共查询到20条相似文献,搜索用时 62 毫秒
1.
B. V. Cockeram 《Metallurgical and Materials Transactions A》2002,33(12):3685-3707
Oxide dispersion-strengthened (ODS) and molybdenum-0.5 pct titanium-0.1 pct zirconium (TZM) molybdenum have excellent creep
resistance and strength at high temperatures in inert atmospheres. Fracture toughness and tensile testing was performed at
temperatures between − 150 °C and 450 °C to characterize 6.35-mm-thick plate material of ODS and TZM molybdenum. A transition
from low fracture-toughness values (5.8 to 29.6 MPa√m) to values >30 MPa√m is observed for TZM molybdenum in the longitudinal
orientation at 100 °C and in the transverse orientation at 150 °C. These results are consistent with data reported in literature
for molybdenum. A transition to low fracture-toughness values (<30 MPa√m) was not observed for longitudinal ODS molybdenum
at temperatures >−150 °C, while a transition to low fracture-toughness values (12.6 to 25.4 MPa√m) was observed for the transverse
orientation at room temperature. The fine spacing of La-oxide precipitates which are present in ODS molybdenum results in
a transition temperature that is significantly lower than any molybdenum alloy reported to date, with upper-bound fracture-toughness
values that bound the literature data. A comparison of fracture-toughness values obtained using 1T, 0.5T, and 0.25T three-point
bend specimens shows that a 0.5T bend specimen could be used as a subsized geometry. 相似文献
2.
3.
B.V. Cockeram 《Metallurgical and Materials Transactions A》2009,40(12):2843-2860
Commercially available molybdenum and oxide dispersion strengthened (ODS) molybdenum produced by powder metallurgy (PM) methods
were subjected to tensile testing, fracture toughness testing, and examination of the toughening mechanism. Both PM and ODS
molybdenum have an equiaxed grain size that is larger in scale than comparable wrought products. This results in lower tensile
strength and a higher tensile ductile-to-brittle transition temperature (DBTT) for PM and ODS molybdenum compared to wrought
product forms. Although the grain size for PM molybdenum is large and the oxygen content is relatively high, both attributes
tending to embrittle molybdenum, the transition temperature and fracture toughness values are comparable to those observed
for wrought molybdenum. Crack initiation at grain boundaries and the center of grains where pores are present were observed
to leave ligaments for the PM molybdenum that are similar in scale to those observed for wrought molybdenum. This is a similar
toughening mechanism to the ductile laminate mechanism observed for wrought molybdenum. The larger oxide particle size for
PM ODS molybdenum produces larger cracks that result in lower fracture toughness values and a higher DBTT in comparison to
PM molybdenum. The impact of the grain size, grain shape, and oxide particles on the toughening mechanism and resulting properties
is discussed. 相似文献
4.
H. Choe J. H. Schneibel R. O. Ritchie 《Metallurgical and Materials Transactions A》2003,34(2):225-239
The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature
toughness for potential use at temperatures above ∼1000 °C has remained a persistent challenge in materials science. In this
work, one promising class of intermetallic alloys is examined, namely, boron-containing molybdenum silicides, with compositions
in the range Mo (bal), 12 to 17 at. pct Si, 8.5 at. pct B, processed using both ingot (I/M) and powder (P/M) metallurgy methods.
Specifically, the oxidation (“pesting”), fracture toughness, and fatigue-crack propagation resistance of four such alloys,
which consisted of ∼21 to 38 vol. pct α-Mo phase in an intermetallic matrix of Mo3Si and Mo5SiB2 (T2), were characterized at temperatures between 25 °C and 1300 °C. The boron additions were found to confer improved “pest”
resistance (at 400 °C to 900 °C) as compared to unmodified molybdenum silicides, such as Mo5Si3. Moreover, although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those
of MoSi2, for the I/M processed microstructures with coarse distributions of the α-Mo phase, fracture toughness properties were far superior, rising from values above 7 MPa √m at ambient temperatures to almost
12 MPa √m at 1300 °C. Similarly, the fatigue-crack propagation resistance was significantly better than that of MoSi2, with fatigue threshold values roughly 70 pct of the toughness, i.e., rising from over 5 MPa √m at 25 °C to ∼8 MPa √m at 1300 °C. These results, in particular, that the toughness and cyclic
crack-growth resistance actually increased with increasing temperature, are discussed in terms of the salient mechanisms of
toughening in Mo-Si-B alloys and the specific role of microstructure. 相似文献
5.
J. J. Kruzic J. H. Schneibel R. O. Ritchie 《Metallurgical and Materials Transactions A》2005,36(9):2393-2402
Ambient- to elevated-temperature fracture and fatigue-crack growth results are presented for five Mo-Mo3Si-Mo5SiB2-containing α-Mo matrix (17 to 49 vol pct) alloys, which are compared to results for intermetallic-matrix alloys with similar compositions.
By increasing the α-Mo volume fraction, ductility, or microstructural coarseness, or by using a continuous α-Mo matrix, it was found that improved fracture and fatigue properties are achieved by promoting the active toughening mechanisms,
specifically crack trapping and crack bridging by the α-Mo phase. Crack-initiation fracture toughness values increased from 5 to 12 MPa√m with increasing α-Mo content from 17 to 49 vol pct, and fracture toughness values rose with crack extension, ranging from 8.5 to 21 MPa√m at
ambient temperatures. Fatigue thresholds benefited similarly from more α-Mo phase, and the fracture and fatigue resistance was improved for all alloys tested at 1300 °C, the latter effects being
attributed to improved ductility of the α-Mo phase at elevated temperatures. 相似文献
6.
Chongmin Kim A. Richard Johnson William F. Hosford 《Metallurgical and Materials Transactions A》1982,13(9):1595-1605
The influence of microstructural variations on the fracture toughness of two tool steels with compositions 6 pct W-5 pct Mo-4
pct Cr-2 pct V-0.8 pct C (AISI M2 high-speed steel) and 2 pct W-2.75 pct Mo-4.5 pct Cr-1 pct V-0.5 pct C (VASCO-MA) was investigated.
In the as-hardened condition, the M2 steel has a higher fracture toughness than the MA steel, although the latter steel is
softer. In the tempered condition, MA is softer and has a higher fracture toughness than M2. When the hardening temperature
is below 1095 °C (2000 °F), tempering of both steels causes embrittlement,i.e., a reduction of fracture toughness as well as hardness. The fracture toughness of both steels was enhanced by increasing
the grain size. The steel samples with intercept grain size of 5 (average grain diameter of 30 microns) or coarser exhibit
2 to 3 MPa√m (2 to 3 ksi√in.) higher fracture toughness than samples with intercept grain size of 10 (average grain diameter
of 15 microns) or finer. Tempering temperature has no effect on the fracture toughness of M2 and MA steels as long as the
final tempered hardness of the steels is constant. Retained austenite has no influence on the fracture toughness of as-hardened
MA steel, but a high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. There
is a temperature of austenitization for each tool steel at which the retained austenite content in the as-quenched samples
is a maximum. The above described results were explained through changes in the microstructure and the fracture modes.
CHONGMIN KIM, formerly with Climax Molybdenum Company of Michigan, Ann Arbor, MI. 相似文献
7.
The temperature dependence of fatigue crack propagation is considered in an Fe-1 pct Cr-0.5 pct Mo alloy steel. This material
was tested at temperatures between 425 and 550 °C, a frequency of 1 Hz, and anR-ratio of 0.1. It is shown that the effect of temperature can be explained in terms of a thermal activation energy for fatigue.
The magnitude of this activation energy is a function of ΔK and varies from more than 150 kJ/mole at 15 MPa√m to 30 kJ/mole
above 30 MPa√m. The magnitude of these activation energies supports the idea that oxidation, and not creep, is the rate-controlling
time-dependent process for the test conditions studied. 相似文献
8.
Sunghak Lee Je Won Rhyu Kyung-Mox Cho Jacques Duffy 《Metallurgical and Materials Transactions A》1993,24(4):901-912
An investigation was conducted into the effects of test temperature and loading rate on the initiation of plane strain fracture
of an HY-100 steel. Fracture toughness tests were conducted using fatigue precracked round bars loaded in tension to produce
a quasi-static stress intensity rate of ·K1
= 1 MPa√m/s and a dynamic rate of ·K1 = 2 × 106 MPa√m/s. Testing temperatures covered the range from -150 °C to 200 °C, which encompasses fracture initiation modes involving
quasi-cleavage to fully ductile fracture. The results of toughness tests show that the lower-shelf values of fracture toughness
were substantially independent of loading rate, while the dynamic values exceeded the quasi-static values by about 50 pct
on the upper shelf. In analyzing these results, phenomenological fracture initiation models were adopted based on the requirement
that, for fracture to occur, a critical strain or stress must be achieved over a critical distance. In separate tests, the
observation of microfracture processes was investigated using fractography and anin situ scanning electron microscope (SEM) fracture technique. The layered ppearance of the fracture surfaces was found to be associated
with a banded structure which generally contains many MnS inclusions, probably resulting in a reduction of the fracture toughness
values. 相似文献
9.
S. Hariprasad S. M. L. Sastry K. L. Jerina R. J. Lederich 《Metallurgical and Materials Transactions A》1994,25(5):1005-1014
The room-temperature fatigue crack growth rates (FCGR) and fracture toughness were evaluated for different crack plane orientations
of an Al-8.5 Pct Fe-1.2 Pct V-1.7 Pct Si alloy produced by planar flow casting (PFC) and atomized melt deposition (AMD) processes.
For the alloy produced by the PFC process, properties were determined in six different orientations, including the short transverse
directions S-T and S-L. Diffusion bonding and adhesive bonding methods were used to prepare specimens for determining FCGR
and fracture toughness in the short transverse direction. Interparticle boundaries control fracture properties in the alloy
produced by PFC. Fracture toughness of the PFC alloy varies from 13.4 MPa√m to 30.8 MPa√m, depending on the orientation of
the crack plane relative to the interparticle boundaries. Fatigue crack growth resistance and fracture toughness are greater
in the L-T, L-S, and T-S directions than in the T-L, S-T, and S-L orientations. The alloy produced by AMD does not exhibit
anisotropy in fracture toughness and fatigue crack growth resistance in the as-deposited condition or in the extruded condition.
The fracture toughness varies from 17.2 MPa√m to 18.5 MPa√m for the as-deposited condition and from 19.8 MPa√m to 21.0 MPa√m
for the extruded condition. Fracture properties are controlled by intrinsic factors in the alloy produced by AMD. Fatigue
crack growth rates of the AMD alloy are comparable to those of the PFC alloy in the L-T orientation. The crack propagation
modes were studied by optical metallographic examination of crack-microstructure interactions and scanning electron microscopy
of the fracture surfaces. 相似文献
10.
The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class
of forged damage-tolerant niobium aluminide (Nb3Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti
(10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages
unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa√m at room temperature.
Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed
at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature
strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa√m), and fatigue crack growth resistance
(comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural
applications of the 40Ti alloy in the intermediate-temperature (∼700 °C to 750 °C) regime. 相似文献
11.
K. T. Venkateswara Rao J. C. McNulty R. O. Ritchie 《Metallurgical and Materials Transactions A》1993,24(10):2233-2245
Aluminum-lithium alloys are currently being considered for applications at moderately elevated temperatures; accordingly,
a study has been made on the effects of prolonged (100 and 1000 hours overaging) thermal exposure at 149 °C and 260 °C on
the mechanical properties of a peakaged Al-Li-Cu-Mg-Zr alloy 8090-T8771. In the as-received T8771 temper, the alloy exhibits
an excellent combination of strength (˜500 MPa) and toughness (35 MPa√m) with moderate tensile elongation (4 pct). Overaging
at 149 °C results in a ˜50 pct reduction in ductility and toughness, primarily associated with the growth of equilibrium phases
along grain/subgrain boundaries, resulting in formation of solute-depleted precipitate-free zones and coarsening of matrix8' andS precipitates; strength levels and fatigue-crack growth rates, however, remain largely unchanged. Thermal exposures at 260
°C, conversely, lead to dramatic reductions in strength (by ˜50 to 80 pct), toughness (by ˜30 pct) and fatigue-crack propagation
resistance; crack-growth rates at all ΔK levels above ~5 MPa√m are 2 to 3 orders of magnitude faster. Microstructurally, this was associated with complete dissolution
of δ′, severe coarsening ofS andT
2
precipitates in the matrix, and formation of equilibrium Cu- and Mg-rich intermetallic phases in the matrix and along grain
boundaries. The resulting lack of planar-slip deformation and low yield strength of 8090 following overaging exposures at
260 °C increase the cumulative crack-tip damage per cycle and reduce the tendency for crack-path deflection, thereby accelerating
fatigue-crack growth rates. Despite this degradation in properties, the 8090-T8771 alloy has better strength retention and
generally superior fatigue-crack growth properties compared to similarly overaged Al-Li-Cu-Zr 2090 and Al-Cu-Zn-Mg 7150 alloys.
formerly with the University of California,
formerly with the University of California, 相似文献
12.
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
13.
Michael J. Haynes Richard P. Gangloff 《Metallurgical and Materials Transactions A》1997,28(9):1815-1829
The plane-strain initiation fracture toughness (K
JICi
) and plane-stress crack growth resistance of two Al-Cu-Mg-Ag alloy sheets are characterized as a function of temperature
by a J-integral method. For AA2519 +Mg+Ag, K
JICi
decreases from 32.5 MPa√m at 25 °C to 28.5 MPa√m at 175 °C, while K
JICi
for a lower Cu variant increases from 34.2 MPa√m at 25 °C to 36.0 MPa√m at 150 °C. Crack-tip damage in AA2519+Mg+Ag evolves
by nucleation and growth of voids from large undissolved Al2Cu particles, but fracture resistance is controlled by void sheeting coalescence associated with dispersoids. Quantitative
fractography, three-dimensional (3-D) reconstruction of fracture surfaces, and metallographic crack profiles indicate that
void sheeting is retarded as temperature increases from 25 °C to 150°C, consistent with a rising fracture resistance. Primary
microvoids nucleate from smaller constituent particles in the low Cu alloy, and fracture strain increases. A strain-controlled
micromechanical model accurately predicts K
JICi
as a function of temperature, but includes a critical distance parameter (l*) that is not definable a priori. Nearly constant initiation toughness for AA2519+Mg+Ag is due to rising fracture strain with temperature, which balances
the effects of decreasing flow strength, work hardening, and elastic modulus on the crack-tip strain distribution. Ambient
temperature toughnesses of the low Cu variant are comparable to those of AA2519+Mg+Ag, despite increased fracture strain,
because of reduced constituent spacing and l*. 相似文献
14.
A. L. McKelvey K. T. Venkateswara Rao R. O. Ritchie 《Metallurgical and Materials Transactions A》2000,31(5):1413-1423
A study has been made of the effect of temperature (between 25 °C and 800 °C) on fracture toughness and fatigue-crack propagation
behavior in an XD-processed, γ-based titanium aluminide intermetallic alloy, reinforced with a fine dispersion of ∼1 vol pct TiB2 particles. It was found that, whereas crack-initiation toughness increased with increasing temperature, the crack-growth
toughness on the resistance curve was highest just below the ductile-to-brittle transition temperature (DBTT) at 600 °C; indeed,
above the DBTT, at 800 °C, no rising resistance curve was seen. Such behavior is attributed to the ease of microcrack nucleation
above and below the DBTT, which, in turn, governs the extent of uncracked ligament bridging in the crack wake as the primary
toughening mechanism. The corresponding fatigue-crack growth behavior was also found to vary inconsistently with temperature.
The fastest crack growth rates (and lowest fatigue thresholds) were seen at 600 °C, while the slowest crack growth rates (and
highest thresholds) were seen at 800 °C; the behavior at 25 °C was intermediate. Previous explanations for this “anomalous
temperature effect” in γ-TiAl alloys have focused on the existence of some unspecified environmental embrittlement at intermediate temperatures or
on the development of excessive crack closure at 800 °C; no evidence supporting these explanations could be found. The effect
is now explained in terms of the mutual competition of two processes, namely, the intrinsic microstructural damage/crack-advance
mechanism, which promotes crack growth, and the propensity for crack-tip blunting, which impedes crack growth, both of which are markedly enhanced by increasing temperature. 相似文献
15.
This paper presents a study of dynamic fracture initiation behavior of 2124-T6 aluminum matrix composites containing 0, 5.2,
and 13.2 vol pct SiC whiskers. In the experiment, an explosive charge is detonated to produce a tensile stress wave to initiate
the fracture in a modified Kolsky bar (split Hopkinson bar). This stress wave loading provided a stress intensity rate, KI,, of about 2 × 106 MPa√m/s. The recorded data are then analyzed to calculate the critical dynamic stress intensity factor,K
Id, of the composite, and the values obtained are compared with the corresponding quasi-static values. The test temperatures
in this experiment ranged from −196 °C to 100°C, within which range the fracture initiation mode was found to be mostly ductile
in nature. The micromechanical processes involved in void and microcrack formation were investigated using metallographic
techniques. As a general trend, experimental results show a lower toughness as the volume fraction of the SiC whisker reinforcement
increases. The results also show a higher toughness under dynamic than under static loading. These results are interpreted
using a simple dynamic fracture initiation model based on the basic assumption that crack extension initiates at a certain
critical strain developed over some microstructurally significant distance. This model enables us to correlate tensile properties
and microstructural parameters, as, for instance, the interspacing of the SiC whiskers with the plane strain fracture toughness. 相似文献
16.
This article presents the results of a combined experimental and theoretical study of the effects of loading rate (1, 10,
and 100 MPa√m · s−1) on the resistance-curve behavior and toughening in cast lamellar gamma-based titanium aluminides (Ti-48Al-2Cr-2Nb, Ti-45Al-2Mn-2Nb
+ 0.8 vol pct TiB2, and Ti-47Al-2Mn-2Nb + 0.8 vol pct TiB2). Note that compositions are quoted in at. pct unless stated otherwise. The fracture-initiation toughness and resistance-curve
behavior in Ti-48Al-2Cr-2Nb are shown to be similar at the three loading rates examined. In the case of the Mn-containing
alloys, stronger resistance-curve behavior is observed as the loading rate increases from 1 to 10 MPa√m · s−1. However, the fracture-initiation toughness and resistance-curve behavior of the Mn-containing alloys are similar at loading
rates of 10 and 100 MPa√m · s−1. The observed resistance-curve behavior is attributed largely to the role of ligament bridging and, to a lesser extent, to
the effects of cracktip plasticity. Small- and large-scale bridging models are also shown to predict the measured resistance
curves when the observed/measured bridging parameters and material properties are used in the micromechanical modeling of
crack bridging. The implications of the results are also discussed for the design of damage-tolerant gamma alloys and microstructures. 相似文献
17.
The effects of changes in test temperature (−196 °C to 25 °C) and grain size (40 to 165 μm) on the dynamic cleavage fracture toughness (K
ID
) and Charpy impact toughness of polycrystalline niobium (Nb) have been investigated. The ductile-to-brittle transition was
found to be affected by both changes in grain size and the severity of stress concentration (i.e., notch vs fatigue-precrack). In addition to conducting impact tests on notched and fatigue-precracked Charpy specimens, extensive fracture
surface analyses have been performed in order to determine the location of apparent cleavage nucleation sites and to rationalize
the effects of changes in microstructure and experimental variables on fracture toughness. Existing finite element analyses
and the stress field distributions ahead of stress concentrators are used to compare the experimental observations with the
predictions of various fracture models. The dynamic cleavage fracture toughness, K
ID
, was shown to be 37±4 MPa√m and relatively independent of grain size (i.e., 40 to 105 μm) and test temperature over the range −196 °C to 25 °C. 相似文献
18.
D. L. Davidson 《Metallurgical and Materials Transactions A》1987,18(12):2115-2128
Subcritical crack growth and rapid fracture of the mechanically alloyed aluminum alloy IN-9052* reinforced with SiC particles
have been investigated. Fatigue crack growth rates for the composite exceed those of the unreinforced alloy, except that the
threshold stress intensity for growth is higher for the composite. Fracture toughness of the composite is about 9 MPa√m compared
to a (reported) value of 29 MPa√m for the unreinforced alloy. The contributions to fracture toughness from work done within
the plastic zone and in formation of the void sheet have been computed using analytical models. Fracture toughness is shown
to result almost entirely from work done within the plastic zone of the growing crack. The matrix microstructure and the particulate
characteristics are found to account for the elastic and fracture properties of this composite. 相似文献
19.
Cheng-Hsun Hsu Shen-Chih Lee Yih-Hsun Shy Hui-Ping Feng 《Metallurgical and Materials Transactions A》2001,32(2):295-303
The effect of testing temperature (− 150 °C, 25 °C, and + 150 °C) on the fracture toughness of austempered ductile iron (ADI)
was studied. Specimens were first austenitized at 900 °C for 1.5 hours and then salt-bath quenched to 360 °C or 300 °C, for
1, 2, or 3 hours of isothermal holding before cooling to room temperature. The resulting matrices of the iron were of upper-ausferrite
and lower-ausferrite. It was found that raising the testing temperature to 150 °C from ambient improved the fracture toughness
by 18, 30, and 7 pct for the as-cast/lower-ausferrite ADI/upper-ausferrite ADI, respectively. Lowering the testing temperature
to −150 °C produced a decrease of −15, −35, and −48 pct. Optical microscopy, X-ray diffraction analysis, and scanning electron
microscopy (SEM) fractography were applied to correlate the toughness variation with testing temperatures. 相似文献
20.
Fatigue crack growth rate behavior in CORONA 5, an alloy developed for applications requiring high fracture toughness, has
been examined for eight material conditions. These conditions were designed to give differences in microstructure, strength
level (825 to 1100 MPa [120 to 160 ksi]), and oxygen content (0.100 to 0.174 wt pct), in such a manner that the separate effects
of these variables could be defined. For all eight conditions, fatigue crack growth rates (da/dN) are virtually indistinguishable
over the full spectrum of stress-intensity range (ΔK) examined,viz., 8 to 40 MPa√m (7 to 36 ksi√in). Concomitantly, it is noted that over the sizable solution annealing range studied (830°
to 915 °C [1525° to 1675 °F]), the primary α-phase morphology was substantially invariant. Eachda/dN curve exhibits a bilinear form with a transition point (ΔKT) between 16 and 19 MPa√m (15 and 17 ksi√in). A change in microfractographic appearance occurs at ΔKT, as extensive secondary cracking along α/β interfaces is observed at all hypertransitional levels ofAK, but not for AK < ΔKT. For each material condition, the mean length of primary α platelets is approximately the same as the cyclic plastic zone
size at ΔKT. Accordingly, locations ofAKT (and their similarity for the different material conditions) are rationalized in conformance with a cyclic plastic zone model
of fatigue crack growth. Finally, the difference in behavior of CORONA 5, as compared to conventional α/β alloys such as Ti-6A1-4V,
is rationalized in terms of crack path behavior. 相似文献