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1.
Fatigue-crack growth rates for different simulated ocean environments and loading conditions have been investigated for beta-annealed
Ti-6Al-4V-0.1Ru (extra-low interstitials, (ELI)), a candidate material for oil production risers; the focus was on uncovering
whether certain combinations of conditions could produce unexpectedly high crack growth rates. A two-level, one-quarter-fraction
factorial design-of-experiments (DOE) approach was used to ascertain which testing variables and environmental conditions
warranted further study. This study used eight different combinations of variables: parent/deformed material, 27 °C/85 °C
temperature, 2 Hz/20 Hz loading frequency, 0.1/0.6 load ratio (R=σ
min/σ
max, where σ
min is the minimum and σ
max is the maximum stress during a fatigue cycle), and aerated/deaerated seawater. Comparisons were based on crack growth rates
at ΔK=17 MPa
, roughly the middle of the Paris portion of the da/dN vs ΔK curves. The da/dN vs ΔK curves were also examined, and conclusions based upon these data were compared with those from the DOE. Consideration of
the microstructure’s influence on the crack path is postponed until Part II of this article. Samples tested at the higher
load ratio showed a statistically significant increase in the crack propagation rate compared to those tested at R=0.1; the same was true of specimens tested at 20 Hz vs those tested at 2 Hz, but the level of significance was lower. The parent material had somewhat higher crack growth rates
than the deformed samples. Changes in environmental conditions other than frequency produced little effect on the crack growth
rate. Comparison of crack growth rates over the ΔK range measured revealed details that would have not been uncovered in comparisons at a single ΔK value. The Paris exponent ranged between 3.7 and 6.7, and the only systematic variation observed was an increase in the exponent
with increasing test frequency. In seawater, cold work (a 5 pct reduction in thickness by rolling) reduced fatigue-crack growth
rates by a factor of 2 (compared to the parent material) at intermediate and high ΔK values. There was a crossover of crack growth rates for low ΔK values: below 10 MPa
, growth rates were lower for the parent material than for the cold-rolled material, suggesting a higher ΔK
th
for the parent material, while above this value, fatigue cracks grew more rapidly in the parent material than in the cold-rolled
material. Crack growth rates were slightly higher in seawater than in air, but only slightly more than the sample-to-sample
variation of crack growth rates, and cold work reduced fatigue-crack growth rates in air by about the same amount as in seawater.
Somewhat more scatter was observed for the R=0.1 tests than for the R=0.6 tests. Differences in temperature (27 °C, 53 °C, and 85 °C) do not appear to affect fatigue-crack growth rates. For ΔK<20 MPa
, crack growth rates were similar for 0.2 and 2 Hz but were higher for 20 Hz; above 20 MPa
, the crack growth rates were similar for all three frequencies.
One explanation for the unusual frequency dependence relies on the possibility that the environment produces different amounts
of closure for different test frequencies. According to this view, closure is effective in air and in seawater at 0.2 and
2 Hz but not at 20 Hz: perhaps the higher loading rate breeches the passive layer at a rate more rapid than it can reform.
Because the crack growth rate appeared independent of temperature, it is unlikely that there is a significant influence of
thermally activated corrosion-fatigue mechanisms for the conditions tested.
The results demonstrate that beta-annealed Ti-6V-4Al-0.1Ru (ELI) possesses a robust response to the combinations of environment
and loading expected in oil production riser service. The value of the DOE approach was clear, and supplementary tests verified
the main effects predicted by the DOE results. Comparison of the single-value DOE results with the da/dN curves reveals a limitation in the former: different slopes of the Paris curves and crossover effects are or would be missed
for DOE comparing crack growth rates derived from constant ΔK tests. The use of constant Δσ data and a second level of interrogation, following DOE analysis and based on the da/dN curves, addressed this limitation effectively. A DOE comparison based, for example, on three ΔK values (the lower, middle, and upper portions of the Paris regime) might be another way of proceeding. 相似文献
2.
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
3.
D. B. Dawson 《Metallurgical and Materials Transactions A》1981,12(5):791-800
Differences in the corrosion fatigue crack growth behavior of anα–β titanium alloy in chloride-containing aqueous and methanol environments are reported, and discussed in relation to differences
in repassivation behavior for the two types of environments. Experiments have been conducted with various solution mixtures
of water (a passive film-forming environment) and methanol (a nonfilm-forming environment) to define the role of repassivation
in controlling fracture modes and crack growth rates at different frequencies. The critical event in determining whether the
repassivation process can suppress environmental fatigue fracture is the interaction between the rate of exposure of fresh
metal surfaces at the crack tip and the rate at which they can be repassivated. The out-come of this mechano-chemical interaction
is shown to be dependent on the frequency and stress intensity(ΔK) level as well as the chemistry of the environment. As a result, differences in repassivation behavior for methanol-water
solutions can be correlated with major differences in fatigue crack growth rates and fracture modes at low ΔK levels, whereas
repassivation differences have little effect at high ΔK levels. Based on these low-ΔK corrosion fatigue characteristics, methanol
solutions are concluded to be far more detrimental to titanium alloys than aqueous solutions. 相似文献
4.
G. F. Pittinato 《Metallurgical and Materials Transactions B》1972,3(1):235-243
The effects of a hydrogen environment on the fatigue crack growth rates in Ti-6A1-4V ELI (STA) and weld material were determined
in the temperature range of ambient to -200°F. The hydrogen environment resulted in an acceleration of the crack growth rate
and a change in the fracture mode for both materials in the temperature range of ambient to -100°F. At -200°F, there was no
significant difference between the crack growth rates obtained in helium and hydrogen gas. The degree of hydrogén-enhanced
crack growth was found to be dependent on the crack tip stress-intensity range, temperature, and microstructure of the material.
The data is consistent with an embrittlement mechanism involving hydrogen diffusing ahead of the crack front. 相似文献
5.
Fatigue crack propagation studies in vacua of 1.33 mN m-2, on Ti-6 A1-4V, at growth rates of 10-7 to 10-4 mm/cycle have shown that a threshold for growth exists at ‡K values of 6.3 to 7.6 MN m-3/2. The value of the threshold level is microstructure dependent, but growth above this value was structure insensitive according
to both growth rates and fracture surface observations. Some slow (≈ 10-8 mm/cycle) crack extension was observed below the threshold values but prolonged cycling reduced the growth rate to a vanishingly
small level. Fracture surface observations indicated that growth in this region was microstructure sensitive. Comparison with
previously performed air work on the same material showed that while structure insensitive growth rates in vacuum were slower
than those in air by a factor of 3 to 4, the low ‡K value structure sensitive rates were slower than the air ones by at least
three orders of magnitude. A hypothesis is proposed to explain this in terms of a propagation mechanism for the structure
sensitive mode of fatigue crack growth.
Research Fellow, Department of Physical Metallurgy, University of Birmingham, England 相似文献
6.
The effects of environment and cyclic stress intensity factor on crack opening displacement and crack tip strain have been
measured, and discontinuous crack growth has been observed directly under high resolution conditions in the Scanning Electron
Microscope. This information is used in a crack tip failure model together with cyclic stress-strain, low cycle fatigue, and
microstructural characteristics of the material to derive the expected crack growth increment. Agreement with measured striation
spacings is reasonable. 相似文献
7.
采用冷等静压–真空烧结法制备Ti-6Al-4V-2Cr-1Nd合金,然后进行固溶及时效热处理,通过实验与最小错配度理论计算,研究Nd元素对该合金组织细化的影响,并分析细化机理。结果表明,添加1%(质量分数)的稀土元素Nd后,析出相Nd2O3能有效促进晶粒细化。二维错配度的计算结果证明析出相Nd2O3是有效的形核剂,可促进非均匀形核,增加形核率,从而使晶粒细化。通过对合金试样薄区进行高分辨率观察,发现另一种絮状的、非常细小的、弥散分布的Nd2Ti4O11相,由于其界面错配度较低,也可作为非均匀形核的核心,促进形核,起到细化晶粒的作用。 相似文献
8.
9.
10.
S. Ankem D. Banerjee D. J. McNeish J. C. Williams S. R. Seagle 《Metallurgical and Materials Transactions A》1987,18(12):2015-2025
TEM, including convergent beam electron diffraction (CBED), SEM, and EDX techniques, were used to] characterize the silicide
(Ti, Zr)5 Si3 which forms in Beta-C (Ti-3Al-8V-6Cr-4Zr-4Mo) and other similar titanium alloys. Space group of this silicide is determined
as P6/mmm. In addition, solvus of this silicide was determined and a complete phase field diagram was constructed for Beta-C
alloy. It was found that morphology and distribution of the silicide strongly depends on thermal and thermomechanical treatments.
Continuous grain boundary silicide was found to be detrimental for tensil ductility which results in intergranular fracture.
Thermal and thermomechanical treatments were identified which can reduce or eliminate continuous grain boundary silicide precipitation. 相似文献
11.
The thermal history developed in laser metal deposition (LMD) processes has been shown to be quite complex and results in the evolution of an equally complex microstructure. A companion article (Part I. Microstructural Characterization) discussed the LMD of Ti-6Al-4V, where the resultant microstructure consists of a periodic, scale-graded layer of basketweave Widmanstätten alpha and a banding that consists of colony Widmanstätten alpha. In order to understand the microstructural evolution in Ti-6Al-4V, a numerical thermal model based on the implicit finite-difference technique was developed to model LMD processes. The effect of different laser-scan velocities on the characteristics of the thermal history was investigated using an eight-layer single-line build. As the laser-scan speed decreases and the position within a layer increases, the peak temperature increases. The heating rate and the peak thermal gradient within a deposited layer were shown to follow the same trend as the peak temperature after two layers were deposited on top of the substrate. In general, the laser-scan speed or z-position within a layer did not have a significant effect on the cooling rate. The cooling rate in a newly deposited layer decreases as the number of layer additions increases. Given the predicted temperature vs time profile from the thermal model, the evolution of phase transformations occurring in the deposit is mapped as each layer is deposited. As a result of the thermal cycling imposed by the periodic deposition of material, a characteristic layer, consisting of two regions heated above and below the beta transus, forms in layer n due to the deposition of layer n+1. 相似文献
12.
W. Shen W. O. Soboyejo A. B. O. Soboyejo 《Metallurgical and Materials Transactions A》2004,35(1):163-187
This article presents the results of an experimental and theoretical study of the effects of microstructure on room-temperature
fatigue and dwell-fatigue crack growth in Ti-6242. The crack growth rates and micromechanisms of long crack growth are elucidated
for equiaxed, elongated, and colony microstructures. The article shows that dwell and pure fatigue crack growth rates are
generally similar in the long crack growth regime. The underlying mechanisms of long crack growth are also generally similar
under pure fatigue and dwell-crack growth conditions. However, differences in dwell and fatigue sensitivities are observed
between the different microstructure in the mid-and high-ΔK regimes. These are explained by considering the possible interactions
between fatigue and creep during dwell fatigue loading at room temperature. Linearized fracture mechanics crack growth laws
are presented for the modeling of crack growth in the near-threshold, Paris, and high-ΔK regimes. 相似文献
13.
A previous study (Part I) showed that the solution-treated and aged (STA) (i.e., peak-aged) condition of Beta-C Ti, (σ
0.2 pct y = 1260 MPa) possesses an enhanced hydrogen (H) embrittlement susceptibility compared to the solution-treated (ST) condition,
(σ
0.2 pct y = 865 MPa), as measured by reductions in the fracture initiation stress with predissolved H content and the introduction
of an intergranular (IG) fracture mode. It was also shown that yield-strength elevation and the subsequent enhancement in
the local hydrostatic stresses within the notch root are not the controlling factors in the H-assisted* IG fracture initiation of the STA condition. Previous work (Part I) implicates
a microstructural feature or condition associated with the 500 °C aging treatment. In this study, it is shown that localized
internal hydride precipitation at the grain boundaries or alpha beta interfaces was not detected by a variety of experimental
methods over the range of internal H contents for which IG fracture initiation was observed. It was also shown that grain-boundary
alpha colonies or films are not responsible for the IG fracture initiation in the STA condition. A measured increase in hydrogen
embrittlement (HE) susceptibility as a function of aging time at 500 °C is consistent with the segregation or depletion of
a critical species at the grain boundary. However, grain-boundary segregation/depletion could not be detected with Auger electron
spectroscopy (AES) of specimens fracturing in a vacuum. Compression tests used to characterize and compare the alloys’ slip
behavior showed that plastic deformation is concentrated at or near the grain boundaries in the STA condition. Therefore,
a possible intergranular fracture initiation mechanism that includes the effects of hydrogen and localized deformation is
discussed. 相似文献
14.
R. J. H. Wanhill 《Metallurgical and Materials Transactions A》1976,7(9):1365-1373
A study was made of environmental fatigue crack propagation in 2.5 mm thick Ti-6A1-4V sheet conforming to AMS 4911, and 2.2 mm thick IMI 318 conforming to BS TA 10. The environments were dry argon, normal air, distilled water and 3.5 pct aqueous NaCl. There were three alloy/orientation combinations: Ti-6A1-4V L-T, IMI 318 L-T and IMI 318 T-L. Test frequencies were 30 and 50 Hz, at which there was a general trend of higher crack growth rates in the order: argon, air, distilled water, 3.5 pct aqueous NaCl. For both dry argon and 3.5 pct aqueous NaCl there were large differences in crack growth rates at low δK values between the three types of specimen. There was a correlation between the texture and cleavage fracture and crack growth rates in 3.5 pct aqueous NaCl. This result is of considerable practical importance. For dry argon the ranking of specimen types could be explained by the relative importance of mechanical and environmental crack growth, using the structure-sensitive to structure-insensitive transition concept of Irving and Beevers. 相似文献
15.
R. J. H. Wanhill 《Metallurgical and Materials Transactions A》1976,7(8):1365-1373
A study was made of environmental fatigue crack propagation in 2.5 mm thick Ti-6A1-4V sheet conforming to AMS 4911, and 2.2
mm thick IMI 318 conforming to BS TA 10. The environments were dry argon, normal air, distilled water and 3.5 pct aqueous
NaCl. There were three alloy/orientation combinations: Ti-6A1-4V L-T, IMI 318 L-T and IMI 318 T-L. Test frequencies were 30
and 50 Hz, at which there was a general trend of higher crack growth rates in the order: argon, air, distilled water, 3.5
pct aqueous NaCl. For both dry argon and 3.5 pct aqueous NaCl there were large differences in crack growth rates at low δK values between the three types of specimen. There was a correlation between the texture and cleavage fracture and crack growth
rates in 3.5 pct aqueous NaCl. This result is of considerable practical importance. For dry argon the ranking of specimen
types could be explained by the relative importance of mechanical and environmental crack growth, using the structure-sensitive
to structure-insensitive transition concept of Irving and Beevers. 相似文献
16.
17.
W. Chen C. J. Boehlert J. Y. Howe E. A. Payzant 《Metallurgical and Materials Transactions A》2011,42(10):3046-3061
This work studied the effect of processing on the elevated-temperature [728 K (455 °C)] fatigue deformation behavior of Ti-6Al-4V-1B for maximum applied stresses between 300 to 700 MPa (R = 0.1, 5 Hz). The alloy was evaluated in the as-cast form as well as in three wrought forms: cast-and-extruded, powder metallurgy (PM) rolled, and PM extruded. Processing caused significant differences in the microstructure, which in turn impacted the fatigue properties. The PM-extruded material exhibited a fine equiaxed α + β microstructure and the greatest fatigue resistance among all the studied materials. The β-phase field extrusion followed by cooling resulted in a strong α-phase texture in which the basal plane was predominately oriented perpendicular to the extrusion axis. The TiB whiskers were also aligned in the extrusion direction. The α-phase texture in the extrusions resulted in tensile-strength anisotropy. The tensile strength in the transverse orientation was lower than that in the longitudinal orientation, but the strength in the transverse orientation remained greater than that for the as-cast Ti-6Al-4V. The ratcheting behavior during fatigue is also discussed. 相似文献
18.
The fatigue crack growth behavior of TiC/Ti-alloy composite was examined at 450 °C and compared to the room-temperature behavior.
Contrary to the temperature-dependent fatigue crack growth behavior of the monolithic alloy, fatigue crack growth resistance
of the composite was improved at the elevated temperature. At 450 °C, the fatigue threshold of the composite was found to
be 50 Pct higher than at room temperature. Such an improved fatigue crack growth resistance is shown to result from extensive
microcracking of reinforcing particles, which promotes fatigue crack closure at the elevated temperature. 相似文献
19.
M. Niinomi B. Gong T. Kobayashi Y. Ohyabu O. Toriyama 《Metallurgical and Materials Transactions A》1995,26(5):1141-1151
The hydrogenation behavior of Ti-6Al-4V, with the starting microstructures of coarse equiaxed α and coarse Widmanstätten α, respectively, was investigated under a hydrogen pressure of 0.1 MPa at temperatures between 843 and 1123 K. The hydrogen content was determined as a function of hydrogenation time, hydrogenation temperature, and hydrogen flow rate. The phases presented in the alloy of after hydrogenation were determined with X-ray and electron diffraction analysis in order to define the effect of Thermochemical Processing (TCP) on the microstructure of the alloy. Mechanical properties and fracture toughness of Ti-6Al-4V and Ti-5Al-2.5Fe subjected to the various TCP were then investigated. Hydrogenation of Ti-6Al-4V with the starting microstructure of coarse equiaxed α at 1023 K, just below hydrogen saturated β (denoted β″ (H)) transus temperature, produces a microstructure of a, orthohombic martensite (denoted α″ (H)) and β (H). Hydrogenation at 1123 K, above β (H) transus, results in a microstructure of α″ (H) and β (H). Microstructure refinement during TCP results mainly from decomposition of α″ (H) and ;β (H) into a fine mixture of α + β during dehydrogenation. An alternative TCP method is below β (H) transus hydrogenation (BTH), consisting of hydrogenation of the alloy below the hydrogenated β (H) transus temperature, air cooling to room temperature, and dehydrogenation at a lower temperature, which is found to improve mechanical properties significantly over a conventional TCP treatment. Compared with the untreated material, the BTH treatment increases the yield strength and increases the ultimate tensile strength significantly without decreasing the tensile elongation in the starting microstructure of coarse equiaxed α or with a little decrease in the tensile elongation in the starting microstructure of coarse Widmanstätten α, although the conventional TCP treatment results in a large decrease in elongation over the unprocessed material in Ti-6Al-4V. In Ti-5Al-2.5 Fe, both conventional TCP and BTH result in a increase in yield strength, ultimate tensile strength, and elongation; however, the BTH gives the best balance between strength and elongation. The TCP-treated Ti-6Al-4V shows smaller fracture toughness compared with the unprocessed material, while TCP-treated Ti-5Al-2.5Fe shows greater fracture toughness compared with the unprocessed material. The BTH treatment results in a improvement in fatigue strength in both Ti-6Al-4V and Ti-5Al-2.5Fe. 相似文献
20.
G. R. Yoder L. A. Cooley T. W. Crooker 《Metallurgical and Materials Transactions A》1977,8(11):1737-1743
Fatigue crack growth rates in commercial purity Ti-6A1-4V can be substantially reduced with a beta anneal from levels associated with the mill anneal, owing primarily to crystallographic crack bifurcation in the Widmanstätten packets. This microstructurally sensitive fatigue crack growth occurs when the reversed plastic zone is less than the packet size and results in a fracture surface with a faceted morphology. It appears from replica and scanning electron microscopic examination that the facets are comprised of three superposed features,viz cleavage-like river-line patterns, very fine striations and traces of slip lines (and slip-band cracks). Limited X-ray evidence suggests a facet orientation some 8 to 10 degrees off the basal plane. 相似文献