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J.B. Jordon M.F. Horstemeyer N. Yang J.F. Major K.A. Gall J. Fan D.L. McDowell 《Metallurgical and Materials Transactions A》2010,41(2):356-363
We examine the dependence of fatigue properties on the different size scale microstructural inclusions of a cast A356 aluminum
alloy in order to quantify the structure-property relations. Scanning electron microscopy (SEM) analysis was performed on
fatigue specimens that included three different dendrite cell sizes (DCSs). Where past studies have focused upon DCSs or pore
size effects on fatigue life, this study includes other metrics such as nearest neighbor distance (NND) of inclusions, inclusion
distance to the free surface, and inclusion type (porosity or oxides). The present study is necessary to separate the effects
of numerous microstructural inclusions that have a confounding effect on the fatigue life. The results clearly showed that
the maximum pore size (MPS), NND of gas pores, and DCS all can influence the fatigue life. These conclusions are presumed
to be typical of other cast alloys with similar second-phase constituents and inclusions. As such, the inclusion-property
relations of this work were employed in a microstructure-based fatigue model operating on the crack incubation and MSC with
good results. 相似文献
3.
Fatigue properties of cast aluminum components are controlled by maximum defect size in the material. The larger the maximum
defect size, the lower the fatigue strength and life. In the presence of casting defects, crack initiation can be ignored
and fatigue life is mainly spent in crack propagation. Therefore, fatigue life of aluminum castings can be predicted by long
or short crack growth models. The main problem is defining a starting defect size from readily available data, such as twodimensional
(2-D) pore size measurements on metallographic sections. In this article, an extremevalue statistics (EVS) method was used
to estimate the maximum defect size in 319 castings from conventional metallographic data. The maximum defect size predicted
by EVS agrees quite well with the initiation defect sizes measured from fracture surfaces, and the predicted fatigue life
is in reasonable agreement with the experimental data. 相似文献
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Luke H. Rettberg J. Brian Jordon Mark F. Horstemeyer J. Wayne Jones 《Metallurgical and Materials Transactions A》2012,43(7):2260-2274
The influence of microstructure and artificial aging response (T6) on the low-cycle fatigue behavior of super vacuum die-cast (SVDC) AZ91 and AM60 has been investigated. Fatigue lifetimes were determined from the total strain-controlled fatigue tests for strain amplitudes of 0.2?pct, 0.4?pct, 0.6?pct, 0.8?pct, and 1.0?pct under fully reversed loading at a frequency of 5?Hz. Cyclic stress?Cstrain behavior was determined using an incremental step test (IST) and compared with the more traditional constant amplitude test. Two locations in a prototype casting were investigated to examine the role of microstructure and porosity on fatigue behavior. At all total strain amplitudes microstructure refinement had a negligible impact on fatigue life because of significant levels of porosity. AM60 showed an improvement in fatigue life at higher strain amplitudes when compared with AZ91 because of higher ductility. T6 heat treatment had no impact on fatigue life. Cyclic stress?Cstrain behavior obtained via the incremental step test varied from constant amplitude test results due to load history effects. The constant amplitude test is believed to be the more accurate test method. In general, larger initiation pores led to shorter fatigue life. The fatigue life of AZ91 was more sensitive to initiation pore size and pore location than AM60?at the lowest tested strain amplitude of 0.2?pct. Fatigue crack paths did not favor any specific phase, interdentritic structure or eutectic structure. A multistage fatigue (MSF) model showed good correlation to the experimental strain-life results. The MSF model reinforced the dominant role of inclusion (pore) size on the scatter in fatigue life. 相似文献
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Microstructure-based fatigue life prediction for cast A356-T6 aluminum-silicon alloys 总被引:1,自引:0,他引:1
J. Z. Yi Y. X. Gao P. D. Lee T. C. Lindley 《Metallurgical and Materials Transactions B》2006,37(2):301-311
Fatigue life prediction and optimization is becoming a critical issue affecting the structural applications of cast aluminum-silicon
alloys in the aerospace and automobile industries. In this study, a range of microstructure and porosity populations in A356
alloy was created by controlling the casting conditions and by applying a subsequent hot isostatic pressing (“hipping”) treatment.
The microstructure and defects introduced during the processing were then quantitatively characterized, and their effects
on the fatigue performance were examined through both experiment and modeling. The results indicated that whenever a pore
is present at or near the surface, it initiates fatigue failure. In the absence of large pores, a microcell consisting of
α-Al dendrites and associated Si particles was found to be responsible for crack initiation. Crack initiation life was quantitatively
assessed using a local plastic strain accumulation model. Moreover, the subsequent crack growth from either a pore or a microcell
was found to follow a small-crack propagation law. Based on experimental observation and finite-element analysis, a unified
model incorporating both the initiation and small crack growth stages was developed to quantitatively predict the dependency
of fatigue life on the microstructure and porosity. Good agreement was obtained between the model and experiment. 相似文献
6.
The effects of various microconstituents on crack initiation and propagation in high-cycle fatigue (HCF) were investigated
in an aluminum casting alloy (A356.2). Fatigue cracking was induced in both axial and bending loading conditions at strain/stress
ratios of −1, 0.1, and 0.2. The secondary dendrite arm spacing (SDAS) and porosity (maximum size and density distribution)
were quantified in the directionally solidified casting alloy. Using scanning electron microscopy, we observed that cracks
initiate at near-surface porosity, at oxides, and within the eutectic microconstituents, depending on the SDAS. When the SDAS
is greater than ∼ 25 to 28 μm, the fatigue cracks initiate from surface and subsurface porosity. When the SDAS is less than
∼ 25 to 28 μm, the fatigue cracks initiate from the interdendritic eutectic constituents, where the silicon particles are
segregated. Fatigue cracks initiated at oxide inclusions whenever they were near the surface, regardless of the SDAS. The
fatigue life of a specimen whose crack initiated at a large eutectic constituent was about equal to that when the crack initiated
at a pore or oxide of comparable size. 相似文献
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连铸车轴钢能否达到模铸车轴钢的性能水平是其能否应用的一个关键。对此,采用旋转弯曲疲劳试验及疲劳裂纹扩展速率试验对比研究了连铸与模铸工艺生产的高铁车轴钢的高周疲劳破坏行为。结果表明,工业试制的连铸车轴钢的强度和疲劳极限均低于模铸车轴钢,且前者的疲劳裂纹扩展速率略高于后者。疲劳断口分析表明,疲劳断裂大部分起源于试样表面基体。微观组织分析表明,尽管两者的微观组织均为高温回火马氏体,但连铸车轴钢中原奥氏体晶粒尺寸及碳化物均略大于模铸车轴钢。金相评级法及夹杂物极值统计法的结果均表明,连铸车轴钢中的夹杂物尺寸明显大于模铸车轴钢。因此,为了以连铸工艺取代模铸工艺,还需要进一步优化连铸车轴钢的成分、冶金生产和热处理等工艺,以获得优良的冶金质量和组织性能。 相似文献
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AbstractPorosity in sintered powder metals may contribute to fatigue strength degradation in two ways. First, pores will act as local stress concentrators and, second, they may act act as fatigue crack precursors. Accordingly, the effect of porosity on fatigue crack initiation was chosen as the thrust of the present study. Conventional powder metallurgical techniques were employed to generate various levels of porosity in a heat treatable steel of the AISI 4600 type. Porous steel specimens, in a modified compact tension configuration, were cyclically loaded and cycles to initiation noted. Initiation was defined as the generation of a fatigue crack 0·10 mm in length at the notch root. As expected, the greater the porosity content, the earlier the crack developed. There are two interdependent variables in porosity character for a given porosity content: these are the average interpore spacing and the average pore diameter. The region of concentrated stress around each pore is proportional to the cube of the diameter of the pore, whereas the total volume of material to be damaged between pores is proportional to the cube of the interpore spacing. The present study found that cycles to initiation clearly depended on the volume of highly stressed material adjacent to pores, relative to the volume of void free material between pores. The correlation suggests that porosity effects on fatigue crack initiation are primarily stress concentration effects: pores as crack precursors seem less important. PM/0323 相似文献
9.
《Journal of Light Metals》2001,1(1):73-84
The influence of casting defects on the room temperature fatigue performance of a Sr-modified A356-T6 casting alloy has been studied using un-notched polished cylindrical specimens. The numbers of cycles to failure of materials with various secondary arm spacings (SDAS) were investigated as a function of stress amplitude, stress ratio, and casting defect size. To produce pore-free samples, HIP-ed and Densal™ treatments were applied prior to the T6 heat treatment. It was observed that casting defects have a detrimental effect on fatigue life by shortening not only the crack propagation period, but also the initiation period. Castings with defects show at least an order of magnitude lower fatigue life compared to defect-free ones. The decrease in fatigue life is directly correlated to the increase of defect size. HIP-ed alloys show much longer fatigue lives compared to non-HIP-ed ones. There seems to exist a critical defect size for fatigue crack initiation, below which fatigue crack initiates from other competing initiators such as eutectic particles and slip bands. A fracture mechanics approach has been used to determine the number of cycles necessary to propagate a fatigue crack from a casting defect to final failure. Fatigue life of castings containing defects can be quantitatively predicted using the size of the defects. Moreover, the fatigue fracture behavior of aluminum castings is well described by Weibull statistics. Crack originating from different defects (such as porosity and oxide films) can be readily identified from the Weibull modulus and the characteristic fatigue life. Compared with oxide films, porosity is more detrimental to fatigue life. 相似文献
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Jimin Zhang Marek A. Przystupa Annetta J. Luévano 《Metallurgical and Materials Transactions A》1998,29(3):727-737
Although qualitative relationships between fatigue lives and the sizes of the microstructural features, such as pores and
particles, are well known, the quantitative models are lacking because of the unavailability of the required detailed microstructural
data. The purpose of this work was to obtain such data for the high porosity (HP) and reduced porosity (RP) variants of the
aluminum 7050-T7451 thick-plate alloys. Both alloys had similar tensile and fracture properties, but the reduced porosity
variant showed superior fatigue performance attributed to the smaller sizes of the fatigue crack initiating particles and
pores. Those size differences, as well as the differences in the through-thickness size gradients, have been characterized
in this work. The sizes, shapes, and orientations of particles and pores were analyzed first on the plane sections and then
converted to the true three-dimensional (3-D) characteristics using the moment method. In the conversions, the particle and
pore shapes have been assumed as triaxial ellipsoids and their size distributions as lognormal. The spatial distributions
were quantified using the nearest neighbor spacing method. Results confirmed that the reduced porosity alloy had smaller particles
and pores than the high porosity variant. The size distributions in the former were also more confined. In both alloys, the
largest particles and pores were at the plate centers and the smallest at the surface. Their spatial distributions could be
categorized as random with clusters. 相似文献
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W. Hoffelner 《Metallurgical and Materials Transactions A》1982,13(7):1245-1255
The high cycle fatigue (HCF) properties of two cast nickel base-superalloys, IN 738 LC and IN 939, were investigated using
both fracture mechanics samples and smooth specimens. The crack propagation behavior was studied in terms of linear fracture
mechanics at RT and at 850 °C. In addition to the influence of temperature, the influences of frequency, mean stress, and
environment (vacuum, air, sulfidizing atmosphere) were studied. At 850 °C, the fatigue thresholds were found to be higher
in air than in vacuum. This could be explained by crack branching. The high scatter of fatigue crack propagation rates could
be related also to this phenomenon. The S/N curves at 850 °C can be predicted treating crack growth from casting pores as
the predominant failure mechanism. At RT the same method is not as successful. The reason for this may be that crack growth
laws measured on long, branched cracks are not applicable to short, unbranched cracks. At RT, no significant influence of
frequency on S/N-curves and fatigue crack growth rates was observed for frequencies up to 20 kHz. 相似文献
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Yakub Tijani André Heinrietz Wolfram Stets Patrick Voigt 《Metallurgical and Materials Transactions A》2013,44(12):5408-5415
In the current study, test bars of cast aluminum alloys EN AC-AlSi8Cu3 and EN AC-AlSi7Mg0.3 were produced with a defined amounts of shrinkage pores and oxides. For this purpose, a permanent mold with heating and cooling devices for the generation of pores was constructed. The oxides were produced by contaminating the melt. The specimens and their corresponding defect distributions were examined and quantified by X-ray computer tomography (CT) and quantitative metallography, respectively. A special test algorithm for the simultaneous image analyses of pores and oxides was developed. Fatigue tests were conducted on the defective samples. It was found that the presence of shrinkage pores lowers the fatigue strength, and only few oxide inclusions were found to initiate fatigue cracks when shrinkage pores are present. The results show that the pore volume is not sufficient to characterize the influence of shrinkage pores on fatigue life. A parametric model for the calculation of fatigue life based on the pore parameters obtained from CT scans was implemented. The model accounts for the combined impact of pore location, size, and shape on fatigue life reduction. 相似文献
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Lu Yao Steve Cockcroft Jindong Zhu Carl Reilly 《Metallurgical and Materials Transactions A》2011,42(13):4137-4148
Porosity is one of the most common defects to degrade the mechanical properties of aluminum alloys. Prediction of pore size,
therefore, is critical to optimize the quality of castings. Moreover, to the design engineer, knowledge of the inherent pore
population in a casting is essential to avoid potential fatigue failure of the component. In this work, the size distribution
of the porosity was modeled based on the assumptions that the hydrogen pores are nucleated heterogeneously and that the nucleation
site distribution is a Gaussian function of hydrogen supersaturation in the melt. The pore growth is simulated as a hydrogen-diffusion-controlled
process, which is driven by the hydrogen concentration gradient at the pore liquid interface. Directionally solidified A356
(Al-7Si-0.3Mg) alloy castings were used to evaluate the predictive capability of the proposed model. The cast pore volume
fraction and size distributions were measured using X-ray microtomography (XMT). Comparison of the experimental and simulation
results showed that good agreement could be obtained in terms of both porosity fraction and size distribution. The model can
effectively evaluate the effect of hydrogen content, heterogeneous pore nucleation population, cooling conditions, and degassing
time on microporosity formation. 相似文献
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Many of the targeted applications for powder-metallurgy materials, particularly in the automotive industry, undergo cyclic
loading. It is, therefore, essential to examine the fatigue mechanisms in these materials. The mechanisms of fatigue-crack
initiation and propagation in ferrous powder-metallurgy components have been investigated. The fatigue mechanisms are controlled
primarily by the inherent porosity present in these materials. Since most, if not all, fatigue cracks initiate and propagate
at the specimen surface, surface replication was used to determine the role of surface porosity in relation to fatigue behavior.
Surface replication provides detailed information on both initiation sites and on the propagation path of fatigue cracks.
The effect of microstructural features such as pore size and pore shape, as well as the heterogeneous microstructure on crack
deflection, was examined and is discussed. Fracture surfaces were examined to elucidate a mechanistic understanding of fatigue
processes in these materials. 相似文献
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Hot-isostatically-pressed powder-metallurgy Astroloy was obtained which contained 1.4 pct, fine porosity at the grain boundaries
produced by argon entering the powder container during pressing. The pores averaged about 2μ,m diam and 20 μ m spacing. This
material was tested at 650 °C in fatigue, creep-fatigue, tension, and stress-rupture and the results compared with previous
data on sound Astroloy. The pores influenced fatigue crack initiation and produced a more intergranular mode of propagation.
However, fatigue life was not drastically reduced. A large 25 μm pore in one specimen resulting from a hollow particle did
reduce life by 60 pct, however. Fatigue behavior of the porous material showed typical correlation with tensile behavior.
The plastic strain range-life relation was reduced proportionately with the reduction in tensile ductility, but the elastic
strain range-life relation was little changed reflecting the small reduction in strength divided by modulus for the porous
material. 相似文献
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Mari Koike Kwai S. Chan Susan K. Hummel Robert L. Mason Toru Okabe 《Metallurgical and Materials Transactions A》2013,44(2):1034-1044
The objective of the study was to evaluate the hypothesis that the mechanical properties and fatigue behavior of removable partial dentures (RPD) made from cast titanium alloys can be improved by alloying with low-cost, low-melting elements such as Cu, Al, and Fe using commercially pure Ti (CP-Ti) and Ti-6Al-4V as controls. RPD specimens in the form of rest-shaped, clasp, rectangular-shaped specimens and round-bar tensile specimens were cast using an experimental Ti-5Al-5Cu alloy, Ti-5Al-1Fe, and Ti-1Fe in an Al2O3-based investment with a centrifugal-casting machine. The mechanical properties of the alloys were determined by performing tensile tests under a controlled displacement rate. The fatigue life of the RPD specimens was tested by the three-point bending in an MTS testing machine under a cyclic displacement of 0.5 mm. Fatigue tests were performed at 10 Hz at ambient temperature until the specimens failed into two pieces. The tensile data were statistically analyzed using one-way ANOVA (α = 0.05) and the fatigue life data were analyzed using the Kaplan-Meier survival analysis (α = 0.05). The experimental Ti-5Al-5Cu alloy showed a significantly higher average fatigue life than that of either CP-Ti or Ti-5Al-1Fe alloy (p < 0.05). SEM fractography showed that the fatigue cracks initiated from surface grains, surface pores, or hard particles in surface grains instead of the internal casting pores. Among the alloys tested, the Ti-5Al-5Cu alloy exhibited favorable results in fabricating dental appliances with an excellent fatigue behavior compared with other commercial alloys. 相似文献