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1.
Dong Hyuk Shin Duck-Young Hwang Yong-Jun Oh Kyung-Tae Park 《Metallurgical and Materials Transactions A》2004,35(3):825-837
Tensile tests were carried out at temperatures of 673 to 773 K and strain rates of 1×10−3 to 1 s−1 on an ultrafine-grained (UFG) 5083 Al alloy containing 0.2 wt pct Sc fabricated by equal-channel angular pressing, in order
to examine its high-strain-rate superplastic characteristics. The mechanical data for the alloy at 723 and 773 K exhibited
a sigmoidal behavior in a double logarithmic plot of the maximum true stress vs true strain rate. The strain-rate sensitivity was 0.25 to 0.3 in the low-(
<5×10−3 s−1) and high- (
>5×10−2 s−1) strain-rate regions, and ∼0.5 in the intermediate-strain-rate region (5×10−3 s−1<
<5 × 10−2 s−1). The maximum elongation to failure of ∼740 pct was obtained at 1×10−2 s−1 and 773 K. By contrast, no sigmoidal behavior was observed at 673 K. Instead, the strain-rate sensitivity of 0.3 was measured
in both intermediate-and low-strain-rate regions, but it was about 0.25 in the high-strain-rate region. High-strain-rate superplasticity
(HSRS) in the intermediate-strain-rate region at 723 and 773 K was dominated by grain-boundary sliding (GBS) associated with
continuous recrystallization and preservation of fine recrystallized grains by second-phase particles. However, the activation
energy for HSRS of the present alloy was lower than that predicted for any standard high-temperature deformation mechanism.
The low activation energy was likely the result of the not-fully equilibrated microstructure due to the prior severe plastic
deformation (SPD). For 673 K, the mechanical data and the microstructural examination revealed that viscous glide was a dominant
deformation mechanism in the intermediate- and low-strain-rate regions. Deformation in the high-strain-rate region at all
testing temperatures was attributed to dislocation breakaway from solute atmospheres. 相似文献
2.
Shogo Komura Zenji Horita Minoru Furukawa Minoru Nemoto Terence G. Langdon 《Metallurgical and Materials Transactions A》2001,32(3):707-716
An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain
size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723 K, with elongations
up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10−2 s−1. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673
K. An analysis of the experimental data for the equal-channel angular (ECA)-pressed samples shows consistency with conventional
superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary
diffusion in pure Al and interdiffusion in Al-Mg solid solution alloys. 相似文献
3.
Shogo Komura Zenji Horita Minoru Furukawa Minoru Nemoto Terence G. Langdon 《Metallurgical and Materials Transactions A》2001,32(13):707-716
An Al-3 pct Mg-0.2 pct Sc alloy was fabricated by casting and subjected to equal-channel angular pressing to reduce the grain
size to ∼0.2 μm. Very high tensile elongations were achieved in this alloy at temperatures over the range from 573 to 723
K, with elongations up to >2000 pct at temperatures of 673 and 723 K and strain rates at and above 10−2 s−1. By contrast, samples of the same alloy subjected to cold rolling (CR) yielded elongations to failure of <400 pct at 673
K. An analysis of the experimental data for the equal-channel angular (ECA)—pressed samples shows consistency with conventional
superplasticity including an activation energy for superplastic flow which is within the range anticipated for grain boundary
diffusion in pure Al and interdiffusion in Al−Mg solid solution alloys.
MINORU NEMOTO, formerly Professor, Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University. 相似文献
4.
Nb-1 wt pct Zr/218W long-fiber composite monotapes, nominally containing 0 to 70 vol pct of 218 tungsten fibers, were fabricated
by arc spraying the Nb-1 pct Zr matrix onto the tungsten fibers. The monotapes were consolidated by hot pressing and hot isostatic
pressing techniques. Tensile tests conducted between 1400 and 1600 K, under engineering strain rates varying between 1.5×10−5 and 1.5×10−3 s−1, demonstrated that composites containing 70 vol pct of fibers had the highest strength-to-density ratio. Microstructural
observations of specimens tested at 1400 K revealed that composites containing less than 50 vol pct of fibers showed extensive
matrix cavitation, fiber-matrix debonding, and necking of the fibers. Above 50 vol pct, the composite matrix was less prone
to cavitation, with an increasing tendency toward shear deformation of the fibers as the fiber volume fraction increased.
No fiber damage was observed at 1400 K away from the fractured end, but significant fiber damage was observed at higher temperatures.
A phenomenological model is presented to rationalize these observations.
L.J. GHOSN, formerly Researcher with Case Western Reserve University, Cleveland, OH 44115
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
5.
Tensile and creep tests were conducted to characterize the deformation behavior of four dilute SnBi alloys: SnBi0.5 at. pct,
SnBi1.5 at. pct, SnBi3 at. pct, and SnBi6 at. pct, the last two being supersaturated solid solutions at room temperature.
The test temperatures were − 20 °C, 23 °C, 90 °C, and 150 °C, and the strain rates ranged from approximately 10−8 to 10−1 1/s. In the tensile tests, all the alloys showed strain-hardening behavior up to room temperature. At higher temperatures,
only the higher-Bi-content alloys exhibited strain softening. The deformation behavior of the alloys can be divided into two
stress regimes, and the change from the low-stress regime to the high-stress regime occurred at around 6 × 10−4<σ/E<7.5 × 10−4. The results suggest that, at the low-stress regime, the rate-controlling deformation mechanism changes from dislocation
climb to viscous glide with the increasing Bi content of the alloy. At the high-stress regime, the activation energy of deformation
is about equal in all the alloys (∼60 kJ/mol) and the stress exponents are high (10<n<12.5). Unlike in the other alloys, bismuth precipitated at room temperature from the solution-annealed and quenched SnBi6
at. pct alloy by the discontinuous mechanism. This strongly affects the mechanical properties and makes the alloy brittle
at lower test temperatures. A comparison of the deformation behavior of the dilute SnBi alloys to that of the eutectic SnBi
alloy suggests that the deformation of eutectic structure is controlled by the Sn-rich phase containing the equilibrium amount
of dissolved Bi. 相似文献
6.
R. W. Hayes P. B. Berbon R. S. Mishra 《Metallurgical and Materials Transactions A》2004,35(12):3855-3861
The creep behavior of a cryomilled Al-10Ti-2Cu nanocomposite has been studied at temperatures of 533, 588, and 644 K at initial
applied stresses ranging from 55 to 117 MPa. Although the strain rates fall within the 10−10 to 10−9 S−1 regime, we observe no evidence of threshold-type creep behavior in this material. We attribute this to the unique microstructure
of the present material combined with the mechanism of dislocation slip in ultrafine grain size materials. In particular,
the very fine AIN precipitates present within the microstructure are ineffective as obstacles to dislocations during high-temperature
deformation. The coherent nature of these fine particles along with their extremely small size prevents a strong dislocation-particle
attraction. The inability of the activation energy for self-diffusion in Al to successfully collapse the present creep data
onto a single slope combined with the fact that the true activation energy for creep exceeds the value for lattice self-diffusion
are both features found in materials containing second-phase particles, which deform simultaneously with the matrix during
high-temperature deformation. In the present case, these particles are likely to be Al3Ti. 相似文献
7.
The stress-strain response of polycrystalline, γ-quenched U-7.5 wt pct Nb-2.5 wt pct Zr alloy was studied as a function of
strain rate and compared to equilibrium stress-strain tests performed by allowing the strain to reach a maximum value at incrementally
increasing stresses. Equilibrium stress-strain tests were also performed on prestressed samples. Sheet tensile specimens were
held at various states of strain in an X-ray diffractometer to determine crystal structural changes during deformation. Prestressed
tensile bars were sectioned and examined metallographically and with the X-ray diffractometer. Two linear regions were observed
in the equilibrium stress-strain tests: a low stress region with a slope of 5.3 to 5.5 x 106 psi, and a region above 40,000 psi with a slope of 3.3 x 106 psi. Finite strain rates tended to increase both slopes. The diffractometer experiments yielded plots of lattice parameter
vs strain which showed a shift from a bcc structure of the γs phase, to a bct structure of the γ0 phase between 1 and 3 pct deformation. It is postulated that this is a thermoelastic martensite transformation. A semiempirical
equation was developed which describes the equilibrium stress-strain behavior of this alloy in terms of a stress induced phase
transformation. 相似文献
8.
Nb-1 wt pct Zr/218W long-fiber composite monotapes, nominally containing 0 to 70 vol pct of 218 tungsten fibers, were fabricated
by arc spraying the Nb-1 pct Zr matrix onto the tungsten fibers. The monotapes were consolidated by hot pressing and hot isostatic
pressing techniques. Tensile tests conducted between 1400 and 1600 K, under engineering strain rates varying between 1.5×10−5 and 1.5×10−3 s−1, demonstrated that composites containing 70 vol pct of fibers had the highest strength-to-density ratio. Microstructural
observations of specimens tested at 1400 K revealed that composites containing less than 50 vol pct of fibers showed extensive
matrix cavitation, fiber-matrix debonding, and necking of the fibers. Above 50 vol pct, the composite matrix was less prone
to cavitation, with an increasing tendency toward shear deformation of the fibers as the fiber volume fraction increased.
No fiber damage was observed at 1400 K away from the fractured end, but significant fiber damage was observed at higher temperatures.
A phenomenological model is presented to rationalize these observations.
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.
H. N. Azari G. S. Murty G. S. Upadhyaya 《Metallurgical and Materials Transactions A》1994,25(10):2153-2160
The superplastic behavior of thermomechanically treated P/M 7091 aluminum alloy was assessed in the temperature range of 573
to 773 K. The thermomechanical treatment (TMT) comprised of three steps of solution treatment, overaging, and warm rolling.
There are large η-phase (MgZn2) precipitate particles of average size of 1.30 μm in the overaged condition. The warm-rolled alloy undergoes continuous recrystallization
at the test temperatures of 573 and 623 K, exhibiting a maximum tensile elongation of 450 pct at 573 K and a strain rate of
8 × 10−5 s−1. The precipitate particles play a major role in the process of continuous recrystallization. For a given volume fraction
of precipitate particles and constant amount of warm rolling (in the course of TMT), an optimum precipitate particle size
is expected to maximize the rate of continuous recrystallization and render the finest recrystallized grain size. The warm-rolled
alloy undergoes static recrystallization at temperatures above 673 K. The grain growth accompanying the deformation at these
test temperatures limits the tensile ductility to a lower value. Irrespective of the test temperature and strain rate, the
specimens undergo extensive cavitation when deformed at elevated temperatures. 相似文献
10.
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
11.
The Ostwald ripening of Al3Sc precipitates in an Al-0.28 wt pct Sc alloy during aging at 673, 698, and 723 K has been examined by measuring the average
size of precipitates by transmission electron microscopy (TEM) and the reduction in Sc concentration in the Al matrix with
aging time, t, by electrical resistivity. The coarsening kinetics of Al3Sc precipitates obey the t
1/3 time law, as predicted by the Lifshitz-Slyozov-Wagner (LSW) theory. The kinetics of the reduction of Sc concentration with
t are consistent with the predicted t
−1/3 time law. Application of the LSW theory has enabled independent calculation of the Al/Al3Sc interface energy, γ, and volume diffusion coefficient, D, of Sc in Al during coarsening of precipitates. The Gibbs-Thompson equation has been used to give a value of γ using coarsening data obtained from TEM and electrical resistivity measurements. The value of γ estimated from the LSW theory is 218 mJ m−2, which is nearly identical to 230 mJ m−2 from the Gibbs-Thompson equation. The pre-exponential factor and activation energy for diffusion of Sc in Al are determined
to be (7.2±6.0)×10−4 m2 s−1 and 176±9 kJ mol−1, respectively. The values are in agreement with those for diffusion of Sc in Al obtained from tracer diffusion measurements. 相似文献
12.
The effect of quenching condition on the mechanical properties of an A356 (Al-7 wt pct Si-0.4 wt pct Mg) casting alloy has
been studied using a combination of mechanical testing, differential scanning calorimetry (DSC), and transmission electron
microscopy (TEM). As the quench rate decreases from 250 °C/s to 0.5 °C/s, the ultimate tensile strength (UTS) and yield strength
decrease by approximately 27 and 33 pct, respectively. The ductility also decreases with decreasing quench rate. It appears
that with the peak-aged condition, both the UTS and yield strength are a logarithmic function of the quench rate,i.e., UTS orσ
y
=A logR +B. The termA is a measure of quench sensitivity. For both UTS and yield strength of the peak-aged A356 alloy,A is approximately 32 to 33 MPa/log (°C/s). The peak-aged A356 alloy is more quench sensitive than the aluminum alloy 6063.
For 6063,A is approximately 10 MPa/log (°C/s). The higher quench sensitivity of A356 is probably due to the high level of excess Si.
A lower quench rate results in a lower level of solute supersaturation in the α-Al matrix and a decreased amount of excess
Si in the matrix after quenching. Both of these mechanisms play important roles in causing the decrease in the strength of
the peak-aged A356 with decreasing the quench rate. 相似文献
13.
M. Eddahbi O. A. Ruano T. R. McNelley 《Metallurgical and Materials Transactions A》2001,32(5):1093-1102
The evolution of microstructure, texture, and microtexture in an Al-6 pct Cu-0.4 pct Zr alloy was studied during mechanical
testing at 480 °C and a strain rate of 5·10−4 s−1. The as-processed material had an elongated, banded microstructure and a deformation texture with orientation distribution
along the β-orientation fiber. The true strain vs true stress curve exhibited three stages: I, II, and III. Work hardening occurred in stages I and III, whereas nearly steady-state
behavior was observed in stage II. A bimodal distribution of boundary disorientation angles was evident in as-processed material
and persisted into stage I, with peaks at 5–15 deg in the low-angle boundary (LAB) regime and at 45–60 deg in the high-angle
boundary (HAB) regime. An increase in strain rate sensitivity coefficient, m, in stage I was accompanied by fragmentation of the initial microstructure, leading to the formation of new grains. During
stage II the strain rate sensitivity coefficient, m, attained a value of 0.5, which is consistent with the onset of grain boundary sliding. In stage III, the texture and the
grain boundary disorientation distribution became randomized while the m value decreased. Grain elongation and cavity formation at second-phase particles and along grain boundaries were seen in
samples deformed to failure. The as-processed microstructure is described in terms of deformation banding, and a model for
the evolution of such a structure during superplastic deformation is proposed. 相似文献
14.
L. Yu G. L. Ding J. Reye S. N. Tewari S. N. Ojha 《Metallurgical and Materials Transactions A》2000,31(9):2275-2285
The Pb-5.8 wt pct Sb alloy was directionally solidified with a positive thermal gradient of 140 K cm−1 at a growth speed ranging from 0.8 to 30 μm s−1, and then it was quenched to retain the mushy zone morphology. The morphology of the mushy zone along its entire length has
been characterized by using a serial sectioning and three-dimensional image reconstruction technique. Variation in the cellular/dendritic
shape factor, hydraulic radius of the interdendritic region, and fraction solid along the mushy zone length has been studied.
A comparison with predictions from theoretical models indicates that convection remarkably reduces the primary dendrite spacing
while its influence on the dendrite tip radius is not as significant. 相似文献
15.
R. A. Perkins 《Metallurgical and Materials Transactions B》1973,4(7):1665-1669
The tracer diffusion of63Ni in Fe-17 Cr-12 Ni by both volume and grain boundary transport has been studied from 600° to 1250°C. The use of an RF sputtering
technique for serial sectioning allowed the determination of very small volume diffusion coefficients at the lower temperatures.
Volume diffusion of nickel in this alloy was observed to be much slower than in pure iron or austenitic stainless steel at
comparable temperatures. The volume diffusion coefficient is described byD
v
=8.8 exp (−60,000/RT) cm2/s and grain boundary diffusion is described by σD
gb
=3.7×10−9 exp (−32,000/RT) cm3/s.
R. A. PERKINS, formerly Presidential Intern, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak, Ridge, Tenn.
37830, is 相似文献
16.
Kyung-Tae Park Duck-Young Hwang Si-Young Chang Dong Hyuk Shin 《Metallurgical and Materials Transactions A》2002,33(9):2859-2867
A submicrometer-grained structure was introduced in a commercial 5083 Al alloy by imposing an effective strain of ∼8 through
equal channel angular pressing. In order to examine the low-temperature superplastic behavior, the as-equal channel angular
pressed (as-ECAP) samples were tensile tested in the strain rate range of 10−5 to 10−2 s−1 at temperatures of 498 to 548 K corresponding to 0.58 to 0.65 T
m, where T
m is the incipient melting point. The mechanical data of the alloy at 498 and 548 K exhibited a sigmoidal behavior in a double
logarithmic plot of the maximum true stress vs true strain rate. The strain rate sensitivity was 0.1 to 0.2 in the low- and high-strain rate regions and 0.4 in the intermediate-strain
rate region, indicating the potential for superplasticity. At 523 K, instead of the sigmoidal behavior, a strain rate sensitivity
of 0.4 was maintained to low strain rates. A maximum elongation of 315 pct was obtained at 548 K and 5×10−4 s−1. The activation energy for deformation in the intermediate-strain rate region was estimated as 63 kJ/mol. Low-temperature
superplasticity of the ultrafine grained 5083 Al alloy was attributed to grain boundary sliding that is rate-controlled by
grain boundary diffusion, with a low activation energy associated with nonequilibrium grain boundaries. Cavity stringers parallel
to the tensile axis were developed during deformation, and the failure occurred in a quasi-brittle manner with moderately
diffusive necking. 相似文献
17.
J. A. Jimenez J. Wesemann G. Frommeyer 《Metallurgical and Materials Transactions A》1998,29(5):1425-1430
A study of the high-temperature deformation behavior of a binary α
2+γTi-47 at. pct Al-3 at. pct Cr alloy was undertaken. The alloy was produced by induction melting and exhibited a structure
of coarse columnar grains oriented in the radial direction. After a solution treatment at 1653 K for 3600 seconds and aging
at 1223 K for 28,800 seconds, a nearly lamellar structure was formed. Deformation behavior was investigated by compression-strain-rate-change
tests at strain rates ranging from 10−6 to 10−3 s−1 and temperatures ranging from 1073 to 1373 K. This alloy shows at low temperature/high stress a stress exponent of about
5. The deformation behavior is explained in this regimen by a dislocation climb mechanism, which includes a threshold stress.
Finally, at the lowest stress levels and highest temperatures of testing, a stress exponent of about 3 is observed, which
suggests that deformation is controlled by the viscous glide of dislocations. 相似文献
18.
C. Zhang M. Enomoto T. Yamashita N. Sano 《Metallurgical and Materials Transactions A》2004,35(4):1263-1272
The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good
formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening
behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter
hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays
a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model.
Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion
microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness
in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations
or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced
by prestraining. 相似文献
19.
Transition of dominant diffusion process during superplastic deformation in AZ61 magnesium alloys 总被引:2,自引:0,他引:2
The superplastic behavior of the AZ61 magnesium alloy sheet, processed by one-step hot extrusion and possessing medium grain
sizes of ∼12 μm, has been investigated over the temperature range of 523 to 673 K. The highest superplastic elongation of
920 pct was obtained at 623 K and a deformation rate of 1×10−4 s−1. In the lower and higher strain rate regimes, with apparent m values of ∼0.45 and ∼0.25, respectively, grain-boundary sliding (GBS) and dislocation creep appeared to dominate the deformation,
consistent with the scanning electron microscopy (SEM) examination. The SEM examination also revealed that individual GBS
started to operate from the very initial deformation stage in the strain rate range with m∼0.45, which was attributed to the relatively high fraction (88 pct) of high-angle boundaries. The analyses of the superplastic
data over 523 to 673 K and 5×10−5 to 1×10−3 s−1 revealed a true stress exponent of ∼2, and the activation energy was close to that for grain-boundary and lattice diffusion
of magnesium at 523 to 573 K and 573 to 673 K, respectively. The transition temperature of activation energy is ∼573 K, which
is attributed to the change in the dominant diffusion process from grain-boundary diffusion to lattice diffusion. It is demonstrated
that the effective diffusion coefficient is a valid parameter to characterize the superplastic behavior and the dominant diffusion
process. 相似文献
20.
F. C. Levey M. B. Cortie L. A. Cornish 《Metallurgical and Materials Transactions A》2000,31(8):1917-1923
A gold alloy with 18 wt pct Cu and 6 wt pct Al undergoes a reversible displacive phase transformation between an incompletely
ordered L21 parent phase and a tetragonal product. The characteristics of these transformations were studied using acoustic emission,
dilatometry, X-ray diffraction, and metallography. The morphology of the transformation products, the structure of the parent
phase, and the generation of significant acoustic emission during the transformations indicate that they are at least quasi-martensitic,
if not martensitic, and that this system is an example of a β-phase shape-memory alloy (SMA). The onset temperatures of the transformations depend on the prior thermal history of the
sample. The martensite start (M
s
) temperature is between 30 °C and 20 °C. The system exhibits hysteresis and will revert to the parent phase when reheated,
with an austenite start (A
s
) temperature between 55 °C and 80 °C. However, freshly cast or solution-annealed and quenched samples of the alloy do not
transform to the tetragonal phase. Aging of such material at temperatures between 30 °C and 200 °C is required before they
will manifest the displacive transformation. The “martensite” phase is considerably more resistant to aging-induced stabilization
than that of most other SMAs. 相似文献