献
10.
Creep specimens of IN-X750 with U and V notch geometries have been tested at 700 °C under constant load. The do potential
drop technique was employed to measure local deformation, detect crack initiation, and measure crack propagation rates in
the notched specimens. Rupture of the U-notched bars and crack initiation in the V-notched bars were found to be controlled
by creep deformation. The time to crack initiation and the rupture time can be predicted by using the Monkman-Grant relationship,
which was shown to apply to localized crack initiation and multiaxial loading conditions. Sustained load crack growth rates
were measured in the V-notched specimens. The dependence of the stage II crack growth rates on the stress intensity factor,
the crack initiation, and rupture behavior of IN-X750 can be unified and interpreted with the continuous nucleation and constrained
cavity growth model.
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
11.
The effects of hydrogen on crack growth rates in a moderate-strength pipeline steel subjected to cyclic loads were studied.
Fatigue crack growth experiments were conducted in high-pressure hydrogen and nitrogen environments, and the influences of
stress ratio, stress intensity, and cyclic loading frequency on hydrogen-accelerated fatigue crack growth were investigated.
Hydrogen acceleration of intermediate-rate (Stage II) crack growth was greatest at low stress ratios and decreased to approximately
zero at a stress ratio of about 0.5. However, hydrogen promoted the premature onset of accelerated (Stage III) crack growth.
This appeared to be related to a hydrogen-induced reduction of fracture toughnessJ
IC. 相似文献
12.
The effect of orientation, applied stress state, environment, and temperature on the crack growth and crack-path behavior
of single-crystal specimens of UDIMET 720 has been examined. Stage I cracking has been promoted by mixed-mode loading, plane
stress, and vacuum conditions; increasing the test temperature to 600 °C does not suppress stage I crack growth in vacuum.
Consideration of the local resolved shear-stress intensity and local resolved normal-stress intensity for each slip system
as it intersects the nominal crack-growth plane allows the prediction of stage I crack paths, clarifying the importance of
secondary single-crystal testing orientation (i.e., nominal crack-growth direction as well as effective tensile axis). A combination of both opening and shearing are found to promote stage I crack growth, and boundary conditions have been established within
which stage I cracking is promoted. Highly deflected stage I cracking gives rise to significant shielding effects, but under
suitable mixed-mode loading, highly oriented, coplanar stage I crack growth can be produced. Intrinsic stage I cracking under
mixed-mode loading appears to be greatly accelerated compared with mode I-dominated stage II crack growth for comparable stress-intensity
levels. 相似文献
13.
The microscopic fatigue damage characteristics and short fatigue crack growth of an unnotched SiC(SCS-6) fiber-reinforced
Ti-15-3 alloy composite were investigated in tension-tension fatigue tests (R = 0.1) carried out at room temperature for applied maximum stress of 450, 670, and 880 MPa.In situ observation of the damage-evolution process was done using optical and scanning laser microscopies, which were attached in
the fatigue machine. The first damage for the composite started from a cracking of the reaction layer followed by fiber fracture.
The matrix cracking initiated near the broken fiber when the microhardness of the matrix just to the side of the fracture
fiber reached ≈6 GPa, and the number of cycles for the initiation of this cracking decreased with the increase of applied
stress. The slope of the relation of surface crack growth lengthvs number of cycles fell into two characteristic stages; in the first stage, the rate was lower than the second stage and accelerated.
The surface crack growth rate,d(2c)/dN,vs surface crack length relation also fell into two stages (stages I and II). With the increase in surface crack length, the
crack-growth rate,d(2c)/dN, decreased in stage I and increased in stage II. The transition from stage I to stage II occurred due to the fracture of
fibers located around the first fractured fiber. It was concluded that the fatigue crack growth resistance of the composite
in the short-crack region was controlled by the fiber fracture and matrix work hardening near the fractured fiber. When the
fiber fracture occurred, the surface crack growth rate was accelerated and became faster than that of the monolithic matrix. 相似文献
14.
The kinetics of sustained-load subcritical crack growth in hydrogen were determined for 18Ni(200) and 18Ni(250) maraging steels
over a range of hydrogen pressures and temperatures. Crack growth in each steel was characterized by an apparent threshold
stress intensity, a domain where the growth rate increased sharply with stress intensity (K) (Stage I), and a range where the growth rate was independent ofK (Stage II). The rate-limited Stage II crack growth in these steels exhibited three distinct regions of temperature dependency,
with a different isothermal pressure dependence in each region. In the low temperature region, Stage II crack growth was thermally
activated with δH = 18.2 ±1.7 kj/mol; (δH being independent of hydrogen pressure and yield strength). The growth rates at a givenK were proportional to the square root of hydrogen pressure. In the intermediate temperature region, Stage II growth rates
increased at slower rates, passed through a maximum and then decreased with increasing temperature. Within this region, the
pressure dependence for crack growth increased from 1/2-power to 2.0-power with increasing temperature. Above a transition
temperature, each grade of maraging steel became essentially immune to gaseous hydrogen embrittlement for the hydrogen pressure
range considered. The transition temperature was strongly affected by yield strength and hydrogen pressure. Plausible explanations
for these phenomenological results are considered.
Formerly a graduate student in the Department of Metallurgy and Materials Science, Lehigh University 相似文献
15.
Stress corrosion cracking behavior of 300M steel under various heat treated conditions was studied. Threshold stress intensity
was slightly dependent upon the martensitic substructure, the KIc value, and amounts of retained austenite. The prior austenite grain size exerted the maximum influence. The crack growth
rate was directly related to the number of constraint points and, hence, the prior austenite grain size. Increasing the prior
austenite grain size increased theK
Iscc, although Stage II crack growth rate also increased. The actual crack growth rate in Stage II was intermittent and decreased
slightly with increasing applied stress intensity. 相似文献
16.
R. C. McClung 《Metallurgical and Materials Transactions A》1991,22(7):1559-1571
Experimental and analytical evidence for the influence of applied stress, crack length, and stress intensity factor on crack
closure is critically compared and evaluated. Fatigue crack opening behaviors are broadly catalogued into three classes. Class
I comprises “near-threshold” behavior, where crack closure levels increase with decreasing stress intensity factor. In class
II, the “stable” regime, the crack opening level is independent of the stress intensity factor and crack length but is influenced
by the applied stress. Class III is characterized by the loss of elastic constraint accompanying extensive yielding at the
crack tip or in the remaining ligament, especially with further crack growth. Here, the crack opening level decreases with
increasing crack length until little or no closure occurs. These three different classes of closure behavior are extensively
illustrated with both experimental data and the results of numerical closure simulations, particularly original finite element
(FE) analyses. No single relationship between crack opening levels and the fundamental fatigue parameters is found to hold
universally, due to the wide range of mechanisms which cause or influence closure. 相似文献
17.
A new test specimen configuration, designated the T-notch double cantilever beam (TNDCB), was developed, calibrated and employed
for a fracture mechanics study of stress corrosion cracking (SCC) of cold worked Type-316 austenitic stainless steel exposed
to hot aqueous solutions of 44.7 wt pct MgCl2. The effects of stress intensity (K
I
), temperature (T) and electrochemical potential (E) upon the crack velocity (v) and fractography were investigated. The stress intensity (K
ISCC
) below whichv became immeasurably small was ∼12 MN·m−3/2. Above this value, three regions of behavior were observed. Region I exhibitedK
I
dependent cracking followed by Region II which exhibitedK
I
independent cracking and an apparent activation energy of 63 to 67 kJ/mol, followed by Region III where cracking again became
dependent uponK
I
. The relative proportions of intergranular and transgranular crack paths were markedly dependent upon bothK
I
andE, and less sensitive toT. Crack velocity was insensitive to small changes inE with respect to the free corrosion potentials (E
corr), but could be terminated by an applied active potential of ∼−0.35 VSCE. The pH within the propagating crack was estimated to be <1.0 atE
corr, rising to ∼4.5 at −0.35 VSCE. The mechanism of SCC was discussed with respect to film rupture events caused by crack tip plastic deformation, adsorption
controlled processes on the metal surface, and hydrogen diffusion in the metal lattice.
Alan J. RUSSELL, formerly Research Student, University of British Columbia 相似文献
18.
Single-ply composites containing both laminate and continuous Nb fiber reinforcement coated with A12O3 debond coatings in an MoSi2 matrix are used as model systems for investigating bridge toughening concepts for various precrack configurations. When cracks
are introduced symmetrically on either side of the ductile phase with zero crack offset spacing (S = 0), a minimum amount of energy is expended in plastic deformation and the local rupture process in the metal, as measured
by the area of the force displacement curve in tension. For asymmetric precracks introduced on either side of the ductile
reinforcement, as the offset spacing,S, was varied from 1 to 20R (R being the ductile phase half-thickness), the overall extension continuously increased within the bridging ligament. The effective
ligament gage length was nearly equal to the crack spacing in the limiting case of a weak interface. However, the ductile
Nb phase developed a Nb5Si3 reaction layer on its surface which was strongly bonded to the Nb and was found to undergo periodic cracking, leading to
numerous shear bands within the ductile phase. This unique and previously unreported mode of metal deformation in shear loading
has been analyzed using a simple geometric model. The results indicate that the profusion of shear bands is the primary source
of toughening enhancement in the case of asymmetric crack geometry, which was not recognized in prior work of this type.
This article is based on a presentation made at the “High Temperature Fracture Mechanisms in Advanced Materials” symposium
as a part of the 1994 Fall meeting of TMS, October 2-6, 1994, in Rosemont, Illinois, under the auspices of the ASM/SMD Flow
and Fracture Committee. 相似文献
19.
B. L. Averbach Bingzhe Lou P. K. Pearson R. E. Fairchild E. N. Bamberger 《Metallurgical and Materials Transactions A》1985,16(7):1267-1271
The growth rates of fatigue cracks propagating through the case and into the core have been studied for carburized X-2M steel
(0.14 C, 4.91 Cr, 1.31 Mo, 1.34 W, 0.42 V). Fatigue cracks were propagated at constant stress intensities, ΔK, and also at
a constant cyclic peak load, and the crack growth rates were observed to pass through a minimum value as the crack traversed
the carburized case. The reduction in the crack propagation rates is ascribed to the compressive stresses which were developed
in the case, and a pinched clothespin model is used to make an approximate calculation of the effects of internal stress on
the crack propagation rates. We define an effective stress intensity, Ke = Ka
+ Ki, where Ka is the applied stress intensity, Ki = σid
i
1/2
, σi is the internal stress, and di is a characteristic distance associated with the depth of the internal stress field. In our work, a value of di = 11 mm (0.43 inch) fits the data quite well. A good combination of resistance to fatigue crack propagation in the case and
fracture toughness in the core can be achieved in carburized X-2M steel, suggesting that this material will be useful in heavy
duty gears and in aircraft gas turbine mainshaft bearings operating under high hoop stresses. 相似文献
20.
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