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1.
The hydrogen-environment embrittlement (HEE)-controlled stage II crack growth rate of AA 7050 (6.09 wt pct Zn, 2.14 wt pct
Mg, and 2.19 wt pct Cu) was investigated as a function of temper and alloyed copper level in a humid air environment at various
temperatures. Three tempers representing the underaged (UA), peak-aged (PA), and overaged (OA) conditions were tested in 90
pct relative humidity (RH) air at temperatures between 25 °C and 90 °C. At all test temperatures, an increased degree of aging
(from UA to OA) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed
an Arrhenius-type temperature dependence, with activation energies between 58 and 99 kJ/mol. For both the normal-copper and
low-copper alloys, the fracture path was predominately intergranular at all test temperatures (25 °C to 90 °C) in each temper
investigated.
Comparison of the stage II HEE crack growth rates for normal- (2.19 wt pct) and low- (0.06 wt pct) copper alloys in the peak
PA aged and OA tempers showed a beneficial effect of copper additions on the stage II crack growth rate in humid air. In the
2.19 wt pct copper alloy, the significant decrease (∼10 times at 25 °C) in the stage II crack growth rate upon overaging is
attributed to an increase in the apparent activation energy for crack growth. In the 0.06 wt pct copper alloy, overaging did
not increase the activation energy for crack growth but did lower the pre-exponential factor ( v
0), resulting in a modest (∼2.5 times at 25 °C) decrease in the crack growth rate. These results indicate that alloyed copper
and thermal aging affect the kinetic factors that govern stage II HEE crack growth rates. The OA, copper-bearing alloys are
not intrinsically immune to hydrogen-environment-assisted cracking, but are more resistant due to an increased apparent activation
energy for stage II crack growth. 相似文献
2.
The hydrogen-environment embrittlement (HEE)-controlled stage II crack growth rate of AA 7050 (6.09 wt pct Zn, 2.14 wt pct
Mg, and 2.19 wt pct Cu) was investigated as a function of temper and alloyed copper level in a humid air environment at various
temperatures. Three tempers representing the underaged (UA), peak-aged (PA), and overaged (OA) conditions were tested in 90
pct relative humidity (RH) air at temperatures between 25 °C and 90 °C. At all test temperatures, an increased degree of aging
(from UA to OA) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed
an Arrhenius-type temperature dependence, with activation energies between 58 and 99 kJ/mol. For both the normal-copper and
low-copper alloys, the fracture path was predominately intergranular at all test temperatures (25 °C to 90 °C) in each temper
investigated.
Comparison of the stage II HEE crack growth rates for normal- (2.19 wt pct) and low- (0.06 wt pct) copper alloys in the peak
PA aged and OA tempers showed a beneficial effect of copper additions on the stage II crack growth rate in humid air. In the
2.19 wt pct copper alloy, the significant decrease (∼10 times at 25 °C) in the stage II crack growth rate upon overaging is
attributed to an increase in the apparent activation energy for crack growth. In the 0.06 wt pct copper alloy, overaging did
not increase the activation energy for crack growth but did lower the pre-exponential factor ( v
0), resulting in a modest (∼2.5 times at 25 °C) decrease in the crack growth rate. These results indicate that alloyed copper
and thermal aging affect the kinetic factors that govern stage II HEE crack growth rates. The OA, copper-bearing alloys are
not intrinsically immune to hydrogen-environment-assisted cracking, but are more resistant due to an increased apparent activation
energy for stage II crack growth.
An erratum to this article is available at . 相似文献
3.
Creep crack growth (CCG) has been investigated in an 8009 (Al-Fe-V-S) P/M alloy at 175 °, 250 °, and 316 ° and in a 2618 ingot
alloy at 150 °, 175 °, and 200 °. Under sustained load, subcritical crack growth is observed at stress intensity levels lower
than K
ic
; for 2618, at 200 °, crack growth is observed at stress intensities more than 40 pct lower than K
ic
. Alloys 8009 and 2618 exhibit creep brittle behavior, i.e., very limited creep deformation, during CCG. The CCG rates do not correlate with CCG parameters C* and C but correlate with
the stress intensity factor, K, and the J integral. Generally, crack growth rates increase with increasing temperature. Micromechanisms of CCG have been studied with
regard to microstructural deg-radation, environmental attack, and creep damage. Although theoretical estimation indicates
that CCG resistance decreases with second-phase coarsening, such coarsening has not been observed at the crack tip. Also,
no evidence is found for hydrogen- or oxygen-induced crack growth in comparing test results in moist air and in vacuum. Creep
deformation and cavitation ahead of crack tip are responsible for observed time-dependent crack growth. Based on the cavitation
damage in the elastic field, a micromechanical model is proposed which semiquantitatively explains the correlations between
the creep crack growth rate and stress intensity factor, K. 相似文献
4.
The influence of chromium additions on crack growth and oxidation have been examined in the nickel aluminide, Ni 3Al. Crack growth rates were measured in a chromium containing alloy as a function of stress intensity at temperatures between 600 and 760°C in air, together with rates of oxide film growth, and compared with previous measurements taken from Ni 3Al. The mechanisms of crack propagation and oxidation were investigated with a range of analytical techniques, including SEM, AES, XPS, SIMS, TEM and STEM. An addition of 8% chromium had a significantly beneficial effect on both crack growth resistance and oxidation resistance between 600 and 760°C. Low oxidation rates were associated with the formation of Cr 2O 3 together with Al 2O 3 at the metal/oxide interface, consistent with chromium acting initially as an oxygen getter, and promoting the formation of a protective Al 2O 3 layer, with little internal oxidation. It is proposed that chromium was also responsible for inhibiting oxygen access to and diffusion along grain boundaries at crack tips, modifying the mechanism of crack propagation from “step-wise cracking”, dominated by oxygen embrittlement (observed in the absence of chromium), to a more conventional creep crack growth process. 相似文献
6.
A study has been made of the effect of temperature (between 25 °C and 800 °C) on fracture toughness and fatigue-crack propagation
behavior in an XD-processed, γ-based titanium aluminide intermetallic alloy, reinforced with a fine dispersion of ∼1 vol pct TiB 2 particles. It was found that, whereas crack-initiation toughness increased with increasing temperature, the crack-growth
toughness on the resistance curve was highest just below the ductile-to-brittle transition temperature (DBTT) at 600 °C; indeed,
above the DBTT, at 800 °C, no rising resistance curve was seen. Such behavior is attributed to the ease of microcrack nucleation
above and below the DBTT, which, in turn, governs the extent of uncracked ligament bridging in the crack wake as the primary
toughening mechanism. The corresponding fatigue-crack growth behavior was also found to vary inconsistently with temperature.
The fastest crack growth rates (and lowest fatigue thresholds) were seen at 600 °C, while the slowest crack growth rates (and
highest thresholds) were seen at 800 °C; the behavior at 25 °C was intermediate. Previous explanations for this “anomalous
temperature effect” in γ-TiAl alloys have focused on the existence of some unspecified environmental embrittlement at intermediate temperatures or
on the development of excessive crack closure at 800 °C; no evidence supporting these explanations could be found. The effect
is now explained in terms of the mutual competition of two processes, namely, the intrinsic microstructural damage/crack-advance
mechanism, which promotes crack growth, and the propensity for crack-tip blunting, which impedes crack growth, both of which are markedly enhanced by increasing temperature. 相似文献
7.
The tensile deformation, crack-tip micromechanics, and fracture behaviors of a two-phase (γ + α 2) gamma titanium aluminide alloy, Ti-47Al-2.6Nb-2(Cr+V), heat-treated for the microstructure of either fine duplex (gamma
+ lamellar) or predominantly lamellar microstructure were studied in the 25 °C to 800 °C range. In situ tensile and fracture toughness tests were performed in vacuum using a high-temperature loading stage in a scanning electron
microscope (SEM), while conventional tensile tests were performed in air. The results revealed strong influences of microstructure
on the crack-tip deformation, quasi-static crack growth, and the fracture initiation behaviors in the alloy. Intergranular
fracture and cleavage were the dominant fracture mechanisms in the duplex microstructure material, whose fracture remained
brittle at temperatures up to 600 °C. In contrast, the nearly fully lamellar microstructure resulted in a relatively high
crack growth resistance in the 25 °C to 800 °C range, with interface delamination, translamellar fracture, and decohesion
of colony boundaries being the main fracture processes. The higher fracture resistance exhibited by the lamellar microstructure
can be attributed, at least partly, to toughening by shear ligaments formed as the result of mismatched crack planes in the
process zone. 相似文献
8.
The fatigue crack growth behavior of aluminum alloy 5083-H131 has been systematically studied as a function of degree of sensitization for aging at 448?K (175?°C). Fatigue crack growth rates were measured at load ratios of 0.1 and 0.85, in vacuum, air, and a corrosive aqueous environment containing 1?pct NaCl with dilute inhibitor. Sensitization does not affect the fatigue crack growth behavior of Al 5083-H131 significantly in vacuum or air, at low- or high-load ratio. For high-load ratio, in the 1?pct NaCl+inhibitor solution, the threshold drops by nearly 50?pct during the first 200?hours of aging, then it degrades more slowly for longer aging times up to 2000?hours. The change in aging behavior at approximately 200?hours seems to be correlated with the transition from partial coverage of the grain boundaries by ?? Al 3Mg 2 phase, to continuous full ?? coverage. ASTM G-67 mass loss levels below approximately 30?mg/cm 2 do not exhibit degraded corrosion-fatigue properties for R?=?0.85, but degradation of the threshold is rapid for higher mass loss levels. 相似文献
9.
In the present study, the diffusion bonding of 17-4 precipitation hardening stainless steel to Ti alloy with and without nickel alloy as intermediate material was carried out in the temperature range of 1073 K to 1223 K (800 °C to 950 °C) in steps of 298 K (25 °C) for 60 minutes in vacuum. The effects of bonding temperature on interfaces microstructures of bonded joint were analyzed by light optical and scanning electron microscopy. In the case of directly bonded stainless steel and titanium alloy, the layerwise α-Fe + χ, χ, FeTi + λ, FeTi + β-Ti phase, and phase mixture were observed at the bond interface. However, when nickel alloy was used as an interlayer, the interfaces indicate that Ni 3Ti, NiTi, and NiTi 2 are formed at the nickel alloy-titanium alloy interface and the PHSS-nickel alloy interface is free from intermetallics up to 1148 K (875 °C) and above this temperature, intermetallics were formed. The irregular-shaped particles of Fe 5Cr 35Ni 40Ti 15 have been observed within the Ni 3Ti intermetallic layer. The joint tensile and shear strength were measured; a maximum tensile strength of ~477 MPa and shear strength of ~356.9 MPa along with ~4.2 pct elongation were obtained for the direct bonded joint when processed at 1173 K (900 °C). However, when nickel base alloy was used as an interlayer in the same materials at the bonding temperature of 1148 K (875 °C), the bond tensile and shear strengths increase to ~523.6 and ~389.6 MPa, respectively, along with 6.2 pct elongation. 相似文献
10.
Several alloys based on Fe-25Cr-6Al and Fe-25Cr-11Al (wt pct) with additions of yttrium, Al 2O 3, and Y 2O 3 have been prepared by mechanical alloying of elemental, master alloy and oxide powders. The powders were consolidated by
extrusion at 1000°C with a reduction ratio of 36:1. The resulting oxide contents were all approximately either 3 vol pct or
8 vol pct of mixed Al 2O 3-Y 2O 3 oxides or of Al 2O 3. The alloys exhibited substantial ductility at 600°C: an alloy containing 3 vol pct oxide could be readily warm worked to
sheet without intermediate annealing; an 8 vol pct alloy required intermediate annealing at 1100°C. The 3 vol pct alloys could
be recrystallized to produce large elongated grains by isothermal annealing of as-extruded material at 1450°C, but the high
temperature strength properties were not improved. However, these alloys, together with some of the 8 vol pct materials, could
be more readily recrystallized after rod (or sheet) rolling; sub-stantially improved tensile and stress rupture properties
were obtained following 9 pct rod rolling at 620°C and isothermal annealing for 2 h at 1350°C. In this condition, the rup-ture
strengths of selected alloys at 1000 and 1100°C were superior to those of competitive nickel-and cobalt-base superalloys.
The oxidation resistance of all the alloys was ex-cellent.
F. G. WILSON and C. D. DESFORGES, formerly with Fulmer Re-search Institute 相似文献
11.
A novel in situ technique has been used to fabricate an Al 2O 3-reinforced Ni 3Al matrix composite. The composite was prepared by first incorporating a low volume fraction of continuous Al 2O 3 fibers in a Ni 3Al alloy containing 0.34 at. pct Zr. Pressure casting was used to embed the fibers. Casting resulted in partial reduction
of the Al 2O 3 fiber by the Zr present in the matrix and the formation of a layer of ZrO 2 on the surface of the fibers. The final composite was then prepared by air annealing the precursor composite at 1100 °C for
10 days. Air annealing led to the formation of networks of Al 2O 3 around the fibers. The matrix in the immediate vicinity of the networks consisted of Ni 3Al particles in a matrix of disordered α-Ni(Al). The Al 2O 3 networks raised the yield and tensile strength of the material by 35 and 18 pct, respectively. The composite had a tensile
ductility of 14 pct. 相似文献
12.
The tensile properties, fracture modes, and deformation mechanisms of two DO 3 alloys, Fe-25 and Fe-31 at. pct Al, have been investigated as a function of temperature up to 600 °C. The first alloy was
produced by powder metallurgy and hot-extrusion, the second by casting and hot-extrusion. At room temperature extensive plastic
deformation occurs in these intermetallics, exhibiting an elongation to fracture of 8 pct and 5.6 pct, respectively. In the
Fe-25Al alloy the deformation process consisted of motion and extensive cross-slip of ordinary dislocations and associated
formation of antiphase-boundary (APB) bands, while in the Fe-31Al alloy, plasticity occurred by the motion of superlattice
dislocations which eventually dissociated to form APB bands. At room temperature both alloys exhibited transgranular cleavage
fracture modes. The variation of tensile properties and fracture modes with temperature is presented.
Formerly with the Materials Laboratory of the Air Force Wright
Formerly with the Materials Laboratory of the Air Force Wright
Formerly with the Materials Laboratory of the Air Force Wright 相似文献
13.
A Ti-4Al-2Fe-3Cu (wt pct) alloy containing only low-cost alloying elements was fabricated by vacuum sintering a blend of TiH2, Al, Fe, and Cu powders at 1200 °C for 1 hour followed by hot extrusion at the same temperature. The as-extruded alloy exhibited a microstructure consisting of mainly α/β lamellar colonies and Ti2Cu as a minor phase. The average colony size and lamella thickness were 118 and 12 µm, respectively, and Fe and Cu were predominantly distributed in the β lamellae. The as-extruded alloy had a high tensile yield strength (YS) and ultimate tensile strength (UTS) of 1248 and 1270 MPa, respectively, but a limited ductility (elongation to fracture: 2.3 pct). Annealing at 750 °C for 4 hour caused the average colony size and lamella thickness of the alloy to increase to 145 and 17 µm, respectively, and the volume fraction of the β phase decreased with the annealing. These microstructural changes resulted in a slight decrease of the YS and UTS to 1221 and 1253 MPa, but a clear increase of the ductility with the elongation to fracture reaching 4 pct. This work demonstrates that a combination of relatively low-temperature vacuum sintering, hot extrusion, and annealing can be effectively utilized to fabricate a low-cost Ti-4Al-2Fe-3Cu alloy with high strength and appreciable tensile ductility. 相似文献
14.
The tensile properties, fracture modes, and deformation mechanisms of two DO 3 alloys, Fe-25 and Fe-31 at. pct Al, have been investigated as a function of temperature up to 600°C. The first alloy was
produced by powder metallurgy and hot-extrusion, the second by casting and hot-extrusion. At room temperature extensive plastic
deformation occurs in these intermetallics, exhibiting an elongation to fracture of 8 pct and 5.6 pct, respectively. In the
Fe-25Al alloy the deformation process consisted of motion and extensive cross-slip of ordinary dislocations and associated
formation of antiphase-boundary (APB) bands, while in the Fe-31 Al alloy, plasticity occurred by the motion of superlattice
dislocations which eventually dissociated to form APB bands. At room temperature both alloys exhibited transgranular cleavage
fracture modes. The variation of tensile properties and fracture modes with temperature is presented.
H. A. LIPSITT, formerly with the Materials Laboratory of the Air Force Wright Aeronautical Laboratories, Wright-Patterson
Air Force Base, OH 45433-6533 相似文献
15.
The fatigue crack growth rates (FCGR) of Inconel X-750 were measured in air and in vacuum at 25 °C and 650 °C as a function
of test frequency. The wave shape was triangular and the frequency varied from 10 Hz to 0.01 Hz. The creep crack growth rates
(CCGR) were also measured on single edge notch specimens at 650 °C in air and in purified argon. For a given AK, the FCGR increases when temperature increases and frequency decreases. At low frequency the FCGR approach the creep crack
growth rates. The mode of fracture changes from transgranular at 10 Hz to intergranular at 0.01 Hz. The effect of air environment
is to accelerate the transition from transgranular to intergranular fracture modes with decreasing frequency. The role of
oxidation in accelerating crack growth rate in fatigue and in creep is discussed in detail.
F. GABRIELLI, was formerly a visiting scientist at Massachusetts Institute of Technology. 相似文献
16.
Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct
Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist
of the intermetallic compounds y-TiAl, α 2-Ti 3Al, and ξ-Ti 5(Si, Al) 3. The microstructure can be described as a duplex structure (i.e., lamellar γ/α 2 regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates
and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature
and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite
is true. Examinations of tensile deformed specimens reveal ξ-Ti 5(Si, Al) 3 particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along
lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the
alloys can tolerate ξ-Ti 5(Si, Al) 3 particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized. 相似文献
17.
The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing
tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully
lamellar microstructure with approximately 150- μm grain size and 1- μm lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile
transition temperature (BDTT), while fracture and fatigue tests were performed at 25 °C and 815 °C. Fracture toughness tests
were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using
ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and
by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified
and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture
toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness,
and fatigue resistance were identified and related to pertinent microstructural variables. 相似文献
18.
Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been
investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were
observed over a strain rate range of 3 × 10 −5 s −1 to 1 × 10 −2 s −1, with a maximum value of 0.65 at 5 × 10 −4 s −1 and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were
obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower
superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum
tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this
condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic
grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more
rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain
rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation,
which produced a tensile elongation of 671 pct at 525 °C.
R. VERMA, formerly Visiting Scientist, Department of Materials Science and Engineering, University of Michigan 相似文献
19.
The influence of gaseous atmosphere on the fatigue crack propagation behavior of a Ti6242 alloy is studied at 550 °C. The
aim of this paper is to obtain reference data in controlled environments at the atmospheric pressure in view of a further
evaluation of the corrosion-fatigue resistance of this alloy in super-critical water medium for a new process for hydrothermal
treatment of organic effluents. Tests were conducted in ambient air, high vacuum, and humidified gaseous atmospheres (80 pct
RH) including pure argon, 80 pct argon + 20 pct oxygen, and 80 pct nitrogen + 20 pct oxygen. The loading specimen was triangular
at a frequency of 0.05 Hz. Some additional tests were performed at frequencies ranging from 0.001 to 35 Hz. The crack propagation
rate is shown to be highly sensitive to the environment, with a predominant detrimental influence of water vapor. A crack
growth model is proposed accounting for the influence of partial pressures of water vapor, oxygen, test, and frequency. 相似文献
20.
We propose a method for developing new quaternary Ir-Nb-Ni-Al refractory superalloys for ultra-high-temperature uses, by mixing
two types of binary alloys, Ir-20 at. pct Nb and Ni-16.8 at. pct Al, which contain fcc/L1 2 two-phase coherent structures. For alloys of various Ir-Nb/Ni-Al compositions, we analyzed the microstructure and measured
the compressive strengths. Phase analysis indicated that three-phase equilibria—fcc, Ir 3Nb-L1 2, and Ni 3Al-L1 2—existed in Ir-5Nb-62.4Ni-12.6Al (at. pct) (alloy A), Ir-10Nb-41.6Ni-8.4Al (at. pct) (alloy B), and Ir-15Nb-20.8Ni-4.2Al (at.
pct) (alloy C) at 1400 °C; at 1300 °C, three phase equilibria—fcc, Ir 3Nb, and Ni 3Al—existed in alloys A and C and four-phase equilibria—fcc, Ir 3Nb, Ni 3Al, and IrAl-B2—existed in alloy B. The fcc/L1 2 coherent structure was examined by using transmission electron microscopy (TEM). At a temperature of 1200 °C, the compressive
strength of these quaternary alloys was between 130 and 350 MPa, which was higher than that of commercial Ni-based superalloys,
such as MarM247 (50 MPa), and lower than that of Ir-based binary alloys (500 MPa). Compared to Ir-based alloys, the compressive
strain of these quaternary alloys was greatly improved. The potential of the quaternary alloys for ultra-high-temperature
use is also discussed. 相似文献
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