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1.
G. M. Vyletel J. E. Allison D. C. Aken 《Metallurgical and Materials Transactions A》1993,24(11):2545-2557
The low-cycle and high-cycle fatigue behavior and cyclic response of naturally aged and artificially aged 2219/TiC/15p and unreinforced 2219 Al were investigated utilizing plastic strain-controlled and stress-controlled testing. The cyclic
response of both the reinforced and un-reinforced materials was similar for all plastic strain amplitudes tested except that
the saturation stress level for the composite was always greater than that of the unreinforced material. The cyclic response
of the naturally aged materials exhibited cyclic hardening and, in some cases, cyclic softening, while the cyclic response
for the artificially aged materials showed no evidence of either cyclic hardening or softening. The higher ductility of the
unreinforced material made it more resistant to fatigue failure at high strains, and thus, at a given plastic strain, it had
longer fatigue life. It should be noted that the tensile ductilities of the 2219/TiC/15p were significantly higher than those previously reported for 2XXX-series composites. During stress-controlled test-ing at
stresses below 220 MPa, the presence of TiC particles lead to an improvement in fatigue life. Above 220 MPa, no influence
of TiC reinforcement on fatigue life could be detected. In both the composite and unreinforced materials, the low-cycle and
high-cycle fatigue lives were found to be virtually independent of matrix microstructure.
G.M. VYLETEL, formerly Graduate Student, Department of Materials Science and Engineering, The University of Michigan
D.C. VAN AKEN, formerly Assistant Professor, Department of Materials Science and Engineering, The University of Michigan 相似文献
2.
G. M. Vyletel D. C. Van Aken J. E. Allison 《Metallurgical and Materials Transactions A》1995,26(12):3155-3162
The 150 °C cyclic response of peak-aged and overaged 2219/TiC/15p and 2219 Al was examined using fully reversed plastic strain-controlled
testing. The cyclic response of peak-aged and overaged particle-reinforced materials showed extensive cyclic softening. This
softening began at the commencement of cycling and continued until failure. At a plastic strain below 5 × 103, the unreinforced materials did not show evidence of cyclic softening until approximately 30 pct of the life was consumed.
In addition, the degree of cyclic softening (†σ) was significantly lower in the unreinforced microstructures. The cyclic softening
in both reinforced and unreinforced materials was attributed to the decomposition of the θ′ strengthening precipitates. The
extent of the precipitate decomposition was much greater in the composite materials due to the increased levels of local plastic
strain in the matrix caused by constrained deformation near the TiC particles.
formerly Research Assistant with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI
48109
This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the
1994 TMS/ASM Spring meeting, held February 28–March 3, 1994 in San Francisco, California, under the auspices of the Joint
TMS-SMD/ASM-MSD Composite Materials Committee. 相似文献
3.
The deformation and failure mechanisms under cyclic deformation in an 8090 Al-Li alloy reinforced with 15 vol pct SiC particles
were studied and compared to those of the unreinforced alloy. The materials were tested under fully reversed cyclic deformation
in the peak-aged and naturally aged conditions to obtain the cyclic response and the cyclic stress-strain curve. The peak-aged
materials remained stable or showed slight cyclic softening, and the deformation mechanisms were not modified by the presence
of the ceramic reinforcements: dislocations were trapped by the S′ precipitates and the stable response was produced by the mobile dislocations shuttling between the precipitates to accommodate
the plastic strain without further hardening. The naturally aged materials exhibited cyclic hardening until failure, which
was attributed to the interactions among dislocations. Strain localization and slip-band formation were observed in the naturally
aged alloy at high cyclic strain amplitudes, whereas the corresponding composite presented homogeneous deformation. Fracture
was initiated by grain-boundary delamination in the unreinforced materials, while progressive reinforcement fracture under
cyclic deformation was the main damage mechanism in the composites. The influence of these deformation and damage processes
in low-cycle fatigue life is discussed. 相似文献
4.
The deformation and failure mechanisms under cyclic deformation in an 8090 Al−Li alloy reinforced with 15 vol pct SiC particles
were studied and compared to those of the unreinforced alloy. The materials were tested under fully reversed cyclic deformation
in the peak-aged and naturally aged conditions to obtain the cyclic response and the cyclic stress-strain curve. The peak-aged
materials remained stable or showed slight cyclic softening, and the deformation mechanisms were not modified by the presence
of the ceramic reinforcements: dislocations were trapped by the S′ precipitates and the stable response was produced by the mobile dislocations shuttling between the precipitates to accommodate
the plastic strain without further hardening. The naturally aged materials exhibited cyclic hardening until failure, which
was attributed to the interactions among dislocations. Strain localization and slip-band formation were observed in the naturally
aged alloy at high cyclic strain amplitudes, whereas the corresponding composite presented homogeneous deformation. Fracture
was initiated by grain-boundary delamination in the unreinforced materials, while progressive reinforcement fracture under
cyclic deformation was the main damage mechanism in the composites. The influence of these deformation and damage processes
in low-cycle fatigue life is discussed. 相似文献
5.
The mechanical behavior under fully reversed cyclic deformation was determined through the incremental step method for two
Al alloys reinforced with 15 vol pct A12O3 particulates in the naturally aged and peak-aged conditions. The composites exhibited cyclic strain hardening in all cases,
but the hardening was more pronounced in the naturally aged condition. This behavior was reflected by the stress-strain curves
in monotonie tension and in fatigue, and the cyclic strain-hardening coefficient was about twice the monotonie one for both
materials and tempers. The tensile and cyclic strengths of the materials were very similar, and the dominant failure mechanism
under both loading conditions was paniculate fracture, which was very localized around the fracture region in fatigue, but
was spread along the specimen length in monotonie tension. In addition, a few A12O3 particulates were broken in compression during cyclic deformation. The final fracture micromechanism was the growth and coalescence
of voids in the matrix from broken ceramic particulates. This last stage in the fracture process was fast and started when
a critical volume fraction of broken reinforcements (between 30 and 45 pct) was reached in a given section of the specimen.
This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the
1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint
TMS-SMD/ASM-MSD Composite Materials Committee. 相似文献
6.
P. E. Krajewski J. W. Jones J. E. Allison 《Metallurgical and Materials Transactions A》1995,26(12):3107-3118
The effect of TiC particle reinforcement on the creep behavior of Al (99.8) and Al-1.5Mg is investigated in the temperature
range of 150 °C to 250 °C. The dislocation structure developed during creep is characterized in these materials. The addition
of TiC increases creep resistance in both alloys. In pure aluminum, the presence of 15 vol pct TiC leads to a factor of 400
to 40,000 increase in creep resistance. The creep strengthening observed in Al/TiC/15p is substantially greater than the direct strengthening predicted by continuum models. Traditional methods for explaining
creep strengthening in particle-reinforced materials(e.g., threshold stress, constant structure, and dislocation density) are unable to account for the increase in creep resistance.
The creep hardening rate(h) is found to be 100 times higher in Al/TiC/15p, than in unreinforced Al. When incorporated into a recovery creep model, this increase inh can explain the reduction in creep rate in Al/TiC/15p. Particle reinforcement affects creep hardening, and thus creep rate, by altering the equilibrium dislocation substructure
that forms during steady-state creep. The nonequilibrium structure generates internal stresses which lower the rate of dislocation
glide. The strengthening observed by adding TiC to Al-1.5Mg is much smaller than that found in the pure aluminum materials
and is consistent with the amount of strengthening predicted by continuum models. These results show that while both direct
(continuum) and indirect strengthening occur in particle-reinforced aluminum alloys, the ratio of indirect to direct strengthening
is strongly influenced by the operative matrix strengthening mechanisms.
This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the
1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint
TMS-SMD/ASM-MSD Composite Materials Committee. 相似文献
7.
The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites
In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with
fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy
was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the
plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 104 cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted.
The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized
in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation
characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and
test temperature.
This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Facture” held October
11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials
Committee. 相似文献
8.
The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites
In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with
fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy
was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the
plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 104 cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted.
The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized
in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation
characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and
test temperature.
This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October
11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials
Committee. 相似文献
9.
P. E. Krajewski J. E. Allison J. W. Jones 《Metallurgical and Materials Transactions A》1993,24(12):2731-2741
The influence of matrix microstructure and reinforcement with 15 vol pct of TiC particles on the creep behavior of 2219 aluminum
has been examined in the temperature range of 150 ‡C to 250 ‡C. At 150 ‡C, reinforcement led to an improvement in creep resistance,
while at 250 ‡C, both materials exhibited essentially identical creep behavior. Precipitate spacing in the matrix exerted
the predominant influence on minimum creep rate in both the unreinforced and the reinforced materials over the temperature
range studied. This behavior and the high-stress dependence of minimum creep rate are explained using existing constant structure
models where, in the present study, precipitate spacing is identified as the pertinent substructure dimension. A modest microstructure-independent
strengthening from particle reinforcement was observed at 150 ‡C and was accurately modeled by existing continuum mechanical
models. The absence of reinforcement creep strengthening at 250 ‡C can be attributed to diffusional relaxation processes at
the higher temperature. 相似文献
10.
L. F. Van Swam R. M. Pelloux N. J. Grant 《Metallurgical and Materials Transactions A》1975,6(1):45-54
The cyclic stress-strain curves, the low cycle and high cycle fatigue lives and the fatigue crack growth rates of annealed
(1 h 820°C) and aged (3 h 480°C) maraging steel 300 were determined. Incremental step testing and stable hysteresis loop tip
measurements were used to determine the cyclic σ-ε curves. Both annealed and aged maraging steels were found to cyclically soften at room temperature over a plastic strain
range from 0.1 to 20 pct. The S-N curves were determined from 10 to 107 cycles to failure by plastic strain controlled low cycle fatigue tests performed in air and load controlled high cycle fatigue
tests performed in dry argon. The test results compared very well with the theoretical lifetime predictions derived from Tomkins’
theory. Fatigue crack growth rates were measured in air and dry argon for the annealed and aged alloys. Crack growth rates
of annealed maraging steel were found to be equal to those of aged maraging steel at rates between 10-7 and 10-5 in./cycle. A significant difference in crack growth rates in the two environments was found at low stress intensity factor
ranges, indicating a high susceptibility to corrosion fatigue in the presence of water vapor. The mechanisms of cyclic softening
in the two alloys are discussed in terms of dislocations rearrangement in the annealed alloy and dislocation-precipitate interactions
in the aged alloy. 相似文献
11.
The cyclic stress-strain response and microstructures (TEM) were studied in specimens of Ni-14.4 at. pct Al alloy heat treated
to obtain a range of coherent precipitate sizes from underaged (very small precipitates, 6 nm) to overaged (large precipitates,
46 nm). Overaged specimens cyclically hardened initially to a plateau or stable stress amplitude until crack initiation while
underaged samples, after initial cyclic hardening, softened prior to crack initiation. There was very slight cyclic softening
in peak-aged samples prior to failure. An inverse exponential equation modeled the cyclic hardening behavior of all specimens
tested. An empirical constant, R, which measured the rate of approach to the cyclic hardening maximum, changed significantly
with precipitate size, being maximum for the peak-aged condition. The total cyclic hardening was also maximum for peak aging.
The microstructural examination showed that the plastic deformation during early cyclic straining was most uniform in the
peak-aged samples compared to under- or overaged samples. This correlates with the faster approach to maximum hardening. Furthermore,
initial paired dislocation motion was replaced by unpaired tangles as cycling progressed.
D. L. ANTON, formerly Graduate Assistant, Materials Science and Engineering Department, Northwestern University, Evanston,
IL 相似文献
12.
The microstructure and tensile properties of an 8090 Al-Li alloy reinforced with 15 vol pct SiC particles were investigated,
together with those of the unreinforced alloy processed following the same route. Two different heat treatments (naturally
aged at ambient temperature and artificially aged at elevated temperature to the peak strength) were chosen because they lead
to very different behaviors. Special emphasis was given to the analysis of the differences and similarities in the microstructure
and in the deformation and failure mechanisms between the composite and the unreinforced alloy. It was found that the dispersion
of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al3Li at ambient temperature. The deformation processes in the peak-aged materials were controlled by the S′ precipitates, which acted as barriers for dislocation motion and homogenized the slip. Homogeneous slip was also observed
in the naturally aged composite, but not in the unreinforced alloy, where plastic deformation was concentrated in slip bands.
The most notorious differences between the alloy and the composite were found in the fracture mechanisms. The naturally aged
unreinforced alloy failed by transgranular shear, while the failure of the peak-aged alloy was induced by grain-boundary fracture.
The fracture of the composite in both tempers was, however, precipitated by the progressive fracture of the SiC reinforcements
during deformation, which led to the early failure at the onset of plastic instability. 相似文献
13.
The low-cycle fatigue behavior of polycrystalline NiAl was determined at 1000 K, a temperature above the monotonic brittle-to-ductile
transition temperature (BDTT). Fully reversed, plastic strain-controlled fatigue tests were conducted on B2 intermetallic
samples prepared by two fab-rication techniques: hot isostatic pressing (HIP) of prealloyed powders and extrusion of vacuum
induction-melted [cast plus extruded (C+E)] castings. At 1000 K, in an air environment both the hot-isostatically pressed
(“hipped”) and C + E samples cyclically softened throughout most of their fatigue lives, though the absolute change in stress
was no greater than about 35 MPa. At this temperature, samples were insensitive to processing defects, which were a source
of failure initiation in room-temperature tests. The processing method had a small effect on fatigue life; the lives of the
hipped samples were about a factor of 3 shorter than the fatigue lives of the C+E NiAl. The C+E material also underwent dynamic
grain growth during testing, while the hipped NiAl maintained a constant grain size. Stable fatigue-crack growth in both materials
was intergranular in nature, while final fracture by tensile overload occurred by transgranular cleavage. However, at plastic
strain ranges below 0.3 pct, the fatigue lives of the hipped NiAl were controlled by intergranular cavitation and creep processes
such that the fatigue lives were shorter than anticipated. Finally, hipped samples tested in vacuum had a factor of 3 longer
life than specimens tested in air. A comparison of NiAl to typical superalloys (which it may replace) showed that NiAl exhibited
a superior fatigue life on a plastic strain basis but was inferior to most superalloys on a stress basis. 相似文献
14.
N. Chawla U. Habel Y. -L. Shen C. Andres J. W. Jones J. E. Allison 《Metallurgical and Materials Transactions A》2000,31(2):531-540
The effect of matrix microstructure on the stress-controlled fatigue behavior of a 2080 Al alloy reinforced with 30 pct SiC
particles was investigated. A thermomechanical heat treatment (T8) produced a fine and homogeneous distribution of S′ precipitates,
while a thermal heat treatment (T6) resulted in coarser and inhomogeneously distributed S′ precipitates. The cyclic and monotonic
strength, as well as the cyclic stress-strain response, were found to be significantly affected by the microstructure of the
matrix. Because of the finer and more-closely spaced precipitates, the composite given the T8 treatment exhibited higher yield
strengths than the T6 materials. Despite its lower yield strength, the T6 matrix composite exhibited higher fatigue resistance
than the T8 matrix composite. The cyclic deformation behavior of the composites is compared to monotonic deformation behavior
and is explained in terms of microstructural instabilities that cause cyclic hardening or softening. The effect of precipitate
spacing and size has a significant effect on fatigue behavior and is discussed. The interactive role of matrix strength and
SiC reinforcement on stress within “rogue” inclusions was quantified using a finite-element analysis (FEA) unit-cell model. 相似文献
15.
P. E. Krajewski J. E. Allison J. W. Jones 《Metallurgical and Materials Transactions A》1993,24(1):2731-2741
The influence of matrix microstructure and reinforcement with 15 vol pct of TiC particles on the creep behavior of 2219 aluminum
has been examined in the temperature range of 150 °C to 250 °C. At 150 °C, reinforcement led to an improvement in creep resistance,
while at 250 °C, both materials exhibited essentially identical creep behavior. Precipitate spacing in the matrix exerted
the predominant influence on minimum creep rate in both the unreinforced and the reinforced materials over the temperature
range studied. This behavior and the high-stress dependence of minimum creep rate are explained using existing constant structure
models where, in the present study, precipitate spacing is identified as the pertinent substructure dimension. A modest microstructure-independent
strengthening from particle reinforcement was observed at 150 °C and was accurately modeled by existing continuum mechanical
models. The absence of reinforcement creep strengthening at 250 °C can be attributed to diffusional relaxation processes at
the higher temperature. 相似文献
16.
The fatigue crack propagation rate,dc/dN, in cold-rolled and annealed 99.99+ Al is about 80 times slower at 77 K than at 298 K. In annealed 1100 Al which contains
constituent particles,dc/dN decreases by a factor of 20 on cooling from 298 to 77 K. At 298 and 77 K, annealed 99.99+ Al and 1100 Al cyclically harden but the amount is greater at 77 K. Cold-rolled 99.99+ Al cyclically hardens at 77 K but cyclically softens at 298 K. The much slower fatigue crack propagation rate at 77 K in
aluminum is attributed in part to the increase in cyclic yield stress, σy′, on cooling. At 77 K the high rate of work hardening at large strains is also thought to result in high plastic work per
unit area of fatigue crack thereby reducing the fatigue crack propagation rate. Rice’s theory for a Mode I plane stress crack
predicts the measured plastic zone size if the local stress corresponding to zero plastic strain in the cyclic stress-strain
curve is employed in the formula. 相似文献
17.
The microstructure and tensile properties of an 8090 Al−Li alloy reinforced with 15 vol pet SiC particles were investigated,
together with those of the unreinforced alloy processed following the same route. Two different heat treatments (naturally
aged at ambient temperature and artificially aged at elevated temperature to the peak strength) were chosen because they lead
to very different behaviors. Special emphasis was given to the analysis of the differences and similarities in the microstructure
and in the deformation and failure mechanisms between the composite and the unreinforced alloy. It was found that the dispersion
of the SiC particles restrained the formation of elongated grains during extrusion and inhibited the precipitation of Al3Li at ambient temperature. The deformation processes in the peak-aged materials were controlled by the S′ precipitates, which acted as barriers for dislocation motion and homogenized the slip. Homogeneous slip was also observed
in the naturally aged composite, but not in the unreinforced alloy, where plastic deformation was concentrated in slip bands.
The most notorious differences between the alloy and the composite were found in the fracture mechanisms. The naturally aged
unreinforced alloy failed by transgranular shear, while the failure of the peak-aged alloy was induced by grain-boundary fracture.
The fracture of the composite in both tempers was, however, precipitated by the progressive fracture of the SiC reinforcements
during deformation, which led to the early failure at the onset of plastic instability. 相似文献
18.
The creep of metal-matrix composites is analyzed by finite element techniques. An axisymmetric unit-cell model with spherical
reinforcing particles is used. Parameters appropriate to TiC particles in a precipitation-hardened (2219) Al matrix are chosen.
The effects of matrix plasticity and residual stresses on the creep of the composite are calculated. We confirm (1) that the
steady-state rate is independent of the particle elastic moduli and the matrix elastic and plastic properties, (2) that the
ratio of composite to matrix steady-state rates depends only on the volume fraction and geometry of the reinforcing phase,
and (3) that this ratio can be determined from a calculation of the stress-strain relation for the geometrically identical
composite (same phase volume and geometry) with rigid particles in the appropriate power-law hardening matrix. The values
of steady-state creep are compared to experimental ones (Krajewskiet al.). Continuum mechanics predictions give a larger reduction of the composite creep relative to the unreinforced material than
measured, suggesting that the effective creep rate of the matrix is larger than in unreinforced precipitation-hardened Al
due to changes in microstructure, dislocation density, or creep mechanism. Changes in matrix creep properties are also suggested
by the comparison of calculated and measured creep strain rates in the primary creep regime, where significantly different
time dependencies are found. It is found that creep calculations performed for a timeindependent matrix creep law can be transformed
to obtain the creep for a time-dependent creep law.
This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the
1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the joint
TMS-SMD/ASM-MSD Composite Materials Committee. 相似文献
19.
K. T. Venkateswara Rao S. C. Siu R. O. Ritchie 《Metallurgical and Materials Transactions A》1993,24(3):721-734
Micromechanisms influencing crack propagation in a unidirectional SiC-fiber (SCS-8) continuously reinforced Al-Mg-Si 6061
alloy metal-matrix composite (SiCf/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 SiCf/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
20.
Deformation and fracture behavior of two Al-Mg-Si alloys in different aging conditions has been studied by tensile testing,
transmission electron microscope (TEM), and scanning electron microscope (SEM) observation. Tensile test results show that
the strain hardening exponents (n values) of the two alloys decrease sharply at the early stage of artificial aging and are only 0.045 and 0.06, respectively,
in the overaged condition. The sharp decrease of work hardening rate is believed to be one major reason that results in the
rapid decrease of elongation to failure at the early stage of artificial aging. In fully aged conditions, dislocations are
concentrated in narrow bands during plastic deformation of these alloys, which is responsible for the very low n values of the Al-Mg-Si alloys in peak aged and overaged conditions. The Si particles formed in the interior of grains of
the higher Si containing alloy reduce the inhomogeneous deformation behavior. The TEM results show that large precipitates
and precipitate-free zones (PFZs) along grain boundaries are formed in peak aged and overaged conditions, and SEM observations
demonstrate that the tensile fracture modes of the two alloys in these aging conditions are completely intergranular with
many small cusps decorated on facets of the fractured grain boundaries. Thus, the fracture process of both alloys is suggested
to be that in which the high local stresses, built up where the slip band impinges on the grain boundaries, nucleate voids
at the grain boundary precipitates by decohesion of the particle/PFZ interface, and then coalescence of these voids within
the PFZ leads to the final fracture of these alloys. 相似文献