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1.
Micromechanisms influencing crack propagation in a unidirectional SiC-fiber (SCS-8) continuously reinforced Al-Mg-Si 6061
alloy metal-matrix composite (SiC f/Al-6061) during monotonie and cyclic loading are examined at room temperature, both for the longitudinal (0 deg or L-T) and
transverse (90 deg or T-L) orientations. It is found that the composite is insensitive to the presence of notches in the L-T
orientation under pure tension loading due to the weak fiber/matrix interface; notched failure strengths are ∼1500 MPa compared
to 124 MPa for unreinforced 6061. However, behavior is strongly dependent on loading configuration, specimen geometry, and
orientation. Specifically, properties in SiC f/Al in the T-L orientation are inferior to unreinforced 6061, although the composite does exhibit increasing crack-growth
resistance with crack extension (resistance-curve behavior) under monotonie loading; peak toughnesses of ∼16 MPa√m are achieved
due to crack bridging by the continuous metal phase between fibers and residual plastic deformation in the crack wake. In
contrast, such bridging is minimal under cyclic loading, as the ductile phase fails subcritically by fatigue such that the
transverse fatigue crack-growth resistance is superior in the unreinforced alloy, particularly at high stress-intensity levels.
Conversely, fatigue cracks are bridged by unbroken SiC fibers in the L-T orientation and exhibit marked crack deflection and
branching; the fatigue crack-growth resistance in this orientation is clearly superior in the composite. 相似文献
2.
Transverse mechanical properties have been measured, and damage mechanisms identified, in three Ti 3Al matrix composites with different interface compositions and residual stress states. Two of the composites contained SiC fibers with weak interfaces. Large improvements in transverse strength and rupture strain were found in one of these composites, in which brittle reaction products in the matrix around the fibers had been avoided by coating the fibers with Ag and Ta before consolidation. The third composite contained sapphire fibers that were strongly bonded to the matrix. Different damage mechanisms were observed in the strongly and weakly bonded composites. Insight into the damage mechanisms and their dependence on residual stress fields and interface properties is gained from comparison of the observations with analytical solutions of elastic stresses. The conditions for optimum transverse properties are discussed; the results indicate that strong interfacial bonding does not necessarily lead to optimum transverse strength of the composite. 相似文献
3.
Fracture and deformation characteristics of the Ti-based metallic glass matrix composite have been studied by the tensile test and the in situ TEM tension test. Typically, the composite exhibits the high strength and considerable plasticity. Microscopically, it was found that shear deformation zone formed at the crack tip in glass phase, which can bring about quick propagation of shear bands. However, the plastic deformation zone nearby the crack tip in dendrites will postpone or retard the crack extension by dislocations. The attributions of micro-deformations to mechanical properties of composites were discussed. 相似文献
5.
Analysis of the tensile and fracture behavior of a composite system consisting of boron carbide particulate-reinforced NiAl
with continuous 304 stainless steel toughening regions was performed. The composite was fabricated by extrusion, with the
toughening regions extending along the length of the plate in the extrusion direction. Mechanical properties were determined
as a function of orientation. Tensile testing revealed that the composite modulus varied only slightly as a function of testing
direction, the strength was approximately 25 pct greater in the longitudinal relative to the transverse orientation, and the
transverse failure strain was only 0.3 pct compared to values in excess of 10 pct for longitudinal testing. Notched Charpy
impact testing indicated that the energy absorption values varied significantly as a function of specimen location and crack
growth direction, ranging from 2 to 40 Joules. In addition, K
IC values measured on subsize compact tension samples were found to range from 17 to 27 MPa ⋅ m 1/2. It was also established that the K
max values determined from the maximum load measured during compact tension testing were similar to the K
Q values calculated from instrumented notched Charpy impact testing. Finally, the fatigue crack growth characteristics of the
composite were determined as a function of orientation. 相似文献
6.
A Zr-based amorphous matrix composite reinforced with tungsten continuous fibers in an amorphous LM2 alloy matrix containing ductile ?? dendrites was fabricated without pores or defects by the liquid pressing process, and its tensile and compressive properties were examined in relation with microstructures and deformation mechanisms. Overall, 68?vol pct of tungsten fibers were distributed in the matrix, in which 35?vol pct of ?? dendrites were present. The LM2 composite had the greatly improved tensile strength and elastic modulus over the LM2 alloy, and it showed a stable crack propagation behavior as cracks stopped propagating at the longitudinal cracks of tungsten fibers or ductile ?? dendrites. According to the compressive test results, fracture did not take place at one time after the yield point, but it proceeded as the applied loads were sustained by fibers, thereby leading to the maximum strength of 2432?MPa and plastic strain of 16.4?pct. The LM2 composite had the higher strength, elastic modulus, and ductility under both tensile and compressive loading conditions than the tungsten-fiber-reinforced composite whose matrix did not contain ?? dendrites. These distinctively excellent properties indicated a synergy effect arising from the mixing of amorphous matrix and tungsten fibers, as well as from the excellent bonding of interfaces between them. 相似文献
7.
The influence of the properties of the fibers, the matrix and the interface on the mechanical properties of fiber reinforced ceramics is analyzed by a simplified method previously developed by the authors for cohesive materials. The method parts from the assumption that crack displacements are known a priori and furnishes, in a simple and easy way, the fracture resistance curves versus crack length. The numerical results from the model are compared with experimental data from the literature. Finally, the model is used to assess the influence of fiber strength, interface slipping shear stress, fiber radius and fiber defect distribution on the fracture resistance and ductility of fiber-reinforced ceramic composites. 相似文献
8.
The transverse creep and stress rupture behavior of a number of Borsic®-aluminum composites was investigated at temperatures from 200° to 400°C. The cpmposites studied consisted of nominally 50 vol pct Borsic fiber and included matrices of 6061, 2024, 2219, and 5052 aluminum alloys. The effect of heat treatment was studied in the heat-treatable alloys. Where transverse composite behavior differed from matrix alloy behavior, the difference was found to be due primarily to a change in fracture mode at higher matrix strength levels from matrix failure to one which involves longitudinal fiber splitting. Of the four basic matrix alloys tested, the best creep resistance was obtained with the 2024 matrix. Additional improvement of transverse creep and stress rupture resistance was realized by incorporating transverse reinforcements such as SAP alloy foil, titanium alloy foil, and 0.002 in. stainless steel wire in the composites. These reinforcements made possible good transverse properties at 400°C with density increases of ≤15 pct. The two best additions were 21 pct β III titanium foil and 6 pct AFC-77 stainless steel wire. A transverse fracture mode incorporating longitudinal fiber splitting was documented and characterized, and its effect on composite behavior determined. The use of nonsplitting fibers such as 5.6 mil B and 5.7 mil Borsic in preventing this fracture mode was investigated. 相似文献
9.
The anisotropic mechanical properties of a Ti alloy composite reinforced with SiC fibers have been investigated and rationalized
using analytical models. The appropriate material model for this composite involves the following features: an interface that
debonds and slides, a flaw insensitive ductile matrix, and high-strength elastic fibers subject to residual compressive stress
caused by thermal expansion mismatch. This, model is broadly consistent with the longitudinal, transverse, and shear properties
of the composite. 相似文献
10.
There is intensive interest in metal matrix composites (MMCs) for automotive components, and the first production applications
in Japan use discontinuous fibers as the reinforcements. These fibers are randomly oriented, resulting in an MMC with isotropic
properties. However, there are conflicting reports on the tensile strengths attainable. In some cases, the strength increases
with increasing volume fraction (V
f) of fibers, while in other cases, there is little or no benefit. A simple method is proposed to calculate the strength of
this type of MMC. It is shown that the fibers oriented perpendicular to the stress direction play a key role, and the strength
depends upon the strength of the interfacial bond. Upper and lower limits of the composite strength are calculated. If the
bond strength is larger than the matrix strength, the composite strength has a maximum value which increases with V
f. If the bond strength is weaker than the matrix, the composite strength has a minimum value which is either weakly dependent
or even independent of V
f. These calculations are in good agreement with examples taken from the literature of aluminum composites reinforced with
either A1 2O 3, graphite, or SiC. The strength of the matrix alloy is shown to be a very important parameter: weak alloys are easily strengthened,
while in certain cases, strong alloys may be weakened. 相似文献
11.
The mechanical properties of NiAl-matrix composites reinforced with 125-μm diameter single-crystal A1 2O 3 (sapphire) fibers have been examined over the temperature range of 300 to 1200 K. Composites were fabricated with either
a strong or weak fiber-matrix interfacial bond strength. During fabrication, a fiber-matrix interaction occurred such that
fibers extracted from the NiAl matrix were fragmented and significantly weaker than the as-received fibers. Tensile results
of the weakly bonded composite demonstrated that the composite stiffness was greater than the monolithic at both 300 and 1200
K in spite of the weak bond. Room-temperature strengths of the composite were greater than that of the monolithic but below
rule-of-mixture predictions (even when the degraded fiber strengths were accounted for). At 1200 K, the ultimate strength
of the composite was inferior to that of the monolithic primarily because of the poor fiber properties. No tensile data was
obtained on the strongly bonded material because of the occurrence of matrix cracking during fabrication. Primarily because
of the fiber strength loss, sapphire-NiAl composite mechanical properties are inferior to conventional high-temperature materials
such as superalloys and are currently unsuitable for structural applications. 相似文献
12.
The high cycle fatigue (HCF) behavior of the oxide dispersion strengthened (ODS) MA 754 alloy has been determined as a function
of specimen orientation. The fatigue life showed anisotropic behavior with the longest and shortest lives in the longitudinal
and short transverse directions, respectively. Surface porosity, due to oxidation, was found to affect fatigue life in the
long transverse orientation more than in the longitudinal orientation. The fatigue crack growth behavior in MA 754 exhibited
a directional dependence. In general, the crack growth rates in the longitudinal direction were lower than those in the long
transverse direction. The Δ K
th
was ∼11 MN · -3/2 and 9 MN · m -3/2 for the longitudinal and the long transverse orientation, respectively. This behavior was explained on the basis of the unusual
grain structure and the texture exhibited by this alloy as well as different crack closure effects. It was found that a consideration
based on the crack growth rates results, obtained from fracture mechanics specimens, could not explain the anisotropic behavior
of the HCF properties of MA 754. However, the anisotropic HCF properties could be rationalized on the basis of the differences
in the modes of crack initiation. 相似文献
13.
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-m 1/2. 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-m 1/2. 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 K IC data obtained from a J-integral evaluation of compact tension results. 相似文献
14.
The creep properties and microstructures of γ-TiAl reinforced with continuous Al 2O 3 fibers have been investigated. Novel fiber coating concepts have been used to create “weak” fiber/matrix interfaces that allow the fibers to impart enhanced creep and fracture resistance, simultaneously. Several major facets of the creep behavior were identified. Under conditions of limited fiber fracture, creep-resistant sapphire fibers were found to limit longitudinal creep to a short transient strain, consistent with model predictions. At the same time, the interlaminar shear creep properties were found to be insensitive to fiber reinforcement, again consistent with predictions. It was also demonstrated that the “weak” interfaces were maintained after creep and resulted in significant levels of toughening. 相似文献
15.
Due to the high melting point and strong chemical reactivity of titanium alloys, titanium matrix composites (TMCs) are usually processed through solid-state routes such as the foil-fiber-foil technique. An alternative method consists in the deposition of the matrix on the fibers. However, techniques such as physical vapor deposition lead to a very low deposition rate, contrary to liquid route processing using a levitating liquid alloy sphere held in a cold crucible. In order to investigate the effects of the resulting thermal shock on carbon-coated SiC fibers, and select an appropriate fiber, fibers are subjected to a pure thermal shock using a laser bench facility. These fibers are then tensile tested to failure in order to evaluate the resulting fiber strength degradation and, thus, the maximum acceptable temperature. Mechanical characterization of the liquid route processed TMC is then investigated through longitudinal and transverse tensile and creep tests at temperatures representative of aeronautical applications. The specimens, unbroken after long-duration creep tests, are then subjected to tensile loading to failure: conditions representative of service, i.e., short-time overspeeding of a gas turbine. Finally, interpretation of the mechanical tests through micrographical and microfractographical examinations is focused on the identification of the deformation and failure mechanisms specific to the liquid route processed composite, e.g., nucleation, under either longitudinal or transverse loadings, of internal cracks in the α-phase of the titanium-based matrix, explained through a physical model involving a high shear stress and normal stress combination, leading to cleavage. 相似文献
16.
The formation of fiber-matrix interfacial reaction zone and its impact on mechanical properties of Gr/201 Al composite (41
vol pct fiber) was evaluated in the as-received condition and after heat treatment in vacuum at 450°C, 500°C, and 545°C temperatures
for one day, and at 545°C for one week. After heat treatment the microstructures of matrix and interface were studied by transmission
electron microscopy. This study revealed the presence of interfacial constituents Al 4C 3, Al 4O 4C, and TiB 2. The mean fiber-matrix reaction zone thickness showed an increase with increasing heat treatment temperature and time. The
effects of heat treatment on interfacial shear strength, monotonic and cyclic tension/compression properties were evaluated.
The results show that the interfacial shear strength not only depends on chemical reaction but also depends on the thickness
of the reaction zone. An increase in reaction zone size reduces mechanical bonding considerably (thermal induced stresses).
The growth of reaction zone was very detrimental to monotonic and cyclic tension/tension fatigue behavior. The mechanism of
failure in tension/tension fatigue was the initiation of cracks at the interface and their subsequent propagation in the matrix.
It was concluded that the reaction zone was the controlling factor in tension/tension fatigue. In contrast, the results showed
that compressional fatigue was matrix dependent and was little sensitive to the size of the fiber/matrix interfacial reaction
zone. 相似文献
17.
Spatially varied interfaces (SVIs) is a design concept for composite materials where the interface mechanical properties are
varied along the length and circumference of the fiber/matrix interface. These engineered interfaces can be used to modify
critical titanium matrix composite properties such as transverse tensile strength and fatigue crack growth resistance in ways
that produce a balanced set of properties. The SVI approach may also be used to probe interface failure mechanisms for the
purpose of understanding complex mechanical phenomena. Single lamina Ti-6Al-4V matrix composites containing strongly bonded
SiC fibers were fabricated both in the as-received condition and with a weak longitudinal stripe along the sides of the fibers.
The striped SVI composites exhibited an increase in the overall fatigue crack growth life of the specimens compared to the
unmodified specimens. This improvement was caused by an increased extent of debonding and crack bridging in SVI composites.
This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the
TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee. 相似文献
18.
The feasibility of Al 2O 3/FeAl + B and Al 2O 3/FeCrAlY composites for high-temperature applications was assessed. The major emphasis was on tensile behavior of both the
monolithics and composites from 298 to 1100 K. However, the study also included determining the chemical compatibility of
the composites, measuring the interfacial shear strengths, and investigating the effect of processing on the strength of the
single-crystal A12O3 fibers. The interfacial shear strengths were low for Al 2O 3/FeAl + B and moderate to high for Al 2O 3/FeCrAlY. The difference in interfacial bond strengths between the two systems affected the tensile behavior of the composites.
The strength of the A1 2O 3 fiber was significantly degraded after composite processing for both composite systems and resulted in poor composite tensile
properties. The ultimate tensile strength (UTS) values of the composites could generally be predicted with either rule of
mixtures (ROM) calculations or existing models when using the strength of the etched-out fiber. The Al 2O 3/FeAl + B composite system was determined to be unfeasible due to poor interfacial shear strengths and a large mismatch in
coefficient of thermal expansion (CTE). Development of the Al 2O 3/FeCrAlY system would require an effective diffusion barrier to minimize the fiber strength degradation during processing
and elevated temperature service. 相似文献
19.
Two directionally solidified carbide-reinforced alloyed nicel eutectics were evaluated; an alloy consisting of monocarbide
fibers in a single phase matrix and one containing monocarbide fibers in a two-phase γ-γ′ matrix. The mechanical properties
and microstructures of these alloys are compared to those of two directionally solified alloys having the same nominal matrix
compositions, but not containing carbide fibers. The calculated strengths of the monocarbide fibers in the γ′-containing eutectic
alloy are 1,400,000 psi (9650 mn/m 2) and 243,000 psi (1680 mn/m 2) at room temperature and 1000°C, respectively, while those in the single phase γ matrix eutectic at the same temperatures
are 590,000 psi (4060 mn/m 2) and 298,000 psi (2050 mn/m 2). At room temperature, the lower strength of fibers from the γ matrix alloy is believed to result from stress concentrations
induced by the presence of growth facets on the fibers. The lower apparent strength at 1000°C of fibers from the γ′-containing
eutectic alloy is related to nucleation of needles believed to be M 23C 6 on the monocarbide fibers during deformation. These needles appear to act as stress raisers and cause early failure of fibers. 相似文献
20.
The mechanical properties of aluminum-graphite composites were measured at room temperature in the as-received condition,
after elevated temperature exposure and after thermal cycling. The composites were fabricated by solid-state diffusion bonding
of liquid-phase Al-infiltrated Thornel 50 fibers. The results showed that the maximum longitudinal tensile strength of the
as-received material was 80,000 psi (552 MN/m 2), which corresponds well with the rule of mixture value. The composite strength was observed to vary widely, depending on
the extent of wetting of the fibers by the aluminum. The strength of the composites in the transverse direction was generally
very low, due to poor interfacial bonding. Aluminum carbide (A1 4C 3) formed at the surface of the fibers at temperatures greater than 500‡C (773 K). Development of the carbide was shown to
be diffusion-controlled and was dependent on the time and temperature used. It was shown that the tensile strength was virtually
unaffected by heat-treatment up to 500‡C (773 K); beyond that temperature a drastic degradation of tensile strength occurred.
The degradation could be correlated with the extent of carbide development at the interface. Thermal cycling of the composites
below 500‡C (773 K) resulted in an observable degradation of the composite strength. Scanning electron microscopy of fractured
surfaces indicated that the relatively weak interface governs the mode of failure in tension. 相似文献
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