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Bing Q. Han Enrique J. Lavernia Farghalli A. Mohamed 《Metallurgical and Materials Transactions A》2005,36(2):345-355
In the present study, the mechanical behavior of a cryomilled Al-7.5 pct Mg-0.3 pct Sc alloy was investigated at temperatures
in the range of 298 to 648 K. The grain size of the as-extruded alloy was determined to be approximately 200 nm by transmission
electron microscope (TEM) and X-ray diffraction (XRD) analysis. The data indicate that as a result of cryomilling, a supersaturated
solid solution with high thermal stability was formed in the Al-Mg-Sc alloy. The high strength at room temperature was primarily
attributed to three types of strengthening: grain size effect, solid solution hardening, and Orowan strengthening. The elevated
temperature mechanical behavior of the Al-Mg-Sc alloy exhibits the following: (a) a strain-rate sensitivity, m, of less than 0.2; and (b) an activation energy, Q, that increases from 139 to 193 kJ/mol with increasing applied stress. An analysis of the experimental data at elevated temperatures
shows that despite the fine-grained structure of the alloy, the deformation characteristics are not consistent with those
arising from a superplastic deformation process that incorporates a threshold stress. On the other hand, the analysis suggests
that the deformation characteristics agree with those associated with the transition in the creep behavior of Al-based solid
solution alloys from that for the intermediate-stress region, where m=0.33 and Q=Q
D
(Q
D
is the activation energy for self-diffusion in Al), to that of the high-stress region, where m<0.2 and Q>Q
D
. 相似文献
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The kinetics of dynamic spheroidization of the lamellar microstructure and the associated flow-softening behavior during isothermal,
constant-strain-rate deformation of a gamma titanium aluminide alloy were investigated, with special emphasis on the role
of the prior-alpha grain/colony size. For this purpose, fully lamellar microstructures with prior-alpha grain sizes between
80 and 900 μm were developed in a Ti-45.5Al-2Nb-2Cr alloy using a special forging and heat-treatment schedule. Isothermal hot compression
tests were conducted at 1093 °C and strain rates of 0.001, 0.1, and 1.0 s−1 on specimens with different grain sizes. The flow curves from these tests showed a very strong dependence of peak flow stress
and flow-softening rate on grain size; both parameters increased with alpha grain/colony size. Microstructures of the upset
test specimens revealed the presence of fine, equiaxed grains of γ + α
2 + β phases resulting from the dynamic spheroidization process that initiated at and proceeded inward from the prior-alpha grain/colony
boundaries. The grain interiors displayed evidence of microkinking of the lamellae. The frequency and severity of kinking
increased with strain, but were also strongly dependent on the local orientation of lamellae with respect to the compression
axis. The kinetics of dynamic spheroidization were found to increase as the strain rate decreased for a given alpha grain
size and to decrease with increasing alpha grain size at a given strain rate. The breakdown of the lamellar structure during
hot deformation occurred through a combination of events, including shear localization along grain/colony boundaries, microbuckling
of the lamellae, and the formation of equiaxed particles of γ + β
2 + α
2 on grain/colony boundaries and in zones of localized high deformation within the microbuckled regions. 相似文献
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A Ni-52 at. pct Ti shape memory alloy, cold drawn to 30 pct, was annealed at 1173 K for 1 hour, water quenched, and then subjected
to differential scanning calorimetry (DSC). No evidence of the premartensitic R transformation was found during either the forward or the reverse transformation. Microstructurally, it was found that the
alloy possessed a relatively large volume fraction (∼0.05) of coarse second-phase brittle particles. These precipitates acted
as preferential sites for martensite plate nucleation and gave rise to a “starlike” morphology. The tensile and compressive
properties of the alloy in the as-received condition were also investigated. The alloy exhibited relatively good ductility
(fracture strain = 0.28), which was attributed to its inherent ability to relieve or delay the development of plastic instabilities
through rapid strain hardening. In addition, X-ray diffraction (XRD) of deformed specimens indicated the presence of an extraintensity
peak corresponding to the B2 phase (110)B2 when the alloy was plastically deformed in compression. Accordingly, it is suggested that plastic deformation induces the
reverse transformation to the B2 phase in highly stressed local regions. Transmission electron microscopy (TEM) of deformed
martensite structures showed slip lines probably due to dislocation slip, as well as variant interpenetration. Besides, optical
and scanning microscopy of regions adjacent to the fractured surfaces indicated that fine martensite plates and/or “apparent”
new grains develop at regions of prior stress intensification (former crack-tip regions) during crack propagation. 相似文献
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A Ni-52 at. pct Ti shape memory alloy, cold drawn to 30 pct, was annealed at 1173 K for 1 hour, water quenched, and then subjected
to differential scanning calorimetry (DSC). No evidence of the premartensiticR transformation was found during either the forward or the reverse transformation. Microstructurally, it was found that the
alloy possessed a relatively large volume fraction (∼0.05) of coarse second-phase brittle particles. These precipitates acted
as preferential sites for martensite plate nucleation and gave rise to a “starlike” morphology. The tensile and compressive
properties of the alloy in the as-received condition were also investigated. The alloy exhibited relatively good ductility
(fracture strain=0.28), which was attributed to its inherent ability to relieve or delay the development of plastic instabilities
through rapid strain hardening. In addition, X-ray diffraction (XRD) of deformed specimens indicated the presence of an extraintensity
peak corresponding to the B2 phase (110)B2 when the alloy was plastically deformed in compression. Accordingly, it is suggested that plastic deformation induces the
reverse transformation to the B2 phase in highly stressed local regions. Transmission electron microscopy (TEM) of deformed
martensite structures showed slip lines probably due to dislocation slip, as well as variant interpenetration. Besides, optical
and scanning microscopy of regions adjacent to the fractured surfaces indicated that fine martensite plates and/or “apparent”
new grains develop at regions of prior stress intensification (former crack-tip regions) during crack propagation. 相似文献
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Prabir K. Chaudhury V. Sivaramakrishnan Farghalli A. Mohamed 《Metallurgical and Materials Transactions A》1988,19(11):2741-2752
The superplastic deformation behavior of two grades of the Zn-22 pct Al alloy was studied in detail under identical conditions
of grain size, temperature, and stress. While these two grades were prepared by the same procedure, they have different impurity
levels; grade 1 is of commercial purity (180 ppm of impurities) and grade 2 is of very high purity (6 ppm of impurities).
The experimental results on the commercial grade show that the relationship between the applied stress and steady-state strain
rate is sigmoidal and is manifested by the presence of three deformation regions; in region I (low-stress region) the stress
exponent,n, is 3.8 and the activation energy for creep,Q, is higher than that for grain boundary diffusion,Q
gb; in region II (intermediate-stress region)n = 2.5 andQ ≃ Q
gb; and in region III (high-stress region)n again increases. The results on the high purity grade, when compared with those on the commercial grade, reveal a significant
difference: the high purity grade, unlike the commercial grade, does not exhibit region I at low stresses. The difference
in creep behavior between the two grades of Zn-22 pct Al at low stresses leads to the implication that the origin of region
I during superplastic flow is related to the presence of impurity atoms. It is suggested, on the basis of consideration of
various impurity-controlled deformation processes, that the creep behavior in region I is most likely a consequence of the
existence of a threshold stress which is caused by impurity atom segregation at boundaries.
Formerly Graduate Student, Department of Mechanical Engineering, University of California, Irvine 相似文献
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Influence of intense plastic straining on grain refinement,precipitation, and mechanical properties of Al-Cu-Li-Based alloys 总被引:5,自引:0,他引:5
H. G. Salem R. E. Goforth K. T. Hartwig 《Metallurgical and Materials Transactions A》2003,34(5):1153-1161
Grain refinement is one of the major interests when an ultrahigh strength/ductility combination is demanded for ambient and
cryogenic temperature applications, especially when superplastic forming (SPF) is involved for the manufacturing of different
aerospace structures. Equal-channel angular extrusion (ECAE) is a relatively new metalworking process, which is capable of
producing an ultrafine, submicron-grained (SMG) structure by means of intense plastic straining without a change in the shape
or dimensions of the worked material. In the current research work, the influence of ECAE processing on the room-temperature
mechanical properties of Al-Cu-Li-Mg-Ag-Zr alloys in the T4 and T6 temper conditions is investigated. An ultrafine SMG structure
of 0.2 to 0.4 μm was produced for the ECAE-processed alloys from an initial grain size of >100 μm, which is compared with a conventionally processed superplastic Weldalite sheet material with an ∼1.5 μm grain size. The ECAE processing eliminates the precipitation-free zones (PFZs) in the T6 temper condition without the need
for prior stretching. A significant improvement in the mechanical properties at room temperature is achieved by ECAE processing
in comparison with conventional processing. 相似文献
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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
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《Scripta Metallurgica et Materialia》1990,24(7):1319-1322
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V. L. Tellkamp E. J. Lavernia A. Melmed 《Metallurgical and Materials Transactions A》2001,32(9):2335-2343
A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured
grain size. The powders were subsequently degassed, hot isostatically pressed, and extruded. The grain size at each processing
step was measured utilizing both X-ray diffraction and transmission electron microscopy (TEM). The mechanical properties of
the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. This processing technique was found to produce a thermally stable nanostructured aluminum alloy which maintained an average
grain size of 30 to 35 nm through several processing steps up to 0.61 T
mp
. The thermal stability was attributed to Zener pinning of the grain boundaries by AIN and Al2O3 particles and solute drag of numerous atomic species. The nanostructured 5083 was found to have a 30 pct increase in yield
strength and ultimate strength over the strongest commercially available form of 5083, with no corresponding decrease in elongation.
The enhanced ductility is attributed to the presence of a few large, single-crystal aluminum grains acting as crack-blunting
objects. 相似文献
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Plastic-flow and microstructure evolution during hot deformation of a gamma titanium aluminide alloy 总被引:3,自引:0,他引:3
The hot workability of a near gamma titanium aluminide alloy, Ti-49.5Al-2.5Nb-1.1Mn, was assessed in both the cast and the
wrought conditions through a series of tension tests conducted over a wide range of strain rates (10−4 to 100 s−1) and temperatures (850 °C to 1377 °C). Tensile flow curves for both materials exhibited sharp peaks at low strain levels
followed by pronounced necking and flow localization at high strain levels. A phenomenological analysis of the strain rate
and temperature dependence of the peak stress data yielded an average value of the strain rate sensitivity equal to 0.21 and
an apparent activation energy of ∼411 kJ/mol. At low strain rates, the tensile ductility displayed a maximum at ∼ 1050 °C
to 1150 °C, whereas at high strain rates, a sharp transition from a brittle behavior at low temperatures to a ductile behavior
at high temperatures was noticed. Dynamic recrystallization of the gamma phase was the major softening mechanism controlling
the growth and coalescence of cavities and wedge cracks in specimens deformed at strain rates of 10−4 to 10−2 s−1 and temperatures varying from 950 °C to 1250 °C. The dynamically recrystallized grain size followed a power-law relationship
with the Zener-Hollomon parameter. Deformation at temperatures higher than 1270 °C led to the formation of randomly oriented
alpha laths within the gamma grains at low strain levels followed by their reorientation and evolution into fibrous structures
containing γ + α phases, resulting in excellent ductility even at high strain rates. 相似文献
16.
Microstructure studies were conducted on grain-refined Mg-5.1 pct Zn-0.6 pct Zr alloy plates, sand-cast and end-chilled. Grains in this alloy are spherical, nondendritic and increase in diameter with distance from the chill. Coring in these, grains is spherical. The volume fraction of interspherical nonequilibrium secondary phase decreases with distance from the chill, whereas the volume fraction of microporosity increases. Solution kinetics of the secondary phase were found to depend on the dimensionless parameterDt/a 2, where:D is the diffusivity of zinc in this alloy,a the sphere radius andt the solutionizing time. The fatigue strength of this alloy at room temperature was measured in reversed bending at a stress level of 10,000 psi and was found to decrease with increasing distance from the chill, both in the as-cast and the cast-and-solutionized conditions. Solutionizing was found to increase fatigue life. Fatigue cracks were found to initiate in shear bands, most frequently at micropores, and to propagate transgranularly. 相似文献
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Experiments were performed on spheroidized 1045 steel, Cu-0.6 pct Cr alloy, and maraging steel containing respectively Fe3C, Cu-Cr, and TiC particles of nearly equiaxed shape. The local interfacial stresses for separation of these particles during
plastic deformation were evaluated by the methods described in the two preceding papers. The results show that the interfacial
strengths for these particles in their respective matrices are 242, 144, and 264 ksi. In the spheroidized steel the average
diam of the separated particles is distinctly larger than the average diam of the whole population. This is quantitatively
explained by the enhanced interfacial stresses developed in regions of above average volume fraction of second phase which
frequently occur in very dense populations of particles. No such effect was observed in the other two systems which is consistent
with their much lower volume fraction of second phase. Some tension experiments have also been performed with the spheroidized
1045 steel at elevated temperature, giving results qualitatively similar to those at room temperature.
This work has been presented in part orally at the Third International Conference on Fracture in Munich, Germany, April 1973. 相似文献
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Experiments were performed on spheroidized 1045 steel, Cu-0.6 pct Cr alloy, and maraging steel containing respectively Fe3C, Cu-Cr, and TiC particles of nearly equiaxed shape. The local interfacial stresses for separation of these particles during plastic deformation were evaluated by the methods described in the two preceding papers. The results show that the interfacial strengths for these particles in their respective matrices are 242, 144, and 264 ksi. In the spheroidized steel the average diam of the separated particles is distinctly larger than the average diam of the whole population. This is quantitatively explained by the enhanced interfacial stresses developed in regions of above average volume fraction of second phase which frequently occur in very dense populations of particles. No such effect was observed in the other two systems which is consistent with their much lower volume fraction of second phase. Some tension experiments have also been performed with the spheroidized 1045 steel at elevated temperature, giving results qualitatively similar to those at room temperature. 相似文献
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《Acta Metallurgica Materialia》1992,40(12):3357-3368
Distributions of equivalent plastic strains in an A16061/SiC fibre composite measured using the electron back scatter pattern (EBSP) technique were compared to plastic strain and stress distributions calculated using a continuum mechanics model solved by finite element analysis (FEA). Close to the interface EBSP measurements indicated higher dislocation densities than expected for strains calculated in such a region using the FEA model, the excess dislocations presumably being necessary to preserve the continuity of the interface. EBSP measurements also indicated considerable dislocation density in matrix regions where the FEA model calculated small plastic strains due to the production of a nearly hydrostatic tensile stress state. Inhomogeneities in the microstructure of the real matrix material can generate local shear stresses and so lead to production of dislocations even though the far field stress state has no shear component. Thus the dislocation density was controlled by the magnitude of the hydrostatic tension rather than the deviatoric stress components. 相似文献
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《Canadian Metallurgical Quarterly》2013,52(2):299-302
AbstractThe high-temperature ductility of a fine-grained commercial copper dispersion alloy (CDA 638 - Cu - 2.8% Al - 1.8% Si - 0.4%. Co) has been investigated in the temperature range 400°–800°C at strain rates of 10-2 to 1 min-I. Ductility was markedly temperature and strain rate dependent with a maximum of 320 % at 550°C and 3.9 × 10?2 min?1. The strain rate sensitivity of the flow stress was about 0.5 under these conditions and the behaviour can be termed ‘superplastic’. Extensive intergranular cavitation was observed which restricts the ductility; to optimize ductility in this alloy the factors of both cavitation and strain rate sensitivity must be evaluated. Résumé La ductilite d'un alliage commercial de cuivre à particules dispersées (CDA 638 – Cu: 2.8% Al, 1.8% Si, 0.4% Co) a été étudiée dans l'intervalle de température 400–800°C pour des vitesses de déformation entre 10?2 et 1 min?1. La ductilityé dépendant fortement de la température et de la vitesse de déformation avec une valeur maximale de 320% à 500°C pour une vitesse de 3.9 × 10?2 min?1. Ce comportement est considéré comme superplastique dans le mesure où l'exposant reliant la contrainte et la vitesse de déformation a une valeur de ~0.5 dans ces conditions. La ductilité est limitée par une cavitation intergranulaire importante et pour l'augmenter il faut tenir compte de ce facteur aussi. 相似文献