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1.
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
2.
N. U. Deshpande A. M. Gokhale D. K. Denzer John Liu 《Metallurgical and Materials Transactions A》1998,29(4):1191-1201
The fracture toughness of Al-Zn-Mg-Cu-based 7XXX aluminum alloys decreases with an increase in the extent of recrystallization.
In this contribution, the fracture path of plane-strain fracture-toughness specimens of 7050 alloy (a typical alloy of the
7XXX series) is quantitatively characterized as a function of degree of recrystallization, specimen orientation, and aging
condition. The fracture path is quantitatively correlated to fracture toughness, and the bulk microstructural attributes estimated
via stereological analysis. In the companion article, these quantitative data are used to develop and verify a multiple-fracture
micromechanism-based model that relates the fracture toughness to a number of microstructural parameters of the partially
recrystallized alloy plate. 相似文献
3.
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
4.
Mechanical behavior of double-aged AA8090 总被引:3,自引:0,他引:3
The short-transverse fracture toughness of AA8090 is dramatically improved by double aging treatments, which produce a transition from coarse planar slip to homogeneous deformation. Although the fracture mode remains intergranular, stress concentrations across the weak, highangle boundaries are reduced by homogeneous deformation, ultimately increasing fracture toughness. This behavior is attributed to dissolution of the shearable phase(δ’) and growth of the strong precipitate (S’). Predictions of slip distribution agree fairly well with observed deformation behavior. A number of tempers with improved strength-toughness relationships were developed, and fatigue crack growth behavior in laboratory air was not affected by double aging. 相似文献
5.
W. O. Soboyejo J. Dipasquale F. Ye C. Mercer T. S. Srivatsan D. G. Konitzer 《Metallurgical and Materials Transactions A》1999,30(4):1025-1038
This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo
alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature
ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V,
and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized
plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of
the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity
completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation
of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along
slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping
crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold
fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and
fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics. 相似文献
6.
R. D. Carter E. W. Lee E. A. Starke C. J. Beevers 《Metallurgical and Materials Transactions A》1984,15(3):555-563
The effects of slip character and grain size on the intrinsic material and extrinsic closure contributions to fatigue crack
growth resistance have been studied for a 7475 aluminum alloy. The alloy was tested in the underaged and overaged conditions
with grain sizes of 18 μm and 80 μm. The fracture surface exhibited increased irregularity and planar facet formation with
increased grain size, underaging, and tests in vacuum. These changes were accompanied by an increased resistance to fatigue
crack growth. In air the 18 μm grain size overaged material exhibited relatively poor resistance to fatigue crack growth compared
with other microstructural variants, and this was associated with a lower stress intensity for closure. All materials exhibited
a marked improvement in fatigue crack growth resistance when tested in vacuum, with the most significant difference being
˜1000× at a ΔK of 10 MPa m1/2 for the 80 μm grain size underaged alloy. This improvement could not be accounted for by either an increase in closure or
increased crack deflection and is most likely due to increased slip reversibility in the vacuum environment. The intrinsic
resistance of the alloy to fatigue crack growth was microstructurally dependent in vacuum, with large grains and planar slip
providing the better fatigue performance. 相似文献
7.
K. S. Chan 《Metallurgical and Materials Transactions A》1990,21(10):2687-2699
The deformation and fracture behaviors of the Ti-24Al-11Nb alloy with an equiaxed α2 + β microstructure have been characterized as a function of temperature by performing uniaxial tension andJ
IC
fracture toughness tests. The micromechanisms of crack initiation and growth have been studied bypost mortem fractographic and metallographic examinations of fractured specimens, as well as byin situ observation of the fracture events in a scanning electron microscope (SEM) equipped with a high-temperature loading stage.
The results indicate that quasistatic crack growth in the Ti-24Al-11Nb alloy occurs by nucleation and linkage of the microcracks
with the main crack, with the latter frequently bridged by ductile β ligaments. Three microcrack initiation mechanisms have
been identified: (1) decohesion of planar slipbands in the α2 matrix, (2) formation of voids and microcracks in β, and (3) cracking at or near the α2 + β interface due to strain incompatibility resulting from impinging planar slip originated in α2. The sources of fracture toughness in the 25 °C to 450 °C range have been attributed to crack tip blunting, crack deflection,
and a bridging mechanism provided by the ductile β phase. At 600 °C, a change of toughening mechanisms leads to a lowering
of the initiation toughness (theK
IC
value) but a drastic increase in the crack growth toughness and the tearing modulus. 相似文献
8.
《Acta Metallurgica Materialia》1991,39(5):793-806
A study of the influence of SiC-particulate reinforcement on ageing and subsequent fatigue crack growth resistance in a powder metallurgy 8090 aluminium alloy-SiC composite has been made. Macroscopic hardness measurements revealed that ageing at 170°C in the composite is accelerated with respect to the unreinforced alloy, though TEM studies indicate that this is not due to the enhanced precipitation of S'. Fatigue crack growth rates in the naturally aged condition of the composite and unreinforced matrix are similar at low to medium values of δK, but diverge above ≈ 8 MPa√m owing to the lower fracture toughness of the composite. As a result of the presence of the reinforcement, planar slip in the composite is suppressed and facetted crack growth is not observed. Ageing at or above 170°C has a deleterious effect on fatigue crack growth. Increased ageing time decreases the roughness of the fracture path at higher growth rates. These effect are though to be due to microstructural changes occurring at or near to the SiC/matrix interfaces, providing sites for static mode failure mechanisms to operate. This suggestion is supported by the observation that as δK increases, crack growth rates become Kmax dependent, implying the crack growth rate is strongly influenced by static modes. 相似文献
9.
Mechanisms of Slow Fatigue Crack Growth in High Strength Aluminum Alloys: Role of Microstructure and Environment 总被引:1,自引:0,他引:1
S. Suresh A. K. Vasudévan P. E. Bretz 《Metallurgical and Materials Transactions A》1984,15(2):369-379
The role of microstructure and environment in influencing ultra-low fatigue crack propagation rates has been investigated
in 7075 aluminum alloy heat-treated to underaged, peak-aged, and overaged conditions and tested over a range of load ratios.
Threshold stress intensity range, ΔK0, values were found to decrease monotonically with increasing load ratio for all three heat treatments fatigue tested in 95
pct relative humidity air, with ΔK
0 decreasing at all load ratios with increased extent of aging. Comparison of the near-threshold fatigue behavior obtained
in humid air with the data forvacuo, however, showed that the presence of moisture leads to a larger reduction in ΔK0 for the underaged microstructure than the overaged condition, at all load ratios. An examination of the nature of crack morphology
and scanning Auger/SIMS analyses of near-threshold fracture surfaces revealed that although the crack path in the underaged
structure was highly serrated and nonlinear, crack face oxidation products were much thicker in the overaged condition. The
apparent differences in slow fatigue crack growth resistance of the three aging conditions are ascribed to a complex interaction
among three mechanisms: the embrittling effect of moisture resulting in conventional corrosion fatigue processes, the role
of microstructure and slip mode in inducing crack deflection, and crack closure arising from a combination of environmental
and microstructural contributions. 相似文献
10.
The strength,fracture toughness,and low cycle fatigue behavior of 17-4 PH stainless steel 总被引:1,自引:0,他引:1
The influence of microstructure on the strength, fracture toughness and low cycle fatigue behavior of 17-4 PH stainless steel
has been examined. Aging hardening involves initial formation of coherent copper-rich clusters which transform to incoherent
fee ∈-copper precipitates upon further aging. The changes in strength level and strain hardening rates observed during aging
are consistent with previously suggested models for precipitation hardening based on differing elastic moduli. The fracture
toughness and fatigue crack growth rates were shown to be a function of microstructure and environment. At equivalent strength
levels overaging resulted in a higher fracture toughness than did underaging. The fatigue crack growth rates increased with
increasing strength level and humidity but were not a function of toughness level. Attempts to correlate the fatigue crack
growth rates with monotonie tensile properties were unsuccessful. However when final failure obeyed a critical strain criteria,
the fracture toughness behavior could be reasonably described and related to preferential void nucleation and growth at δ-ferrite-matrix
interfaces. 相似文献
11.
The role of corrosion deposits in influencing the near-threshold fatigue crack propagation behavior of 2XXX and 7XXX series
aluminum alloys is examined in detail. The composition, thickness, and distribution of fracture surface oxide films are characterized
with the aid of X-ray photoelectron spectroscopy, scanning Auger spectroscopy, and secondary ion mass spectroscopy analyses.
It is found that the extent of crack closure due to corrosion debris in aluminum alloys is strongly dependent on the composition
and aging treatment. The results suggest that environmentally-influenced near-threshold crack propagation in some aluminum
alloys is controlled by twoconcurrent andmutually-compctitive mechanistic processes: a dominant role of crack closure due to corrosion deposits (which tends toarrest completely the near-threshold crack) and a strong embrittling effect (which considerablyincreases near-threshold crack growth rates) concomitantly with the crack tip oxidation phenomenon in the moist medium. The near-threshold
corrosion fatigue characteristics of aluminum alloys are contrasted with those in a wide range of steels in order to gain
an insight into the various 相似文献
12.
13.
The present study compares the fatigue and fracture properties of the high-strength β titanium alloy β-Cez with the conventional α+β titanium alloy Ti-6Al-4V, because of increasing interest in replacing α+β titanium alloys with β titanium alloys for highly stressed airframe and jet engine components. This comparison study includes the Ti-6Al-4V alloy
in an α+ β-processed condition (for a typical turbine blade application) and the β-Cez alloy in two distinctly different α+β-processed and β-processed conditions (optimized for a combination of superior strength, ductility, and fracture toughness). The comparison
principally showed a much lower yield stress for Ti-6Al-4V (915 MPa) than for both β-Cez conditions (1200 MPa). The Ti-6Al-4V material also showed the significantly lower high-cycle fatigue strength (resistance
against crack initiation) of 375 MPa (R=−1) as compared to the β-Cez alloy (∼600 MPa, R=−1). Particularly in the presence of large cracks (>5 mm), the fatigue crack growth resistance and fracture toughness of
the Ti-6Al-4V material is superior when compared to both β-Cez conditions. However, for small crack sizes, the conditions of both the alloys under study show equivalent resistance
against fatigue crack growth. For the β-Cez material, where microstructures were optimized for high fracture toughness (conventional large crack sizes) by thermomechanical
processing, maximum K
Ic-values of 68 MPa√m of the β-processed β-Cez condition (tested in the longitudinal direction) decreased by ∼50 pct in the presence of small cracks (1 mm). A similar
decrease in fracture toughness was obtained by loading the β-processed β-Cez condition perpendicular to the flat surfaces of the pancake-shaped β grain structure (tested in the short transverse direction). These results were discussed in terms of the effectiveness of
the crack front geometry in hindering crack propagation. Further, the results of this study were considered for alloy selection
and optimized microstructures for fatigue and fracture critical applications. Finally, the advantage of the α+β-processed β-Cez condition in highly stressed engineering components is pointed out because of its overall superior combination of fatigue
crack initiation and propagation resistance (especially against small fatigue cracks). 相似文献
14.
D. L. Davidson K. S. Chan R. Loloee M. A. Crimp 《Metallurgical and Materials Transactions A》2000,31(4):1075-1084
The fatigue and fracture resistance of a commercially made, single-phase Nb-base alloy with 35 at. pct Ti, 5 at. pct Cr, 6
at. pct Al, and several elements to increase solid solution strengthening have been investigated. The threshold for fatigue
crack growth was determined to be ≈7 MPa√m and fracture toughness ≈35 MPa√m. Crack growth was intermittent and sporadic; the
fracture path was tortuous, crystallographic, and appeared to favor the {100} and {112} planes. Fatigue crack closure was
measured directly at the crack tip. The fatigue and fracture properties of the commercial alloy are compared against those
of Nb-Cr-Ti and Nb-Cr-Ti-Al alloys. The comparison indicated that Ti addition is beneficial for, but Al addition is detrimental
to, both fracture toughness and fatigue crack resistance. 相似文献
15.
16.
G. Terlinde H. -J. Rathjen K. -H. Schwalbe 《Metallurgical and Materials Transactions A》1988,19(4):1037-1049
Fracture mechanics and tensile tests have been performed on the metastable β-Ti alloy Ti-IOV-2Fe-3AI. A variety of microstructures
was established by several combinations of forging and heat treatment resulting in different types, morphologies, and volume
fractions of the a-phase which precipitates from the matrix-β phase. Both fracture toughness and ductility are strongly reduced
by increasing hardening by the secondary a-phase. An elongated primary a-phase (α
p
) shows higher toughness compared to a globular α
p
-phase. A thick, continuous subgrain boundary a-film lowers the toughness significantly. For microstructures without primary
a a grain boundary α-film does not affect the toughness, while the ductility is drastically reduced. Very attractive combinations
of fracture toughness and ductility were found for a microstructure without primary a and without grain boundary α. The results
are discussed based on the fractographic observations, and a model is proposed which includes the effect of microstructure
and slip distribution on the crack nucleation, the crack growth path, and the crack deviation. 相似文献
17.
18.
C. J. Gilbert V. Schroeder R. O. Ritchie 《Metallurgical and Materials Transactions A》1999,30(7):1739-1753
The fracture and fatigue properties of a newly developed bulk metallic glass alloy, Zr41.2Ti13.8Cu12.5 Ni10Be22.5 (at. pct), have been examined. Experimental measurements using conventional fatigue precracked compact-tension C(T) specimens
(∼7-mm thick) indicated that the fully amorphous alloy has a plane-strain fracture toughness comparable to polycrystalline
aluminum alloys. However, significant variability was observed and possible sources are identified. The fracture surfaces
exhibited a vein morphology typical of metallic glasses, and, in some cases, evidence for local melting was observed. Attempts
were made to rationalize the fracture toughness in terms of a previously developed micromechanical model based on the Taylor
instability, as well as on the observation of extensive crack branching and deflection. Upon partial or complete crystallization,
however, the alloy was severely embrittled, with toughnesses dropping to ∼1 MPa
. Commensurate with this drop in toughness was a marginal increase in hardness and a reduction in ductility (as measured via depthsensing indentation experiments). Under cyclic loading, crack-propagation behavior in the amorphous structure was similar
to that observed in polycrystalline steel and aluminum alloys. Moreover, the crack-advance mechanism was associated with alternating
blunting and resharpening of the crack tip. This was evidenced by striations on fatigue fracture surfaces. Conversely, the
(unnotched) stress/life (S/N) properties were markedly different. Crack initiation and subsequent growth occurred quite readily,
due to the lack of microstructural barriers that would normally provide local crack-arrest points. This resulted in a low
fatigue limit of ∼4 pct of ultimate tensile strength. 相似文献
19.
S. Hariprasad S. M. L. Sastry K. L. Jerina R. J. Lederich 《Metallurgical and Materials Transactions A》1994,25(5):1005-1014
The room-temperature fatigue crack growth rates (FCGR) and fracture toughness were evaluated for different crack plane orientations
of an Al-8.5 Pct Fe-1.2 Pct V-1.7 Pct Si alloy produced by planar flow casting (PFC) and atomized melt deposition (AMD) processes.
For the alloy produced by the PFC process, properties were determined in six different orientations, including the short transverse
directions S-T and S-L. Diffusion bonding and adhesive bonding methods were used to prepare specimens for determining FCGR
and fracture toughness in the short transverse direction. Interparticle boundaries control fracture properties in the alloy
produced by PFC. Fracture toughness of the PFC alloy varies from 13.4 MPa√m to 30.8 MPa√m, depending on the orientation of
the crack plane relative to the interparticle boundaries. Fatigue crack growth resistance and fracture toughness are greater
in the L-T, L-S, and T-S directions than in the T-L, S-T, and S-L orientations. The alloy produced by AMD does not exhibit
anisotropy in fracture toughness and fatigue crack growth resistance in the as-deposited condition or in the extruded condition.
The fracture toughness varies from 17.2 MPa√m to 18.5 MPa√m for the as-deposited condition and from 19.8 MPa√m to 21.0 MPa√m
for the extruded condition. Fracture properties are controlled by intrinsic factors in the alloy produced by AMD. Fatigue
crack growth rates of the AMD alloy are comparable to those of the PFC alloy in the L-T orientation. The crack propagation
modes were studied by optical metallographic examination of crack-microstructure interactions and scanning electron microscopy
of the fracture surfaces. 相似文献
20.
《Acta Metallurgica Materialia》1994,42(3):845-855
Although aluminum-lithium alloys showed initial promise for aerospace applications, implementation has not proceeded swiftly. In this study, efforts were made to design and develop microstructures with good fracture and fatigue crack propagation resistance to achieve a better balance of mechanical properties in the high strength alloy X2095. Lower aging temperatures were employed, resulting in precipitation of shearable δ' (Al3Li) particles and reduced subgrain boundary T1 precipitation. Although fracture toughness was not significantly altered in the 1.6 Li variant, improvements approaching 50% were achieved in the 1.3 Li alloy. Intrinsic fatigue crack propagation resistance was also slightly improved due to reduced environmental interactions. These improvements were made without altering the 660 MPa yield strength. 相似文献