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1.
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
2.
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
3.
Hot ductility and fracture mechanisms of a C-Mn-Nb-Al steel 总被引:1,自引:0,他引:1
Hot-ductility tests of a C-Mn-Nb-Al steel were performed in a tensile machine at different strain rates of 1×10−4, 3×10−4, 1×10−3, and 3×10−3 s−1 and at temperatures of 650 °C, 710 °C, 770 °C, 840 °C, 900 °C, 960 °C, and 1020 °C, which are close to the continuous casting
conditions of steel. Fracture surfaces were examined using a scanning electron microscope. It was found that low strain rates
and coarse austenitic grains decrease hot ductility. At all test temperatures, when the strain rate decreases, the hot ductility
also decreases because the void growth mechanism predominates over void nucleation, giving time for nucleated cracks to grow.
This leads, finally, to the catastrophic failure. The minimum hot ductility was found at 900 °C for all strain rates, and
the fracture was intergranular. Fractographic evidence showed that the voids formed during the deformation surrounded the
austenite grains, indicating that the deformation was concentrated in ferrite bands located in the same places when the testing
temperature was in the two-phase field. 相似文献
4.
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
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.
M. Shanmugavel M. Nandagopal R. Sandhya K. Bhanu Sankara Rao R. Gnanamoorthy 《Transactions of the Indian Institute of Metals》2008,61(4):301-306
Type 316 L(N) Stainless Steel (SS) is being currently used as a structural material for various components of Prototype Fast
Breeder Reactor (PFBR). The possibility of using 316 L(N) electrodes for fabrication of 316 L(N) welding joints is being critically
examined. This paper discusses about the evaluation of activation energy for Dynamic Strain Aging (DSA) process in 316L(N)/316(N)
SS Weld Joints. The Gas Tungsten Arc Welding (GTAW) process was used for the root pass and Gas Metal Arc Welding (GMAW) process
was used for the remaining passes. Tensile tests have been conducted in the wide temperature range from room temperature to
1023 K at a strain rate of 3 × 10−3 s−1. Yield stress showed a continuous decrease with increasing temperature, with a plateau being observed between 823 and 923
K. A minima in elongation was also observed in this temperature range. These two properties being manifestations of dynamic
strain aging, further tests at different strain rates (3 × 10−5 s−1 to 3 × 10−2 s−1) were conducted in this temperature range. Detailed analyses of the results were carried out and the solute responsible for
dynamic strain aging was identified to be substitutional chromium. Post test analysis of fracture surfaces and deformation
substructures were correlated with the changes in tensile properties at different testing temperatures. 相似文献
7.
E. Frás A. Janas S. Wierzbiński H. F. Lopez 《Metallurgical and Materials Transactions A》2002,33(12):3831-3838
The deformation behavior of TiC particulate-reinforced aluminum composites (Al-TiC
p
) was investigated in this work using pure aluminum as the reference matrix material. Uniaxial compression tests were carried
out at 293 and 623 K and at two strain rates (3.7×10−4 and 3.7×10−3 s−1). Yield strengths of up to 127 MPa were found in composites containing 10 vol pct TiC particulates, which were almost 4 times
the yield strength of pure Al. In addition, at 623 K, relatively small reductions in yield strength were found, suggesting
that this property was rather insensitive to temperature for the temperatures investigated in this work. Nevertheless, at
623 K, increasing the rate of straining from 3.7×10−4 s−1 to 3.7×10−3 s−1 lowered the yield strength, particularly in 10 vol pct TiC
p
-Al composites. Two stages of work hardening were identified in pure Al and a 10 vol pct TiC
p
composite during plastic flow through the modified version of the Hollomon equation (σ = Kɛ
n ± Δ). In particular, the work-hardening exponents found in pure Al shifted from high to low values as the extent of plastic
strain was increased while the opposite was true for the 10 vol pct TiC
p
composite. Finally, at 623 K, dynamic recovery mechanisms became dominant at plastic strain levels >0.2 in 10 vol pct TiC
p
-Al composites, with the effect being minor at room temperature. 相似文献
8.
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
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.
The influence of strain rate and environment on the fracture behavior of a two-phase TiAl-alloy, Ti-47Al-2.6Nb-2(Cr + V),
heat-treated to a nearly fully lamellar microstructure has been studied by performing conventional tensile, compression, and
fracture toughness tests in air, argon, and vacuum at 25 °C and 800 °C. Both tensile and compression tests were conducted
at strain rates of 1 × 10−3 and 1 × 10−5 s−1, and fracture toughness tests were performed under displacement rates of 0.25 to 2.5 mm/min. In addition,in situ fracture toughness tests were conducted at slow rates both in vacuum and in air. The results indicated that both strain rate
and environment affected the tensile stress-strain behavior and ductility and the fracture resistance of the TiAl-alloy at
800 °C. In contrast, neither the tensile ductility nor the fracture toughness was significantly affected by the environment
at ambient temperature. For compression in air, the stress-strain behavior was insensitive to both strain rate and test temperature
within the conditions tested. Studies of fracture surfaces revealed that low tensile ductility in this alloy at ambient temperature
is associated with the tendency to delaminate alongγ/γ andγ/α
2 interfaces.
formerly with Metcut-Materials Research Group, Wright-Patterson AFB, Dayton, OH 45433-0511 相似文献
11.
S. Das A. T. Morales A. R. Riahi X. Meng-Burany A. T. Alpas 《Metallurgical and Materials Transactions A》2011,42(8):2384-2401
Friction maps have been developed to explain the behavior of aluminum alloys under dynamic tribological conditions generated
by the simultaneous effects of temperature and strain rate. A specially designed tribometer was used to measure the coefficient
of friction (COF) of AA5083 strips subjected to sliding with a simultaneous application of tensile strain in the temperature
range of 693 K to 818 K (420 °C to 545 °C) and strain rates between 5 × 10−3 s−1 and 4 × 10−2 s−1. The mechanisms of plastic deformation, namely, diffusional flow, grain boundary sliding (GBS), and solute drag (SD), and
their operation ranges were identified. Relationships between the bulk deformation mechanism and COF were represented in a
unified map by superimposing the regions of dominant deformation mechanisms on the COF map. The change in COF (from 1.0 at
693 K (420 °C) and 1 × 10−2 s−1 to 2.1 at 818 K (545 °C) and 4 × 10−2 s−1) was found to be largest in the temperature–strain rate region, where GBS was the dominant deformation mechanism, as a result
of increased surface roughness. The role of bulk deformation mechanisms on the evolution of the surface oxide layer damage
was also examined. 相似文献
12.
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
13.
Austenitic specimens of Fe-15 wt pct Ni-0.8 wt pct C were tested in tension at strain rates of 10−4 s−1 and 10−1 s−1 over the temperature range −20°C to 60 °C. The influence of strain rate and temperature on the deformation behavior depended
on whether stress-assisted or strain-induced martensitic trans-formation occurred during testing. Under conditions of stress-assisted
transformation, the ductility was low and independent of strain rate. However, when strain-induced transformation occurred,
the duc-tility increased significantly and the higher strain rate resulted in greater ductility and more transfor-mation.
Although the ductility increased continuously with temperature, the amount of strain-induced transformation decreased and
no martensite was observed above 40 °C. Microstructural examination showed that the martensite was replaced by intense bands
and that these bands contained very fine (111) fcc twins. The twinning resulted in enhanced plasticity by providing an additional
mode of deformation as slip became more difficult due to dynamic strain aging at the higher temperature. This study confirms
that the substructure following deformation will depend on the proximity of the deformation temperature to theM
s
σ
temperature. At temperatures much greater thanM
s
σ
, austenite twinning will occur, while at temperatures close toM
s
σ
, bcc martensite will form. 相似文献
14.
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
15.
D. Padmavardhani Y. V. R. K. Prasad 《Metallurgical and Materials Transactions A》1991,22(12):2993-3001
The hot deformation behaviors of β brass in the temperature range of 550°C to 800°C and α-β brass in the temperature range
of 450°C to 800°C have been characterized in the strain rate range of 0.001 to 100 s−1 using processing maps developed on the basis of the Dynamic Materials Model. The map for β brass revealed a domain of superplasticity
in the entire temperature range and at strain rates lower than 1 s−1, with a maximum efficiency of power dissipation of about 68 pct. The temperature variation of the efficiency of power dissipation
in the domain is similar to that of the diffusion coefficient for zinc in β brass, confirming that the diffusion-accommodated
flow controls the superplasticity. The material undergoes microstructural instability in the form of adiabatic shear bands
and strain markings at temperatures lower than 700°C and at strain rates higher than 10 s−1. The map for α-β brass revealed a wide domain for processing in the temperature range of 550°C to 800°C and at strain rates
lower than 1 s−1, with a maximum efficiency of 54 pct occurring at about 750°C and 0.001 s−1. In the domain, the α phase undergoes dynamic recrystallization and controls the hot deformation of the alloy, while the
β phase deforms superplastically. At strain rates greater than 1 s−1, α-β brass exhibits microstructural instabilities manifested as flow rotations at lower temperatures and localized shear
bands at higher temperatures. 相似文献
16.
Characterization of superplastic deformation behavior of a fine grain 5083 Al alloy sheet 总被引:3,自引:0,他引:3
R. Verma P. A. Friedman A. K. Ghosh S. Kim C. Kim 《Metallurgical and Materials Transactions A》1996,27(7):1889-1898
Superplastic deformation behavior of a fine grain 5083 Al sheet (Al-4.2 pct Mg-0.7 pct Mn, trade name FORMALL 545) has been
investigated under uniaxial tension over the temperature range of 500 °C to 565 °C. Strain rate sensitivity values >0.3 were
observed over a strain rate range of 3 × 10−5 s−1 to 1 × 10−2 s−1, with a maximum value of 0.65 at 5 × 10−4 s−1 and 565 °C. Tensile elongations at constant strain rate exceeded 400 pct; elongations in the range of 500 to 600 pct were
obtained under constant crosshead speed and variable strain rates. A short but rapid prestraining step, prior to a slower
superplastic strain rate, provided enhanced tensile elongation at all temperatures. Under the two-step schedule, a maximum
tensile elongation of 600 pct was obtained at 550 °C, which was regarded as the optimum superplastic temperature under this
condition. Dynamic and static grain growth were examined as functions of time and strain rate. It was observed that the dynamic
grain growth rate was appreciably higher than the static growth rate and that the dynamic growth rate based on time was more
rapid at the higher strain rate. Cavitation occurred during superplastic flow in this alloy and was a strong function of strain
rate and temperature. The degree of cavitation was minimized by superimposition of a 5.5 MPa hydrostatic pressure during deformation,
which produced a tensile elongation of 671 pct at 525 °C.
R. VERMA, formerly Visiting Scientist, Department of Materials Science and Engineering, University of Michigan 相似文献
17.
18.
Metalloceramic molybdenum is tested in the temperature range from −100 to +350°C and the strain rate range from 3.4 × 10−3 to 4 × 103 s−1. This wide strain rate range, which was used for the first time to study molybdenum, makes it possible to find the laws of
changes in its strength and deformation characteristics. It is found that the ductile-brittle transition temperature increases
from −90 to +250°C in this strain rate range, the dependences of the lower and upper transition temperatures on the strain
rate intersect at $
\dot \varepsilon
$
\dot \varepsilon
= 105.7 s−1, and the effect of the test temperature on the yield strength weakens with increasing strain rate and disappears at $
\dot \varepsilon
$
\dot \varepsilon
= 105.9 s−1. The closeness of these two strain rates calculated using two different approximations suggests the presence of a certain
limiting critical strain rate beginning from which molybdenum is in a brittle state. 相似文献
19.
Tensile tests were conducted at various temperatures (77 to 550 K) and strain rates (10−5 to 10−1 s−1) in order to study the effects of hydrogen on the ductility loss and the intergranular fracture of hydrogen-charged (32 wt
ppm) tensile specimens of alloy 600. The H-induced intergranular cracking was shown to require H segregation to grain boundaries
(GBs) during plastic deformation. The concordance between the temperature/strain rate domains, where H-induced intergranular
rupture of alloy 600 is observed and those of H transport by dislocations, is in favor of a major influence of this mechanism
of H transport on the intergranular rupture of H-charged alloy 600 in the 180 to 500 K temperature range. The possible contribution
of this mechanism to intergranular stress corrosion cracking (IGSCC) of alloy 600 in the pressurized water reactor (PWR) environment
is discussed. 相似文献
20.
This study investigates the effects of the strain rate and the relative sintered density on the mechanical response and fracture
behavior of 316L sintered stainless steel. Low strain rate compression tests are conducted on an MTS 810 servohydraulic machine
at strain rates of 10−3 to 10−1 s−1, while dynamic impact tests are performed using a split-Hopkinson bar at strain, rates of 3×103 to 9×103 s−1. The Taguchi method with an L9 orthogonal array is used to characterize and optimize the sintering process control factors such that the specimens have
three different relative sintered densities, i.e., 83, 88, and 93 pct. It is found that the strain rate and relative sintered density have significant effects on the flow
stress, fracture strain, strain rate sensitivity, and activation volume. The significant differences observed in the strain
rate sensitivity and activation volume in the high and low strain rate tests indicate that the corresponding deformation is
dominated by different rate controlling mechanisms. Furthermore, the changes in strain rate sensitivity and thermal activation
volume observed at different levels of the relative sintered density are related to the work hardening stress. At high strain
rate and relative sintered densities slip deformation in the form of slip bands is frequently observed within the grains.
Therefore, it appears that higher strain rates and relative sintered densities represent favorable conditions for the formation
of shear bands and cracking, and hence lead to premature specimen fracture. The fracture surfaces contain dimplelike structures,
which are indicative of a ductile fracture mode. The depth and the density of these dimples decrease as the strain rate and
relative sintered density increase, indicating a loss of ductility. 相似文献