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1.
2.
Slip behavior, fracture toughness, and fatigue thresholds of a high purity Al-Li-Cu alloy with Zr as a dispersoid forming
element have been studied as a function of aging time. The fracture toughness variation with aging time has been related to
the changes in slip planarity,i.e., slip band spacing and width. Although the current alloy exhibits planar slip for all aging conditions examined, the crack
initiation toughness,Klc, compares favorably with those of 2XXX and 7XXX aluminum alloys. Near threshold fatigue crack growth results in air and vacuum
suggest that irregularities in the crack profile and the fracture surfaces and slip reversibility are some of the major contributing
factors to the crack growth resistance of this alloy. 相似文献
3.
The fatigue mechanisms in a TiAl sheet alloy, heat treated to the lamellar and equiaxed microstructures, were studied to determine
the effects of microstructure on the initiation of microcracks and their subsequent growth into large cracks. The nucleation
and growth history of individual microcracks were followed. For comparison, fatigue crack growth and fracture toughness were
also characterized using specimens containing a machined notch with a fatigue precrack. The results indicated that microcracks
initiated at grain/colony boundaries and at slip bands. Most microcracks were arrested after nucleation, but a few grew at
stress intensity ranges below the large crack threshold. The populations of nonpropagating and propagating cracks varied with
life fractions. Ligaments in the wake of a fatigue crack were more severely strained than the crack-tip region of the main
crack, and, as a result, they were more prone to fatigue failure. The destruction of the crack-wake ligaments is expected
to result in lower fracture resistance in materials under cyclic loading than those under monotonic loading.
This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the
TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations
Committees. 相似文献
4.
Fracture toughness samples of NIMONIC 115 were creep tested at 704°C in Mode I (tension) and Mode III (torsion) loading. In
Mode III loading the rupture lives were two orders of magnitude shorter than in Mode I. The effects of loading mode are shown
to agree with predictions based on a critical strain fracture model. Earlier test results with a number of different superalloys
also are consistent with a strain controlled fracture model. Improved resistance to crack growth during creep at intermediate
temperatures can be achieved by increasing Young’s modulus, yield strength, grain size and the critical strain value. 相似文献
5.
Fracture toughness samples of NIMONIC 115 were creep tested at 704°C in Mode I (tension) and Mode III (torsion) loading. In Mode III loading the rupture lives were two orders of magnitude shorter than in Mode I. The effects of loading mode are shown to agree with predictions based on a critical strain fracture model. Earlier test results with a number of different superalloys also are consistent with a strain controlled fracture model. Improved resistance to crack growth during creep at intermediate temperatures can be achieved by increasing Young’s modulus, yield strength, grain size and the critical strain value. 相似文献
6.
The fatigue and fracture resistance of a Nb-Cr-Ti-Al alloy 总被引:1,自引:0,他引:1
The microstructure, fatigue, and fracture behaviors of a cast and heat-treated Nb-Cr-Ti-Al alloy were investigated. The microstructure
of the cast alloy was manipulated by annealing at a temperature ranging from 500 °C to 1500 °C for 1 to 24 hours. The heat
treatment produced Cr2Nb precipitates along grain boundaries in all cases except in the 500 °C heat-treated material. Fracture toughness tests indicated
low fracture resistance in both the as-cast and heat-treated materials. Fatigue crack growth tests performed on the 500 °C
heat-treated material also indicated a low fatigue crack growth resistance. Direct observations of the near-tip region revealed
a cleavage-dominated fracture process, in accordance with fractographic evidence. The fracture behavior of the Nb-Cr-Ti-Al
alloy was compared to that of other Nb-Cr-Ti alloys. In addition, theoretical calculations of both the unstable stacking energy
(USE) and Peierls-Nabarro (P-N) barrier energy are used to elucidate the role of Al additions in cleavage fracture of the
Nb-Cr-Ti-Al alloy. The results indicate that an Al alloying addition increases the USE, which, in turn, prevents the emission
of dislocations, promotes the nucleation and propagation of cleavage cracks from the crack tip, and leads to a reduction in
the fracture toughness. 相似文献
7.
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
8.
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
9.
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
10.
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
11.
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
12.
Two microscopic ductile fracture processes have been established in a fracture tough superalloy, Inconel 718, aged to five
strength levels. At yield strengths less than 800 MPa, the mechanism is a slow tearing process within large pockets of inhomogeneous
carbides and nitrides, giving rise to plane strain fracture toughness (K
IC)values greater than 120 MPa-m1/2. At yield strengths greater than 900 MPa, the mechanism involves fracture initiation at carbides and nitrides followed by
off crack plane void sheet growth nucleated at the Laves (σ) phases. Here, the fracture toughness drops to about 80 MPa-m1/2. A Mode I normal strain growth model for low yield strength conditions and a shear strain void sheet model for high yield
strength ones are shown to model KIC data obtained from a J-integral evaluation of compact tension results. 相似文献
13.
Varying amounts of hydrogen were dissolved in the titanium aluminide alloy Ti-24Al-llNb (atomic percent). Virtually all of
this hydrogen probably precipitated as hydride on cooling because the terminal solubility in the dominant Ti3Al phase is very low at room temperature. Although the yield strength (YS) increased, the ultimate tensile strength (UTS),
ductility, fracture stress in notched bend bars, and fracture toughness decreased with increasing amounts of hydride. The
strength and fracture properties, for all hydride contents, did not change with testing speed below about 5 to 50 mm/min but
decreased steeply for speeds greater than that. The presence of hydride decreased the critical value of testing speed by about
an order of magnitude. Brittle cracks in bluntly notched bend bars, with or without hydride, nucleated at the notch root or
at a distance below the root which was less than one fifth of the distance to the peak stress location. This result suggests
that the cleavagelike cracking in this material is not controlled by normal stress alone but has some dependence on the applied
strain. The fracture surfaces of notched or precracked specimens, with or without hydride, consisted entirely of cleavagelike
fracture, but these cracks exhibited stable crack propagation. This permitted both the measurement of crack resistance or
R curves and also observation of the initiation and propagation of the crack with increasing KI. The results showed that cracks initiated discontinuously at characteristic sites within the plastic zone and along the slip
bands when the plastic deformation ahead of the precrack developed to a particular and reproducible extent. Literature cleavage
models were compared to results for the present tests.
WU-YANG CHU, Formerly Visiting Professor, Carnegie Mellon University, 相似文献
14.
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
15.
《Acta Metallurgica》1987,35(9):2227-2242
The role of dispersions of pre-existing grain boundary microvoids is investigated in fracture toughness and fatigue crack propagation behavior in a low alloy steel. Microvoid damage is achieved by prior exposure of the steel to gaseous hydrogen atmospheres at high temperatures and pressures, where carbon within the steel reacts with ingressed hydrogen to nucleate methane bubbles along prior austenite grain boundaries (hydrogen attack). It is shown that, whereas the crack initiation and crack growth toughness (i.e. KIc and the tearing modulus) are severely degraded, even for comparatively mild degrees of microvoid damage, rates of sub-critical crack growth by fatigue remain relatively unaffected. Such results are interpreted in terms of a mutual competition between microstructural damage generated by the grain boundary microvoids, which promotes crack growth by lowering the intrinsic resistance of the microstructure, and the resulting tortuous crack paths, which extrinsically retard crack growth at low stress intensities by lowering the local crack tip “driving force” (crack tip shielding). As shielding effects are minimized at high stress intensities, the degradation in intrinsic toughness is related to changes in ductility by means of a stress-modified critical strain model for ductile fracture, where the presence of small microvoid clusters is shown to promote coalescence through the easier onset of plastic strain localization. Fatigue behavior, conversely, is dominated by extrinsic shielding mechanisms and is modeled in terms of two-dimensional models of crack deflection and roughness-induced crack closure. 相似文献
16.
The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class
of forged damage-tolerant niobium aluminide (Nb3Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti
(10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages
unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa√m at room temperature.
Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed
at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature
strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa√m), and fatigue crack growth resistance
(comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural
applications of the 40Ti alloy in the intermediate-temperature (∼700 °C to 750 °C) regime. 相似文献
17.
The flexural peel technique was used to study the fracture resistance of two model A12O3/A1 interfaces. The bimaterial interface was formed by bonding high-purity A12O3 with molten Al-5 pct Cu alloy under pressure. The specimens were then heat treated so that the Al-Cu alloy reached peakaged
and extended-overaged conditions. The fracture resistance curve was established for two interfaces with either the peak-aged
or overaged Al alloy. The fracture resistance of the interface with the peak-aged Al-Cu alloy was higher in terms of both
the initiation and peak toughness. While the peak toughness of the interface scaled with the yield strength of the metal,
the initiation toughness differed by a factor of 8. The difference in the initiation toughness is discussed in terms of the
disparity in the interfacial microstructure. 相似文献
18.
The fracture resistance of a binary TiAl alloy 总被引:6,自引:0,他引:6
Kwai S. Chan Jessica Onstott K. Sharvan Kumar 《Metallurgical and Materials Transactions A》2000,31(1):71-80
The fracture resistance of a binary Ti-47Al (in at. pct) alloy has been investigated. The binary alloy was cast, forged, and
heat treated to a fully lamellar microstructure with a colony size of either 640 or 1425 μm. Fracture toughness tests were performed in a scanning electron microscope (SEM) equipped with a loading stage. Direct observations
of the fracture process indicated that crack extension commenced at a stress intensity level of 1.2 to 4 MPa√m. The crack
path was primarily interlamellar and crack extension across an individual colony or across similarly oriented colonies was
relatively easy. In contrast, crack arrest was prevalent when the crack encountered the boundaries of unfavorably oriented
colonies. To extend into an unfavorably oriented neighboring colony, the K level of the approaching crack had to be increased significantly to renucleate a microcrack at a location away from the crack
tip, resulting in the formation of an interconnecting ligament that must be fractured to further crack growth. This interaction
between the crack and the microstructure led to a large variation in the slope of the K
R
curves. Comparison of the K
R
curves for the binary Ti-47Al alloy against published data for quinary Ti-47Al-xNb-yCr-zV alloys indicates that the initiation toughness of the quinary alloys is higher by a factor of 5 to 10, implying the existence
of a significant beneficial effect of alloying additions on the initiation toughness. 相似文献
19.
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