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1.
Part I of this study described the changes in order and of structure during cold rolling of a Ni3Al(B,Zr) alloy. The textural and microstructural changes that occur during deformation are reported in this part. In addition
to other features, a high density of shear bands start forming in this alloy from a rather early stage of deformation. The
cold rolling texture of the material, which is basically of pure metal type at low strain levels, changes into alloy type after rolling between 35 and 45 pct. The maximum pole density of the alloy type texture is obtained at the {168}〈211〉
location. Transmission electron microscopy (TEM) micrographs show the presence of twins in the material from a deformation
level of 35 pct onward, their density increasing with increase in deformation level. As has been proposed earlier, a structural
transformation from L12 to DO22 appears to take place in the γ′ phase during rolling. This will change the deformation mode from primarily slip to twinning
and this could be responsible for the observed textural change with rolling. The γ phase deforms by slip in a manner similar
to a fcc material with high stacking fault energy. The final texture of the material actually reflects an aggregate of the
components developed in the two phases. 相似文献
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The present article describes the microstructural changes during recrystallization annealing of a 73 pct cold-rolled Ni3Al(B,Zr) alloy along with a study of the recrystallization kinetics. The deformed γ regions, mostly within and near the shear bands, appear to recrystallize first. The recrystallization front leaves behind
a lamellar discontinuous precipitation within the newly formed strain-free γ grains, when annealing is done at lower temperatures. At higher annealing temperatures, the precipitates within γ assume a globular morphology. This precipitate is presumably made up of γ′ particles. When γ recrystallization is nearly complete, the γ′ regions start to recrystallize. The two-stage recrystallization process is also corroborated from the kinetics results,
which show that the activation energy up to 50 pct recrystallization of the material is only 117 kJ/mole, whereas beyond 50
pct until the completion of recrystallization, an activation energy of ∼274 kJ/mole is obtained. 相似文献
3.
This article describes the changes in order and texture of a 73 pct cold-rolled Ni3Al(B, Zr) alloy after it is subjected to recrystallization annealing. In the initial stages of annealing, the deformed starting
material undergoes both recovery and reordering. This is followed by recrystallization with continued annealing. The cold-rolled
material, which has a DO22 structure, undergoes a change in order during the recovery stage, leading to the formation of the earlier L12 structure. This is accompanied by a drastic decrease in the texture severity. The weak texture at the end of the recovery/reordering
stage persists even during recrystallization, and this finally leads to the formation of a weak texture. 相似文献
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《Acta Metallurgica Materialia》1992,40(8):1933-1943
A technique based on hydrogen embrittlement has been used to study the morphology and growth characteristics of grain boundary cavities in a Ni3Al alloy. The technique facilitates brittle intergranular fracture of specimens previously tested at elevated temperatures. This capability makes it possible to distinguish cavities that have formed by cavity coalescence and also determine cavity shape in three dimensions. Contrary to earlier reports of crack-like cavity growth, the results indicate that the shapes of individually growing cavities in Ni3Al are consistent with quasi-equilibrium cavity growth theory. Cavity size measurements have also been compared with predictions of both quasi-equilibrium and crack-like cavity growth models. These results also support the finding that cavity growth in Ni3Al occurs in a quasi-equilibrium manner. The observations further suggest that the formation of crack-like cavities is primarily due to coalescence and, therefore, not representative of the growth mechanism of individual cavities. 相似文献
8.
Oxide-dispersion-strengthened (ODS) alloy tubes with a nominal composition of Fe-19.5 pct Cr-5.5 pct Al-0.5 pct Y2O3 have been cold flow formed to deformation levels between 72 and 92 pct. The deformation structure of the tubes has been studied
using a variety of techniques including transmission electron microscopy (TEM). The deformation cells produced by flow forming
are elongated in both the hoop and axial directions, especially at deformation levels above 80 pct. In this case, most deformation
cells can be regarded as ribbons, lying roughly along the “rolling direction,” which for flow forming is a helix around the
tube surface. No obvious particle alignment was found in the tubes. Although the initial grain size is submicron, transition
bands composed of parallel-sided long deformation cells similar to those in deformed single-crystal specimens have been observed
in the transverse section of the tubes and a macroscopic shear band has been observed in the longitudinal section of the 92
pct deformed tube. 相似文献
9.
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
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A. B. Pandey B. S. Majumdar D. B. Miracle 《Metallurgical and Materials Transactions A》2000,31(13):921-936
Mechanical tests were performed on a powder-metallurgically processed 7093/SiC/15p discontinuously reinforced aluminum (DRA)
composite in different heat-treatment conditions, to determine the influence of matrix characteristics on the composite response.
The work-hardening exponent and the strain to failure varied inversely to the strength, similar to monolithic Al alloys, and
this dependence was independent of the dominant damage mode. The damage consisted of SiC particle cracks, interface and near-interface
debonds, and matrix rupture inside intense slip bands. Fracture surfaces revealed particle fracture-dominated damage for most
of the heat-treatment conditions, including an overaged (OA) condition that exhibited a combination of precipitates at the
interface and a precipitate-free zone (PFZ) in the immediate vicinity. In the highly OA conditions and in a 450°C as-rolled
condition, when the composite strength became less than 400 MPa, near-interface matrix rupture became dominant. A combination
of a relatively weak matrix and a weak zone around the particle likely contributed to this damage mode over that of particle
fracture. Fracture-toughness tests show that it is important to maintain a proper geometry and testing procedure to obtain
valid fracture-toughness data. Overaged microstructures did reveal a recovery of fracture toughness as compared to the peak-aged
(PA) condition, unlike the lack of toughness recovery reported earlier for a similar 7XXX (Al-Zn-Cu-Mg)—based DRA. The PA material exhibited extensive localization of damage and plasticity. The low toughness of
the DRA in this PA condition is explored in detail, using fractography and metallography. The damage and fracture micromechanisms
formed the basis for modeling the strength, elongation, toughness, and damage, which are described in Part II of this work.
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
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A. B. Pandey B. S. Majumdar D. B. Miracle 《Metallurgical and Materials Transactions A》2000,31(3):921-936
Mechanical tests were performed on a powder-metallurgically processed 7093/SiC/15p discontinuously reinforced aluminum (DRA)
composite in different heat-treatment conditions, to determine the influence of matrix characteristics on the composite response.
The work-hardening exponent and the strain to failure varied inversely to the strength, similar to monolithic Al alloys, and
this dependence was independent of the dominant damage mode. The damage consisted of SiC particle cracks, interface and near-interface
debonds, and matrix rupture inside intense slip bands. Fracture surfaces revealed particle fracture-dominated damage for most
of the heat-treatment conditions, including an overaged (OA) condition that exhibited a combination of precipitates at the
interface and a precipitate-free zone (PFZ) in the immediate vicinity. In the highly OA conditions and in a 450 °C as-rolled
condition, when the composite strength became less than 400 MPa, near-interface matrix rupture became dominant. A combination
of a relatively weak matrix and a weak zone around the particle likely contributed to this damage mode over that of particle
fracture. Fracture-toughness tests show that it is important to maintain a proper geometry and testing procedure to obtain
valid fracture-toughness data. Overaged microstructures did reveal a recovery of fracture toughness as compared to the peak-aged
(PA) condition, unlike the lack of toughness recovery reported earlier for a similar 7XXX (Al-Zn-Cu-Mg)-based DRA. The PA material exhibited extensive localization of damage and plasticity. The low toughness of
the DRA in this PA condition is explored in detail, using fractography and metallography. The damage and fracture micromechanisms
formed the basis for modeling the strength, elongation, toughness, and damage, which are described in Part II of this work.
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
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R. Tewari G. K. Dey R. K. Fotedar T. R. G. Kutty N. Prabhu 《Metallurgical and Materials Transactions A》2004,35(1):189-203
The deformation behavior of the Zr3Al-Nb alloys was studied by measuring hardness at different temperatures, by hot rolling, and by hot pressing. Using the hardness
data, elasticity and plasticity parameters were estimated and were used to determine the suitable temperature range of deformation
for these alloys. Hot rolling and hot pressing were applied to determine the optimum temperatures and annealing time for carrying
out deformation successfully in these alloys. Microstrural investigation of the hotdeformed samples revealed that the matrix
β phase has undergone substantial deformation and the second intermetallic phase, Zr2(Al,Nb), underwent dissolution and re-precipitation. The hardness of the fully annealed Zr3Al-Nb alloys showed two types of temperature dependence. Transition temperatures for the change in behavior, intrinsic hardness,
and softening coefficient were determined. The hardness of the fully annealed alloys rolled to different extent at various
temperatures and subsequently heat treated was used to study the recovery process. The microstructural study of room-temperature
deformed binary Zr3Al alloy showed the splitting of the superlattice dislocations into partials containing superlattice intrinsic stacking faults. 相似文献
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J. Wolfenstine H. K. Kim J. C. Earthman 《Metallurgical and Materials Transactions A》1994,25(11):2477-2482
The creep characteristics, including the nature of the creep transient after a stress reduction and activation energy for
creep of single crystalline Ni3Al(Ta,B) in the temperature range 1083 to 1388 K, were investigated. An inverse type of creep transient is exhibited during
stress reduction tests in the creep regime where the stress exponent is equal to 3.2. The activation energy for creep in this
regime is equal to 340 kJ mol−1. A normal type of creep transient is observed during stress reduction tests in the regime where the stress exponent is equal
to 4.3. The activation energy for creep in this regime is equal to 530 kJ mol−1. The different transient creep behavior and activation energies for creep observed in this investigation are consistent with
the previous suggestion that then = 4.3 regime is associated with creep controlled by dislocation climb, whereas then = 3.2 regime is associated with a viscous dislocation glide process for Ni3Al at high temperatures. 相似文献
18.
To understand the mechanical behavior of the Ni?Ni3Nb eutectic composite, it was necessary to determine the operative deformation and fracture mechanisms in the Ni3Nb intermetallic phase. It was found that Ni3Nb deforms primarily by twinning along {112} planes and {011} planes when tension and compression, respectively, are applied parallel to the [100] growth direction. The {112} twins were observed to serve as crack nucleation sites with cracks forming along the twin boundaries. The monotonic response of the Ni?Ni3Nb eutectic composite was investigated with tension and compression tests, metallography, and electron fractography. Room temperature tensile testing of the Ni?Ni3Nb composite revealed this material to be capable of sustaining tensile strains in excess of 11 pct. This large composite ductility was associated with extensive {112} twinning of the Ni3Nb lamellae and subsequent twin boundary cracking. When amassed in sufficient numbers in a given cross-section, these {112} twin boundary fissures initiated composite rupture. The room temperature ultimate tensile and compressive strengths of the alloy were found to be 109 and 235 ksi, respectively. 相似文献
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Deformation and microstructure development during hot-pack rolling of a near-gamma titanium aluminide alloy 总被引:3,自引:0,他引:3
Deformation behavior and microstructure development during hot pack rolling of the near-gamma titanium aluminide alloy Ti-45.5Al-2Cr-2Nb
(atomic percent) were established. Deformation behavior was investigated through rolling at various nominal furnace temperatures
and parallel modeling studies using a finite difference approach to predict temperature transients during workpiece transfer
from the furnace and during the rolling operation itself. Agreement between measured rolling pressures and predictions based
on a rule-of-mixtures (ROM) average of the flow stresses of the pack components (at the predicted temperatures and strain
rates within the roll gap) was excellent. As-rolled microstructures were interpreted in terms of the Ti-xAl-2Cr-2Nb pseudobinary
phase diagram, predicted temperature transients during rolling, and the static (no deformation) phase-transformation behavior
of the program material. These results demonstrated the strong influence of furnace preheat temperature on microstructure
development, as well as the tendency for temperature transients due to radiation heat losses and roll chilling to suppress
phase transformations. 相似文献