共查询到20条相似文献,搜索用时 15 毫秒
1.
Z. Y. Ma R. S. Mishra M. W. Mahoney R. Grimes 《Metallurgical and Materials Transactions A》2005,36(6):1447-1458
The effect of friction stir processing on the superplastic behavior of extruded Al-4Mg-1Zr was examined at 350 °C to 600 °C
and at initial strain rates of 1×10−3 to 1 s−1. A combination of a fine grain size of 1.5 μm and high-angle grain boundaries in the friction stir-processed (FSP) alloy led to considerably enhanced superplastic ductility,
much-reduced flow stress, and a shift to a higher optimum strain rate and lower optimum temperature. The as-extruded alloy
exhibited the highest superplastic ductility of 1015 pct at 580 °C and an initial strain rate of 1×10−2s−1, whereas a maximum elongation of 1280 pct was obtained at 525 °C and an initial strain rate of 1×10−1s−1 for the FSP alloy. The FSP alloy exhibited enhanced superplastic deformation kinetics compared to that predicted by the constitutive
relationship for superplasticity in fine-grained aluminum alloys. A possible origin for enhanced superplastic deformation
kinetics in the FSP condition is proposed. 相似文献
2.
The kinetics of Nb (Cb) nitridation and ε-NbN growth obey a parabolic relationship and their temperature dependence can be
expressed ask
p = 5.19 × 10−7 exp (−125,500/RT) and (k
p
ξ = 1.15 × 10−4 exp (−61,500/RT), respectively, with the activation energies in joules/mole. The nitrogen diffusion coefficient in niobium, obtained from
microhardness traverses, is given byD = 1.02 × 10−5 exp (−77,000/RT). A diffusion model accounting for the partition of nitrogen between ε-NbN and Nb is proposed. The total nitrogen uptake
calculated from the model is compared to that obtained experimentally. 相似文献
3.
K. F. Laneri J. Desimoni R. C. Mercader R. W. Gregorutti J. L. Sarutti 《Metallurgical and Materials Transactions A》2001,32(1):51-58
The evolution of the relative fraction of high-carbon austenite with austempering time and temperature was analyzed in a compacted graphite (CG) cast iron (average composition, in wt pct: 3.40C, 2.8Si, 0.8Mn, 0.04Cu, 0.01P, and 0.02S) at five different austempering temperatures between 573 and 673 K. Samples were characterized by Mössbauer spectroscopy, hardness measurements, and optical microscopy. During the first stage of transformation, the kinetics parameters were determined using the Johnson-Mehl’s equation, and their dependence with temperature in the range from 573 to 673 K indicates that the transformation is governed by nucleation and growth processes. The balance between growth-rate kinetics and nucleation kinetics causes the kinetics parameter (k) to have a maximum at ≈623 K of 3.9×10?3(s?1). The evolution of the C content in the high-carbon austenite was found to be controlled by the volume diffusion of carbon atoms from the ferrite/austenite interface into austenite, with a dependence of t 0.40±0.05 on the austempering time (t). 相似文献
4.
The plastic deformation kinetics of a commercial fine-grained alumina with ∼300 ppm MgO and grain sizes from 1.4 to 2.9 μm were determined in tension at 1475 °C to 1600 °C and strain rates from 10−5 to 10−3 s−1, employing stress relaxation (SR) as the principal test mode. The constants in the Weertman-Dorn (W-D) equation were determined
and had the following values: A=2.9±0.5×109, n=2.2±0.1, p=1.9±0.1, Q=492±3 kJ/mole, and threshold stress σ
0=0. These constants are in accord with grain boundary sliding (GBS) accommodated by dislocation glide and climb with Al3+ ion lattice diffusion as the rate-controlling mechanism. 相似文献
5.
Effects of temperature and strain rate on tensile properties and activation energy for dynamic strain aging in alloy 625 总被引:1,自引:0,他引:1
Vani Shankar M. Valsan K. Bhanu Sankara Rao S. L. Mannan 《Metallurgical and Materials Transactions A》2004,35(10):3129-3139
Alloy 625 ammonia cracker tubes were service exposed for 60,000 hours at 873 K. These were then subjected to a solution-annealing
treatment at 1473 K for 0.5 hours. The effects of temperature and strain rate on the tensile properties of the solution-annealed
alloy were examined in the temperature range of 300 to 1023 K, employing the strain rates in the range of 3×10−5 s−1 to 3×10−3 s−1. At intermediate temperatures (523 to 923 K), various manifestations of dynamic strain aging (DSA) such as serrated flow,
peaks, and plateaus in the variations of yield strength (YS) and ultimate tensile strength (UTS) and work-hardening rate with
temperature were observed. The activation energy for serrated flow (Q) was determined by employing various methodologies for T<823 K, where a normal Portevien-Le Chatelier effect (PLE) was observed. The value of Q was found to be independent of the method employed. The average Q value of 98 kJ/mol was found to be in agreement with that for Mo migration in a Ni matrix. At elevated temperatures (T≥823 K), type-C serrations and an inverse PLE was noticed. The decrease in uniform elongation beyond 873 K for 3×10−5 s−1 and 3×10−3 s−1 and beyond 923 K for 3×10−4 s−1 strain rates seen in this alloy has been ascribed to reduction in ductility due to precipitation of carbides and δ phase on the grain boundaries. 相似文献
6.
J. Daniel Whittenberger 《Metallurgical and Materials Transactions A》1979,10(9):1285-1295
The tensile behavior of the oxide dispersion strengthened iron-base alloy MA 956 was investigated as a function of strain-rate
ranging from 3.3×10−2 to 8.3×10−8 s−1 at 1366 K. All tests were conducted in the longitudinal direction on specimens machined from bar stock. Because of the microstructure
of this alloy, all specimens were either single crystals or bicrystals with the boundary parallel to the gage length. Testing
revealed that the strength was rather insensitive to strain-rate, the tensile ductility decreased with decreasing strain-rate,
and for strain-rates ≤8.3×10−5 s−1, the alloy fractured in brittle manner. Evidence of transgranular cracking perpendicular to the applied stress was observed
at all strain-rates; failure at strain-rates ≤8.3×10−5 s−1 was due to cracks which grow by the joining together of cavities ahead of the running crack. This alloy appears to possess
a critical stress intensity factor for rapid crack growth. 相似文献
7.
S. V. Raj 《Metallurgical and Materials Transactions A》1992,23(6):1691-1703
Tensile tests were conducted on a Ni-30 (at. pct) Al-20Fe-0.05Zr intermetallic alloy in the temperature range 300 to 1300
K under initial strain rates varying between 10−6 and 10−3 s−1. The alloy did not exhibit any room-temperature ductility and failed at an average stress of about 710 MPa. The brittle-to-ductile
transition temperature (BDTT), which was higher than those for Ni-50Al and Ni-50Al-0.05Zr, was relatively insensitive to strain
rate and varied between about 960 K at a nominal strain rate of 1.4×10−5s−1 to about 990 K at a strain rate of 1.4× 10−3s−1. Detailed observations of the fracture surfaces revealed that cleavage failure had originated at a pre-existing defect in
all instances, where the fracture stress, σ
f
, correlated extremely well with the square root of the average defect size, 2c, in accordance with linear elastic fracture mechanics. The average value of the critical stress intensity factor estimated
from the σ
f
− 2c data varied between 4 to 7 MPa m1/2. A comparison of the fracture map for this intermetallic alloy with those for face-centered cubic (fcc) and refractory body-centered
cubic (bcc) metals, alkali halides, refractory oxides, and covalent-bonded ceramics indicated that the low-temperature brittleness
of the alloy is, in part, due to mixed atomic bonding. 相似文献
8.
Felipe P. Calderon Nobuo Sano Yukio Matsushita 《Metallurgical and Materials Transactions B》1971,2(12):3325-3332
The measurement of the diffusivities of manganese and silicon in molten binary ferroalloys over the whole range of composition
was undertaken to clarify existing but conflicting data at lower concentrations, to present new data at higher concentrations
and to indirectly confirm the behavior of both systems observed in thermodynamic studies. The experiments were carried out
under argon atmosphere in a Tammann furnace. The diffusion couples were held in 5 mm ID alumina tubes (98 pct Al2O3). Electron probe microanalysis of the samples led to a diffusion-penetration curve for the system under consideration. Results
obtained over the whole range of composition showed a slight negative deviation for the Fe−Mn system and a very large positive
deviation for the Fe−Si system. At lower concentrations (0 to 4 pct Mn), the temperature dependence of managanese diffusivity
for the Fe−Mn binary alloy in the temperature range 1550° to 1700°C is as follows:D
Fe−Mn=1.8×10−3 exp (−13,000/RT) cm2/sec
The concentration dependence of manganese diffusivity for the same system at 1600°C may be expressed asD
Fe−Mn={5.48−0.0137 (%Mn)+0.000276 (%Mn)2}×10−5 cm2/sec
The temperature dependence of silicon diffusivity for the Fe−Si binary system in the temperature range 1550° to 1725°C at
various concentrations is as follows:D
Fe−Si=2.8×10−3 exp (−11,900/RT) cm2/sec at 20 pct SiD
Fe−Si=2.1×10−3 exp (−13,200/RT) cm2/sec at 12.5 pct SiD
Fe−Si=5.1×10−4 exp (−9,150/RT) cm2/sec at 2.2 pct Si
FELIPE P. CALDERON, formerly Graduate Student. University of Tokyo, Tokyo, Japan.
This paper is based on a portion of a thesis submitted by FELIPE P. CALDERON in partial fulfillment of the requirements for
the degree of Doctor of Engineering at University of Tokyo. 相似文献
9.
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
10.
The reaction between zinc sulfide and water vapor is a component reaction in a new reaction scheme recently developed to transform
zinc sulfide to zinc oxide through the use of lime and water vapor. The intrinsic kinetics of this reaction for ultrafinely
ground (<1 μm) ZnS particles was determined by carrying out measurements in the absence of heat- and mass-transfer effects.
The reaction products were identified to be ZnO and H2S. The kinetics of the reaction can be represented by
and 3.94<C
H2O <9.84 mol/m3 withn=1.28 andk
1=45.3 exp(−14600/T) m3·mol−1·s−1, whereX
B is the fractional conversion.
DAESOO KIM, formerly Granduate Student at the University of Utah 相似文献
11.
A study of high temperature deformation of a commercial aluminum alloy has been undertaken through tensile tests at strain
rates ranging from 5.6×10−5 s−1 to 5.6×10−2 s−1 and load relaxation testing in the temperature range 473 to 873 K. Experiments have established that maximum ductility is
reached at about 623 K and at maximum strain rates. Maximum fracture ductility corresponds to minimum uniform elongation.
The deformation and fracture mechanisms operating in the temperature range 473 to 573 K seem to differ from those between
623 K and 823 K; different strain rate sensitivities are also observed. Dynamic recovery is the dominant softening mechanism
in high temperature plastic deformation—that is, a thermally activated process whose kinetics can be suitably described by
an empirical power relation. 相似文献
12.
The kinetics of hydrogen reduction of thin, dense strips of hematite were investigated in the range 245 °C to 482 °C. Pure
hydrogen gas at 1 atm was used as the reducing agent. Because of the relative thinness (only 136 /μm thick) of the specimens
used, the pore-diffusion of gases offered no significant resistance to the reduction process. The interfacial-reaction-rate
constantk
s
*
, which has been corrected for film-mass-transfer effects, is found to be given by logk
s
*
= −1.032 (±0.138) -[7860 (±200)]/2.303r where k
s
*
is in g · atom O · cm−2 · s−1 · atm−1. The activation energy for the reduction process is found to be 65,325 (±1650) J · mol−1; the rate-controlling step appears to be the Fe3O4 → Fe conversion step. 相似文献
13.
Y. Li J. A. Lucas G. M. Evans I. P. Ratchey G. R. Belton 《Metallurgical and Materials Transactions B》2000,31(5):1049-1057
The interfacial reaction rate between liquid iron oxide and CO-CO2 was determined using a thermogravimetric technique. The measured rates were controlled by the chemical reactions at the gas-slag
interface. The apparent first-order rate constant, for the oxidation of liquid iron oxide by CO2, decreased sharply with the equilibrium CO2/CO ratio. The rate of reduction of liquid iron oxide by CO showed a slight increase with the oxidation state of the melt.
At 1773 K, the apparent first-order rate constants are given by k=4.0×10−5(CO2/CO)−0.8 and k=4.0 × 10−5(CO2/CO)0.18 mol cm−2 s−1 atm−1 for the oxidation and reduction, respectively. The addition of basic oxides, such as BaO and CaO, resulted in an increased
reaction rate, while the addition of acidic oxide, such as SiO2, decreased the rate. The results are consistent with the dissociation or formation of the CO2 molecule, involving the transfer of two charges, being the rate controlling mechanism of the reactions.
This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney,
Australia, under the joint sponsorship of ISS and TMS. 相似文献
14.
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
15.
S. L. Semiatin M. W. Corbett P. N. Fagin G. A. Salishchev C. S. Lee 《Metallurgical and Materials Transactions A》2006,37(4):1125-1136
The dynamic-coarsening behavior of Ti-6Al-4V with an equiaxed α microstructure was established via isothermal hot-compression
testing of cylindrical samples cut from an ultra-fine-grain-size (UFG) billet. Compression experiments were conducted at 900
and 955 °C, strain rates between 10−4 and 1 s−1, and imposed true strains between 0 and 1.4. Following deformation, quantitative metallography revealed marked coarsening
of the primary α particles at low strain rates (10−4 and 10−3 s−1). The dynamic-coarsening rate followed rn
vs time kinetics, in which n was between 2 and 3, or behavior between those of bulk-diffusion and interface-reaction controlled.
An examination of the temperature and strain-rate dependence of theoretical coarsening rates, however, strongly suggested
that bulk diffusion (with n=3) was more important. The dynamic-coarsening behavior was also interpreted in the context of the observed plastic-flow behavior.
At low strain rates, high values of the strain-rate sensitivity (m>0.5) and the overall shape of log stress-log strain rate plots indicated that the majority of the imposed strain was accommodated
by grain-boundary sliding (gbs) and only a small amount via dislocation glide/climb processes. In addition, an analysis of
the flow hardening that accompanied dynamic coarsening indicated that the flow stress varied approximately linearly with the
α particle size, thus providing support for models based on gbs accommodation by dislocation activity in grain-mantle regions. 相似文献
16.
The deformation behavior of an extruded Ni-30 (at. pct) Al−20Fe−0.05Zr intermetallic alloy was studied in the temperature
range of 300 to 1300 K under initial tensile strain rates varying between about 10−6 and 2×10−3 s−1 and in constant load compression creep between 1073 and 1300 K. The deformation microstructures of the fractured specimens
were characterized by transmission electron microscopy (TEM). Three deformation regimes were observed: Region I consisted
of an athermal regime of low tensile ductility (less than 0.3 pct) occurring between 400 and 673 K, where the substructure
consisted of slip bands in a few grains. Exponential creep was dominant in region II between 673 and 1073 K, where the substructure
changed from a mixture of dislocation tangles, loops, and dipoles at 673 K to a microstructure consisting of subgrains and
dislocation tangles at 973 K. The tensile ductility was generally about 2.0 to 2.5 pct below 980 K in this region. A significant
improvement in tensile ductility was observed in region III, which occurred between 1073 and 1300 K. An analysis of the data
suggests that viscous glide creep with a stress exponent,n, of about 3 and high-temperature dislocation climb withn≈4.5 where the two dominant creep mechanisms in this region depending on stress and temperature. The average activation energy
for deformation in this region was about 310±30 kJ mol−1 for both processes. In this case, a transition from viscous glide creep to dislocation climb occurred when σ/E<1.7×10−4, where σ is the applied stress andE is the Young’s modulus. 相似文献
17.
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
18.
The plastic deformation of austenitic iron, represented by a zone-refined iron, an electrolytic iron, an Fe−0.05 C alloy,
and an Fe−5.2 Mn alloy, has been documented for the temperature range 950 to 1350°C (1740 to 2460°F) and the strain-rate range
2.8 × 10−5 to 2.3 × 10−2 s−1. The intrusion of recrystallization during deformation restricts the documentation to initial periods of strain usually less
than 0.10. The general problem of retaining grain structures representative of polycrystals in specimens annealed at temperatures
above 0.95T
m is recognized, and a basis for its solution is presented. Chemical composion appears to influence the plastic-flow behavior
of austenitic iron primarily through its effect on the grain structure. Thus, the large-grained zone-refined iron is relatively
weak, and the difference in behavior between the Fe−0.05 C alloy and the Fe−5.2 Mn alloy is small.
Formerly Associate Scientist, U.S. Steel Corporation. 相似文献
19.
S. N. S. Reddy L. B. Wiggins D. N. Leonard K. T. Jacob 《Metallurgical and Materials Transactions A》2005,36(10):2685-2694
As a model of an internal displacement reaction involving a ternary oxide “line” compound, the following reaction was studied
at 1273 K as a function of time, t:
Both polycrystalline and single-crystal materials were used as the starting NiTiO3 oxide. During the reaction, the Ni in the oxide compound is displaced by Fe and it precipitates as a γ-(Ni-Fe) alloy. The reaction preserves the starting ilmenite structure. The product oxide has a constant Ti concentration
across the reaction zone, with variation in the concentration of Fe and Ni, consistent with ilmenite composition. In the case
of single-crystal NiTiO3 as the starting oxide, the γ alloy has a “layered” structure and the layer separation is suggestive of Liesegang-type precipitation. In the case of polycrystalline
NiTiO3 as the starting oxide, the alloy precipitates mainly along grain boundaries, with some particles inside the grains. A concentration
gradient exists in the alloy across the reaction zone and the composition is >95 at. pct Ni at the reaction front. The parabolic
rate constant for the reaction is k
p
=1.3 × 10−12 m2 s−1 and is nearly the same for both single-crystal and polycrystalline oxides. 相似文献
20.
Superplastic behavior and microstructure evolution in a commercial Al-Mg-Sc alloy subjected to intense plastic straining 总被引:1,自引:0,他引:1
F. Musin R. Kaibyshev Y. Motohashi G. Itoh 《Metallurgical and Materials Transactions A》2004,35(8):2383-2392
A commercial Al-6 pct Mg-0.3 pct Sc-0.3 pct Mn alloy subjected to equal-channel angular extrusion (ECAE) at 325 °C to a total
strain of about 16 resulted in an average grain size of about 1 μm. Superplastic properties and microstructural evolution
of the alloy were studied in tension at strain rates ranging from 1.4 × 10−5 to 1.4 s−1 in the temperature interval 250 °C to 500 °C. It was shown that this alloy exhibited superior superplastic properties in
the wide temperature range 250 °C to 500 °C at strain rates higher than 10−2 s−1. The highest elongation to failure of 2000 pct was attained at a temperature of 450 °C and an initial strain rate of 5.6
× 10−2 s−1 with the corresponding strain rate sensitivity coefficient of 0.46. An increase in temperature from 250 °C to 500 °C resulted
in a shift of the optimal strain rate for superplasticity, at which highest ductility appeared, to higher strain rates. Superior
superplastic properties of the commercial Al-Mg-Sc alloy are attributed to high stability of ultrafine grain structure under
static annealing and superplastic deformation at T ≤ 450 °C. Two different fracture mechanisms were revealed. At temperatures higher than 300 °C or strain rates less than 10−1 s−1, failure took place in a brittle manner almost without necking, and cavitation played a major role in the failure. In contrast,
at low temperatures or high strain rates, fracture occurred in a ductile manner by localized necking. The results suggest
that the development of ultrafine-grained structure in the commercial Al-Mg-Sc alloy enables superplastic deformation at high
strain rates and low temperatures, making the process of superplastic forming commercially attractive for the fabrication
of high-volume components. 相似文献