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1.
A. M. Gokhale N. U. Deshpande D. K. Denzer John Liu 《Metallurgical and Materials Transactions A》1998,29(4):1203-1210
A multiple micromechanisms-based model is developed to quantitatively relate the fracture toughness of partially recrystallized
7XXX aluminum alloys to their fracture surface morphology. The model is verified using the experimental data on partially
recrystallized 7050 alloy reported in the companion article. It is then used to obtain a quantitative relationship between
the fracture toughness and microstructural attributes. The model relates fracture toughness to microstructural parameters
such as degree of recrystallization, grain size of recrystallized grains, thickness of recrystallized regions, total surface
area of the constituent particles per unit volume, and microstructural anisotropy. The model predicts the changes in the fracture
toughness with the specimen orientation. 相似文献
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 and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing
tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully
lamellar microstructure with approximately 150-μm grain size and 1-μm lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile
transition temperature (BDTT), while fracture and fatigue tests were performed at 25 °C and 815 °C. Fracture toughness tests
were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using
ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and
by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified
and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture
toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness,
and fatigue resistance were identified and related to pertinent microstructural variables. 相似文献
4.
The effect of heat treatment on microstructure and cryogenic fracture properties in 5Ni and 9Ni steel 总被引:2,自引:0,他引:2
Heat treatments were utilized in 5Ni and 9Ni steel which resulted in the development of tempered microstructures which contained
either no measurable retained austenite (<0.5 pct) or approximately 4 to 5 pct retained austenite as determined by X-ray diffraction.
Microstructural observations coupled with the results of tensile testing indicated that the formation of retained austenite
correlated with a decrease in carbon content of the matrix. Relative values ofK
IC
at 77 K were estimated from slow bend precracked Charpy data using both the COD and equivalent energy measurements. In addition,
Charpy impact properties at 77 K were determined. In the 9Ni alloy, optimum fracture toughness was achieved in specimens which
contained retained austenite. This was attributed to changes in yield and work hardening behavior which accompanied the microstructural
changes. In the 5Ni alloy, fracture toughness equivalent to that observed in the 9Ni alloy was developed in grain refined
and tempered microstructures containing <0.5 pct retained austenite. A decrease in fracture toughness was observed in grain
refined 5Ni specimens containing 3.8 pct retained austenite due to the premature onset of unstable cracking. This was attributed
to the transformation of retained austenite to brittle martensite during deformation. It was concluded that the formation
of thermally stable retained austenite is beneficial to the fracture toughness of Ni steels at 77 K as a result of austenite
gettering carbon from the matrix during tempering. However, it was also concluded that the mechanical stability of the retained
austenite is critical in achieving a favorable enhancement of cryogenic fracture toughness properties.
Formerly with Union Carbide Corporation, Tarrytown, NY 相似文献
5.
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
6.
B. E. Meacham M. C. Marshall D. J. Branagan 《Metallurgical and Materials Transactions A》2006,37(12):3617-3627
The Palmqvist fracture toughness method was applied in a new fashion to measure the fracture toughness of a new wear-resistant
alloy in both a powder product for plasma-transferred arc (PTA) welding and a wire product for metal inert gas (MIG) welding.
Cracks were observed at high loads with a linear relationship between load and crack length indicating that only Palmqvist
cracks occurred in the alloys. The fracture toughness for the no-preheat and preheat PT40 welds are 15.6±0.3 Mpa m1/2 and 17.6±0.1 MPa m1/2, respectively, and for the no-preheat and preheat MG40 welds are 12±1 MPa m1/2 and 20.5±0.2 MPa m1/2, respectively. Observations of the cracks show evidence of ligaments indicating that the toughness mechanism is due to crack
bridging. The microstructural scale and morphology correlates with the fracture toughness, and as the structural scale decreases,
the fracture toughness increases. 相似文献
7.
8.
H. Fukumasu T. Kobayashi H. Toda K. Shibue 《Metallurgical and Materials Transactions A》2000,31(12):3053-3061
Dynamic fracture toughness of a reactive sintered γ-base TiAl alloy is studied in the temperature range from 298 to 1073 K. The stop block method is employed in order to observe
the crack paths and microcrack distribution ahead of a main crack tip under dynamic loading conditions at high temperature.
Fracture surface, crack path, and microcrack observations are carried out using a scanning electron microscope (SEM). Microcrack
initiation criteria and crack-tip stress shielding effect caused by crack deflection are discussed. The experimental results
demonstrate that the dynamic fracture toughness, J
Id
, increases with increasing temperature, and after attaining the maximum value at 873 K, the toughness decreases. Crack path
morphology varies with temperature. The stress shielding effect at the crack tip caused by main crack deflection was found
to affect the difference in crack extension energy for each temperature. The number of microcracks ahead of a main crack varies
with temperature. The stress shielding effect at the crack tip caused by microcracking was found to contribute to toughening
around 873 K. 相似文献
9.
G. Terlinde H. -J. Rathjen K. -H. Schwalbe 《Metallurgical and Materials Transactions A》1988,19(4):1037-1049
Fracture mechanics and tensile tests have been performed on the metastable β-Ti alloy Ti-IOV-2Fe-3AI. A variety of microstructures
was established by several combinations of forging and heat treatment resulting in different types, morphologies, and volume
fractions of the a-phase which precipitates from the matrix-β phase. Both fracture toughness and ductility are strongly reduced
by increasing hardening by the secondary a-phase. An elongated primary a-phase (α
p
) shows higher toughness compared to a globular α
p
-phase. A thick, continuous subgrain boundary a-film lowers the toughness significantly. For microstructures without primary
a a grain boundary α-film does not affect the toughness, while the ductility is drastically reduced. Very attractive combinations
of fracture toughness and ductility were found for a microstructure without primary a and without grain boundary α. The results
are discussed based on the fractographic observations, and a model is proposed which includes the effect of microstructure
and slip distribution on the crack nucleation, the crack growth path, and the crack deviation. 相似文献
10.
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
11.
The effect of deformation-induced transformation on the fracture toughness of commercial titanium alloys 总被引:2,自引:0,他引:2
M. Niinomi T. Kobayashi I. Inagaki A. W. Thompson 《Metallurgical and Materials Transactions A》1990,21(6):1733-1744
The effect of deformation-induced transformation of metastableβ phase on the ductility and toughness of four commercial titanium alloys was investigated. Tensile tests, Charpy impact tests,
and both static and dynamic fracture toughness tests were carried out at temperatures between 77 and 473 K on four titanium
alloys containing metastableβ phase. Deformation-inducedα″ (orthorhombic martensite) was observed in an (α + β)-type Ti-6Al-2Sn-4Zr-6Mo alloy. The dynamic fracture toughness of this
alloy increased considerably at 223 K compared to those at other temperatures. In another (α + β)-type Ti-6A1-4V alloy, the
static fracture toughness at 123 K and the dynamic fracture toughness at 223 K were increased considerably by the presence
of deformation-induced martensite compared to those at other temperatures. The strength increased as the temperature decreased
in this alloy. An abnormal elongation of aβ-type alloy, Ti-15V-3Al-3Sn-3Cr, at 123 K was attributed to the mechanical twinning of theβ phase. However, the effect of deformation-induced transformation on the fracture toughness of Ti-3Al-8V-6Cr-4Mo-4Zr alloy
was not observed.
Formerly Visiting Associate Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University,
Pittsburgh, PA.
Formerly with the Department of Production Systems Engineering, Toyohashi University of Technology. 相似文献
12.
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
13.
I. Dutta T. R. McNelley R. Nagarajan F. N. Quiles 《Metallurgical and Materials Transactions A》1998,29(9):2433-2446
The evolution of the microstructure and mechanical properties of a 17.5 vol. pct SiC particulate-reinforced aluminum alloy
6092-matrix composite has been studied as a function of postfabrication processing and heat treatment. It is demonstrated
that, by the control of particulate distribution, matrix grain, and substructure and of the matrix precipitate state, the
strength-toughness combination in the composite can be optimized over a wide range of properties, without resorting to unstable,
underaged (UA) matrix microstructures, which are usually deemed necessary to produce a higher fracture toughness than that
displayed in the peak-aged condition. Further, it is demonstrated that, following an appropriate combination of thermomechanical
processing and unconventional heat treatment, the composite may possess better stiffness, strength, and fracture toughness than a similar unreinforced alloy. In the high- and low-strength matrix microstructural conditions, the
matrix grain and substructure were found to play a substantial role in determining fracture properties. However, in the intermediate-strength
regime, properties appeared to be optimizable by the utilization of heat treatments only. These observations are rationalized
on the basis of current understanding of the grain size dependence of fracture toughness and the detailed microstructural
features resulting from thermomechanical treatments. 相似文献
14.
15.
Kwai S. Chan 《Metallurgical and Materials Transactions A》1996,27(9):2518-2531
The fracture resistance of Nb-Cr-Ti alloys orin situ composites of three different compositions, Cr2Nb, and a Nb-10Siin situ composite was studied at ambient temperature. The crack-tip deformation and fracture behaviors were characterized using near-tip
measurement techniques and fractographic analyses. The relevant fracture and toughening mechanisms were identified and related
to the microstructure. Despite fracture by a combination of cleavage and slip band decohesion, the Nb solid-solution alloy
exhibited a resistance-curve behavior with a relatively high toughness and local ductility. The source of toughness was modeled
and explained in terms of a cracking process that involved alternate slip band decohesion and cleavage. Thein situ composites, on the other hand, exhibited cleavage fracture but considerably lower toughness with little or no resistance-curve
behaviors. The difference in the fracture behavior appears to arise from two factors: (1) the presence of a high constraint
in the Nb solid-solution matrix in thein situ composites, and (2) the lack of plastic flow associated with cleavage of the constrained Nb solid-solution matrix. 相似文献
16.
E. D. Sweet C. G. Bennett I. Musulin S. P. Lynch R. B. Nethercott 《Metallurgical and Materials Transactions A》1996,27(11):3530-3541
The effects of alkali-metal impurity (AMI) content, temperature, and crack-mouth-opening displacement (CMOD) rate on the fracture
toughness of 2090-T8 Al-Li-Cu alloy extrusions were studied, particularly for short-transverse (S-L) orientations. Decreasing
AMI content resulted in increasing room-temperature fracture toughness, especially for underaged S-L and T-L specimens. Unlike
most Al-Li based alloys, material with very low (<2 wt. ppm) AMIs produced by vacuum refining had a high S-L fracture toughness
(up to 38 MPa√m for proof strengths ∼440 MPa) as well as high toughness in other orientations. The increase in room-temperature
fracture toughness with decreasing AMI content was associated with a decrease in the proportion of brittle intergranular and
cleavagelike islands, and a corresponding increase in the proportion of high energy dimpled fracture modes, on fracture surfaces.
Both the present and previous studies indicate that the brittle islands result from liquid-metal embrittlement due to the
presence of discrete sodium-potassium rich liquid phases. For medium to high AMI contents (5 to 37 wt ppm), S-L fracture toughness
increased with decreasing temperature due to solidification of these phases and a consequent decrease in the mobility of embrittling
atoms. The ability of embrittling atoms to keep up with crack tips also depended on crack velocity so that CMOD rate influenced
fracture toughness. The grain structure (degree of recrystallization) appeared to be another important parameter affecting
fracture toughness. 相似文献
17.
Correlation of microstructure and fracture properties in weld heat- affected zones of thermomechanically controlled processed steels 总被引:2,自引:0,他引:2
Sunghak Lee Byung Chun Kim DONGIL Kwon 《Metallurgical and Materials Transactions A》1992,23(10):2803-2816
This article presents a correlation study between the microstructural parameters and fracture properties in the weld heat-affected
zones (HAZs) of high-strength low alloy (HSLA) steels,i.e., a normalized steel and four thermomechanically controlled processed (TMCP) steels. The influence of the local brittle zone
(LBZ) on toughness was investigated by means of simulated HAZ tests as well as welded joint tests. The intercritically reheated
coarse-grained HAZ ex-hibited the lowest impact energy over the testing temperature range, indicating that this region was
the LBZ. By comparing the volume fraction of martensite islands with impact energy val-ues, this LBZ was attributed mainly
to the significant increase in the amount of martensite. Niobium was also found to have a deleterious effect on the HAZ fracture
toughness because of martensite hardening. This suggests that the formation of martensite islands must be controlled by proper
design of chemical compositions to reduce the carbon equivalent and by using the proper welding conditions to limit cooling
rates in order to optimize the fracture toughness of welded joints of TMCP steels. 相似文献
18.
Effects of thickness and precracking on the fracture toughness of particle-reinforced al-alloy composites 总被引:4,自引:0,他引:4
A. B. Pandey B. S. Majumdar D. B. Miracle 《Metallurgical and Materials Transactions A》1998,29(4):1237-1243
The effect of specimen thickness on the fracture toughness of a powder metallurgically processed 7093 Al/SiC/15p composite was evaluated in different microstructural conditions. The fracture toughness in the underaged condition increased
significantly with a decrease in specimen thickness, even at thicknesses well below the value specified by ASTM-E 813 for
a valid J
Ic
test. The influence of thickness was considerably lower in the peak-aged (PA) condition. This relative insensitivity is believed
to be due to the low strain to failure associated with severe flow localization in the PA condition. The effect of precracking
on the fracture toughness of discontinuously reinforced aluminum (DRA) was also evaluated. The dependence of fracture toughness
on specimen thickness and precracking is explained in terms of the hydrostatic stress, which has a strong influence on the
plastic straining capability of the DRA material. The fracture toughness was modeled using a critical strain formulation,
with the void growth strain dependent on hydrostatic stress through the Rice and Tracey model. The predicted toughnesses for
the thick and thin specimens were in good agreement with the experimental data. 相似文献
19.
《Acta Metallurgica》1989,37(7):1957-1970
A new criterion is proposed for quasi-brittle fracture based on the recognition that quasi-static fracture toughness measurements are merely a limiting case of dynamic fracture. It is found that a unique relationship exists between the energy stored ahead of a loaded crack or blunt notch and the velocity of a crack injected into the material by the failure of a microstructural element lying in the zone of elevated stress. The criterion establishes a minimum velocity required for the continued propagation of the injected crack as a function of the crack tip opening displacement at the instant of crack nucleation. Comparison of the predictions of the model with existing data on various steels and an aluminum alloy show excellent correlation. The implications of the model for phenomena such as the ductile-to-brittle transition, microstructural effects on the resistance to unstable crack propagation and grain boundary embrittlement/strengthening by solute segregation are also explored. 相似文献
20.
Dependence of fracture toughness of austempered ductile iron on austempering temperature 总被引:1,自引:0,他引:1
Ductile cast iron samples were austenitized at 927 °C and subsequently austempered for 30 minutes, 1 hour, and 2 hours at
260 °C, 288 °C, 316 °C, 343 °C, 371 °C, and 399 °C. These were subjected to a plane strain fracture toughness test. Fracture
toughness was found to initially increase with austempering temperature, reach a maximum, and then decrease with further rise
in temperature. The results of the fracture toughness study and fractographic examination were correlated with microstructural
features such as bainite morphology, the volume fraction of retained austenite, and its carbon content. It was found that
fracture toughness was maximized when the microstructure consisted of lower bainite with about 30 vol pct retained austenite
containing more than 1.8 wt pct carbon. A theoretical model was developed, which could explain the observed variation in fracture
toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and retained
austenite content. A plot of K
IC
2
against σ
y
(X
γ, C
γ)1/2 resulted in a straight line, as predicted by the model. 相似文献