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1.
The stability of an Al2O3 reinforcement in TIMETAL 21S has been investigated by annealing diffusion couples and consolidated fiber composites at 1100
°C, 900 °C, and 750 °C. Diffusion couple studies indicate that γ-TiAl, α
2-Ti3Al, and α-Ti(Al,O) phases can form upon annealing above the β transus of TIMETAL 21S, but γ-TiAl, α
2-Ti3Al, and a ternary T phase form during annealing below the β transus. The phases developed during diffusional interaction define a diffusion path between TIMETAL 21S and Al2O3. A coating of Nb, Mo, or Ta between TIMETAL 21S and Al2O3 acts as a diffusion barrier, but the coatings can diffuse into TIMETAL 21S at high temperature. In agreement with a kinetics
analysis, a 2-μm-thick interface coating of Nb, Mo, or Ta in the TIMETAL 21S/Al2O3 composite can prevent the reaction during processing (2 hours at 850 °C or 900 °C) with no detectable diffusion into the
matrix. If there are imperfections such as pinholes or cracks present in the diffusion barrier, the reaction quickly starts
at the interface and does not remain confined at the imperfection; rather, it progresses along the interface. The mechanism
for progressive development of interface reaction at a discontinuity in the diffusion barrier has been proposed. The analysis
of the diffusional interface reactions in this work has identified some of the governing design concepts for development of
robust high-temperature titanium-based composites. 相似文献
2.
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
3.
Ji Sik Kim Young Won Chang Chong Soo Lee 《Metallurgical and Materials Transactions A》1998,29(1):217-226
A study has been made to investigate boundary sliding and its accommodation mode with respect to the variation of grain size
and α/β volume fraction during superplastic deformation of a two-phase Ti-6Al-4V alloy. A load relaxation test has been performed
at 600 °C and 800 °C to obtain the flow stress curves and to analyze the deformation characteristics by the theory of inelastic
deformation. The results show that grain matrix deformation (GMD) is found to be dominant at 600 °C and is well described
by the plastic state equation. Whereas, at 800 °C, phase/grain boundary sliding (P/GBS) becomes dominant and is fitted well
with the viscous flow equation. The accommodation mode for fine-grained microstructures (3 μm) well agrees with the isostress model, while that for large-grained structures (11 μm) is a mixed mode of the isostress and isostrain-rate models. The sliding resistance analyzed for the different boundaries
is lowest in the α/β boundary, and increases on the order of α/β≪α/α≈β/β, which plays an important role in controlling the superplasticity of the alloys with various α/β phase ratios. 相似文献
4.
Nonequilibrium austenite, γ, or duplex austenite + lath martensite,γ+ α′
L, phase wires with high strengths and large elongation have been produced in Fe-Ni-Cr-Al-C alloy system by the in-rotating-water
spinning method in which a melt stream is ejected into a rotating water layer. These wires have a circular cross section and
a white luster, and the wire diameter is in the range of 80 to 180 μm. The γ phase has a grain size as small as about 1 to
4 μm. The yield strength, Σy, tensile fracture strength, ay, and elongation, εp, are about 340 to 655 MPa, 440 to 975 MPa, and 12 to 22 pct for the γ single phase wires and about 465 to 865 MPa, 640 to
1350 MPa, and 2 to 18 pct for the α′L+γ duplex phase wires. A cold drawing causes significant increases in Σy and Σf, and the attained values are about 3200 MPa and 4030 MPa for Fe-8Ni-12.5Cr-2.5Al-3C wire drawn to about 95 pct reduction
in area owing to the formation of a strain-induced α′L phase and a remarkable work-hardening ability of γ and α′L phases. On the subsequent low-temperature annealing around 673 K, the Σy and Σf increase further to 4000 MPa and 4240 MPa, respectively, probably because of the enhancement of the interaction between dislocations
and interstitial carbon atoms. Around the temperature (≃800 K) where the γ phase decomposes into a stable mixed structure
of α + ordered bec compound + M7C3 on annealing, the εp decreases drastically and the fracture surface morphology changes from a dimple pattern to a cleavage pattern. It has been
therefore inferred that the high strengths and good ductility of the melt-quenched y and γ + α′L wires are due to the suppression of the phase transformation of y to a mixed structure of γ + ordered bec compound + M7C3 carbide by the melt-quenching technique. 相似文献
5.
Nonequilibrium austenite, γ, or duplex austenite + lath martensite,γ+ α′
L, phase wires with high strengths and large elongation have been produced in Fe-Ni-Cr-Al-C alloy system by the in-rotating-water
spinning method in which a melt stream is ejected into a rotating water layer. These wires have a circular cross section and
a white luster, and the wire diameter is in the range of 80 to 180 μm. The γ phase has a grain size as small as about 1 to
4 μm. The yield strength, Σy, tensile fracture strength, ay, and elongation, εp, are about 340 to 655 MPa, 440 to 975 MPa, and 12 to 22 pct for the γ single phase wires and about 465 to 865 MPa, 640 to
1350 MPa, and 2 to 18 pct for the α′L+γ duplex phase wires. A cold drawing causes significant increases in Σy and Σf, and the attained values are about 3200 MPa and 4030 MPa for Fe-8Ni-12.5Cr-2.5Al-3C wire drawn to about 95 pct reduction
in area owing to the formation of a strain-induced α′L phase and a remarkable work-hardening ability of γ and α′L phases. On the subsequent low-temperature annealing around 673 K, the Σy and Σf increase further to 4000 MPa and 4240 MPa, respectively, probably because of the enhancement of the interaction between dislocations
and interstitial carbon atoms. Around the temperature (≃800 K) where the γ phase decomposes into a stable mixed structure
of α + ordered bec compound + M7C3 on annealing, the εp decreases drastically and the fracture surface morphology changes from a dimple pattern to a cleavage pattern. It has been
therefore inferred that the high strengths and good ductility of the melt-quenched y and γ + α′L wires are due to the suppression of the phase transformation of y to a mixed structure of γ + ordered bec compound + M7C3 carbide by the melt-quenching technique. 相似文献
6.
J. E. Wittig 《Metallurgical and Materials Transactions A》2002,33(8):2373-2379
Rapid solidification by twin-anvil splat quenching captures the initial nucleation and growth of the α → γ
m
massive transformation in titanium aluminides. Splat quenching Ti52Al48 and Ti50Al48Cr2 from the liquid at slightly below the melting point produces an equiaxed α solidification structure. Solid-state cooling rates that approach 106 K/s arrest the α → γ
m
massive transformation with 1- to 5-μm-sized γ
m
nuclei, especially in the Ti50Al48Cr2 alloy. Classical massive-transformation heterogenous nucleation occurs at α:α grain boundaries with an orientation relationship of [111]
γ
//[0001]
α
and
. The γ
m
nucleus then grows into the adjacent α grain without the orientation relationship by forming an incoherent α:γ interface with {111}
γ
facets. Orthogonal variants of the tetragonal c-axis in the γ
m
product suggest that the massive transformation initially produces an fcc structure which subsequently orders into the L10 phase. Nucleation of γ
m
is not only observed at α:α grain boundaries and triple points, but also within the α grains. The intragranular γ
m
nucleation, which is believed to be heterogeneous, occurs with the same orientation relationship as for the intergranular
nuclei. However, the intragrain nuclei do not form {111}
γ
facets and retain a curved α:γ
m
interface. Although analysis of the {111}
γ
faceted growth using weak-beam dark-field (WBDF) imaging shows no evidence for any type of misfit-compensating dislocations,
lattice imaging of the {111}
γ
facets with high resolution transmission electron microscopy (HRTEM) reveals that the planar interface exhibits a slight
curvature, produced by atomic steps of (111) planes. These experimental data have been used to estimate a ratio of ledge spacing
(λ) to ledge height (h) for the {111}
γ
facets as λ/h=41, which is similar to calculated values for a ledge growth mechanism of massive transformations in Cu-Zn and Ag-Cd alloys.
This article is based on a presentation made at the symposium entitled “The Mechanisms of the Massive Transformation,” a part
of the Fall 2000 TMS Meeting held October 16–19, 2000, in St. Louis, Missouri, under the auspices of the ASM Phase Transformations
Committee. 相似文献
7.
Ernest Levine Ira Greenhut Harold Margolin 《Metallurgical and Materials Transactions B》1973,4(11):2519-2525
Methods of revealing grain size in a two-phase α-β titanium alloy have been examined and observations on beta grain growth
in the presence of alpha have been carried out. The technique proposed by Greenfield and Margolin1 for revealing β matrix grain sizes has been shown not to produce grain growth. However, for grain sizes of about 10 μm the
G.M. technique does not reveal all the grains because of the similarity in orientation in neighboring grains. These clusters
of similarly oriented grains are shown to persist as grain growth takes place but the misorientation between grains within
a cluster decreases. Both the beta grain growth and alpha particle coarsening follow the same time dependency from which it
is shown that a linear relationship exists between α particle size and β grain size. It is proposed that α particles must
dissolve from theβ grain edges for β grain growth to occur. The linear dependency between beta grain size,D
β, and alpha particle size,d
α, can be rationalized either on the basis of geometrical or surface tension considerations.
Formerly with New York University.
Formerly Graduate Student with New York University. 相似文献
8.
Young Kyu Cho Duk Yong Yoon Michael F. Henry 《Metallurgical and Materials Transactions A》2001,32(12):3077-3090
When an extruded strain-free RENé 88 Ni-base superalloy about 1 to 2 μm in grain diameter is heattreated at 1150°C, abnormal grain growth (AGG) begins after 50 hours. When heat-treated at 1200
°C, AGG occurs at 15 minutes. Some of the grain boundaries are faceted with hill-and-valley structures when observed in transmission
electron microscopy (TEM), and the occurrence of AGG is consistent with the boundary step and dislocation mechanism for the
migration of singular boundaries with faceted shapes, as observed and proposed in other pure metals and alloys. The dissolution
of abundant γ′ precipitates (with a solvus temperature of 1107 °C), which are coherent with the matrix and hence strongly pin the grain
boundaries, does not cause AGG during early stages of heat-treatment at 1150 °C. Small deformations drastically alter the
AGG behavior. When deformed to 4 pct, AGG begins after heat-treating for 10 minutes at 1150 °C, compared to the apparent incubation
time of 50 hours for an undeformed specimen, and very large abnormal grains are produced. With increasing deformation to 6
and 9.2 pct, the abnormal grain size decreases. These results are qualitatively similar to those observed in Cu. This deformation
effect on AGG is attributed to the absorption of lattice dislocations in the grain boundaries, which produces nonequilibrium
structures that, in turn, can apparently cause rapid boundary migration. When heat-treated at 1200 °C, the largest abnormal
grains are found in the specimens deformed to 2 pct. When the initial grain size is increased to about 14 μm by heattreating the extruded alloy at 1150 °C for 30 minutes, similar low deformation effects on AGG are observed. When
these specimens are deformed to 10, 13, and 15.2 pct, primary recrystallization occurs during the heat-treatment at 1150 °C,
and large abnormal grains are again produced because of the small recrystallized grain size. Therefore, there are two peaks
in the grain size vs deformation curve after heat-treating at 1150 °C for 1 hour. A pre-heat-treatment of this alloy at 1050 °C below the solvus
temperature of the γ′ phase greatly reduces the size of the abnormal grains obtained in the specimen deformed to 4 pct after the heat-treatment
at 1150 °C, probably because some recovery of the dislocations takes place at grain boundaries during the pre-heat-treatment.
The deformation effect on AGG observed in this alloy is qualitatively similar to that previously observed in Cu and appears
to be consistent with the boundary step and dislocation mechanism for AGG.
An erratum to this article is available at . 相似文献
9.
The corrosion of a hot-chamber die-cast AZ91D thin plate (1.4 mm in thickness) was investigated in terms of its microstructure,
to elucidate the role of die-chill skin in corrosion. The die-chill skin was composed of a thin layer of chill zone and a
thick layer of an interdendritic Al-rich α-Mg/Al12Mg17
β-phase particle/α-Mg grain composite microstructures. The chill zone (4±1 μm in thickness) had fine columnar and equiaxed grains and contained a distribution of submicron Mg-Al-Zn intermetallic particles.
Beneath the chill zone, Al12Mg17
β particles were irregularly shaped but did not have an interdendritic network morphology. Furthermore, Al-rich α phase (also known as eutectic α) was in the interdendritic network, which occupied a higher volume fraction than the β phase in the die-skin layer. Corrosion characteristics were studied via constant-immersion and electrochemical tests. Although previous studies have ascribed the fine microstructure to good corrosion
resistance for the AZ91D alloy, the present study showed severe corrosion of the sample with a die skin in chloride solution.
Moreover, the sample without the die skin on the surface corroded more slowly. The inferior corrosion performance of the die
skin was considered to be related to the high volume fraction of the interdendritic network of Al-rich α phase contained in the die skin, owing to the high cooling rate during solidification. The Al-rich α phase does not increase the corrosion resistance of the AZ91D alloy. 相似文献
10.
Y. L. Hao R. Yang M. Niinomi D. Kuroda Y. L. Zhou K. Fukunaga A. Suzuki 《Metallurgical and Materials Transactions A》2002,33(10):3137-3144
An investigation on the formation of α″ martensite and its influence on Young’s modulus and mechanical properties of forged Ti-29Nb-13Ta-4.6Zr (wt pct) alloy is
reported in this article. For ice-water-quenched specimens after solution treatment at 1023, 1123, and 1223 K in the single
β-phase field for 1.8, 3.6, 14.4, and 28.8 ks, X-ray diffraction and internal friction measurements showed that the volume
fraction of the α″ martensite changes with both solution temperature and time. This effect has been attributed mainly to the influence of grain
size of the β-parent phase on the stability of the β phase and, consequently, on the martensitic start (M
s) temperature. A critical grain size of 40 μm was identified for the β phase, below which the martensitic transformation is largely suppressed because of low M
S temperature. With the β grain size increasing above this critical value, the volume fraction of the α″ martensite increases significantly at first and then decreases gradually with further grain growth. The α″ martensite was shown to possess good ductility and, compared to the β phase, lower strength and hardness but nearly identical Young’s modulus in the studied alloy. 相似文献
11.
T. Naohara 《Metallurgical and Materials Transactions A》1996,27(11):3424-3430
The crystallization behavior of amorphous Fe84-X
Si6B10MX (M=Nb, Zr, V, or Cu) alloys was examined using differential scanning calorimetry (DSC) and transmission electron microscopy
(TEM) with the aim of clarifying the effect of additional M elements. The compositional dependence of the first crystallization
temperatureT
x1 increased in the order of Zr > Nb > V; however, the addition of 1 at. pct Cu caused a decrease inT
x1. Such an effect of the M elements on the thermal stability of an amorphous phase was interpreted in terms of the difference
in the atomic size. These alloys were composed of a mixed structure ofα-Fe and amorphous phases after aging for 3.6 ks in the first exothermic temperature range. The addition of more than 3 at.
pct Nb or Zr significantly affected the morphology and grain size of theα-Fe phase. However, their particles possessed dendritic morphology with a grain size of 0.1 to 0.3 μm, when the Nb or Zr content
was less than 2 at. pct. Further addition of these elements brought about the formation of sphericalα-Fe particles. The average grain size, for instance, was as small as 20 nm in the aged alloy containing 6 at. pct Nb, which
shows that a remarkable grain refinement occurs with increasing Nb content. 相似文献
12.
The super α
2 Ti3Al-based alloy with a fine grain size of ∼2.2 μm exhibits superplastic elongations over 1000 pct at 920 °C to 1000 °C, 600 pct at 900 °C, 330 pct at 850 °C, and 140 pct
at 750 °C. Mechanical anisotropy is observed in this alloy, and relatively lower flow stresses and higher tensile elongations
are obtained in the 45 deg specimen loaded at 25 °C to 960 °C. The texture characteristics appear to impose significant influence
on the mechanical anisotropy at temperatures below 900 °C (under the dislocation creep condition), and the {111}〈2
〉 and {0001} basal textures evolve in the β and α
2 phases after tensile straining. At loading temperatures higher than 900 °C (under the superplastic flow condition), the anisotropy
effect is less pronounced and the grain orientation distribution becomes basically random in nature. Rationalizations for
the mechanical anisotropy in terms of the Schmid factor calculations for the major and minor texture components in the β and α
2 phases provide consistent explanations for the deformation behavior at lower temperatures as well as the initial straining
stage at higher temperatures. 相似文献
13.
G. Y. Kim L. M. He J. D. Meyer W. Y. Lee A. Quintero J. A. Haynes 《Metallurgical and Materials Transactions A》2004,35(11):3581-3593
A laboratory-scale chemical vapor deposition (CVD) reactor was used to perform “continuous” Hf doping experiments while the
surface of a single-crystal Ni alloy was being aluminized to form an aluminide (β-NiAl) coating matrix for 45 minutes at 1150 °C. The continuous doping procedure, in which HfCl4 and AlCl3 were simultaneously introduced with H2, required a high HfCl4/AlCl3 ratio (>∼0.6) to cause the precipitation of Hf-rich particles (∼0.1 μm) at grain boundaries of the coating layer, with the overall Hf concentration of ∼0.05 to 0.25 wt pct measured in the coating
layer by glow-discharge mass spectroscopy (GDMS). Below this ratio, Hf did not incorporate as a dopant into the growing coating
layer from the gas phase, as the coating matrix appeared to be “saturated” with other refractory elements partitioned from
the alloy substrate. In comparison, the Hf concentration in the aluminide coating layer formed on pure Ni was in the range
of ∼0.1 wt pct, which was close to the solubility of Hf estimated for bulk NiAl. Interestingly, the segregation of Hf and
the formation of a thin γ′-Ni3Al layer (∼0.5 μm) at the coating surface were consistently observed for both the alloy and pure-Ni substrates. The formation of the thin
γ′-Ni3Al layer was attributed to an increase in the elastic strain of the β-NiAl phase, associated with the segregation of Hf as well as other refractory alloying elements at the coating surface. This
phenomenon also implied that the coating layer was actually growing at the interface between the γ′-Ni3Al layer and the β-NiAl coating matrix, not at the gas/coating interface, during the early stage of the coating growth. 相似文献
14.
T. Liapina A. Leineweber E. J. Mittemeijer 《Metallurgical and Materials Transactions A》2006,37(2):319-330
The ▓/γ′-iron nitride (▓-Fe3N1+x
, γ′-Fe4N) compound double layers with thicknesses of about 10 μm were grown on pure α-Fe, by gas nitriding at 823 K, followed by
quenching. The specimens were subsequently annealed at significantly lower temperatures, in the range of 613 to 693 K, for
different periods of time. These heat treatments led to a redistribution of N, within the compound layer as well as between
the compound layer and the adjacent ferrite, inducing thickness changes in the ▓-and γ′-sublayers. The microstructure and
sublayer-thickness changes were analyzed by light microscopy and X-ray diffraction (XRD). The experimentally observed time
and temperature dependences of the layer-thickness changes were compared with the results obtained from numerical simulations,
by adopting a model based on volume diffusion in the ▓- and γ′-phases and on local equilibrium at the phase interfaces. In
this manner, the intrinsic diffusion coefficient of N in the ▓-phase and the integral diffusion coefficient of N in the γ′-phase
were determined for the applied range of annealing temperatures. 相似文献
15.
T. J. Nijdam C. Kwakernaak W. G. Sloof 《Metallurgical and Materials Transactions A》2006,37(3):683-693
Three γ + β NiCoCrAlY alloys (a cast alloy, a laser-surface-melted (LSM) alloy, and a coating as deposited by electron beam-physical
vapor deposition (EB-PVD)) with similar average composition (Ni-20Co-19Cr-24Al-0.2Y in at. pct), but with different microstructures
prior to oxidation, were oxidized for 0.5 and 1 hours at 1373 K in an Ar 20 vol pct O2 atmosphere (i.e., at a partial oxygen pressure of 20 kPa). It was found that on the alloy with β precipitates larger than 20 μm, the oxide
layer was nonuniform in thickness, and had a laterally inhomogeneous composition and phase constitution. In this case, the
oxide layer developed on top of the γ phase was thicker than that formed on top of the β phase and consisted of a NiCr2O4/Cr2O3 outer and an α-Al2O3 inner layer. For the thinner oxide formed on top of the β phase, the outer layer was constituted of a Cr and Co containing
NiAl2O4 spinel and the inner layer also consisted of α-Al2O3. For the alloys with β precipitates smaller than 3 μm, a uniform and laterally homogeneous oxide formed, consisting of a
Cr and Co containing NiAl2O4 outer layer on top of an α-Al2O3 inner layer. After oxidation, Y was distributed as numerous, small precipitates within the oxide layer for a homogeneous
Y distribution prior to oxidation, or as a few, very large pegs along the γ/β phase boundaries of the alloy for an inhomogeneous
Y distribution prior to oxidation. The performance of the alloys upon thermal cycling was improved for smaller β precipitates
and for a more homogeneous Y distribution in the alloy prior to oxidation. 相似文献
16.
M. W. Barsoum T. El-Raghy A. Procopio 《Metallurgical and Materials Transactions A》2000,31(2):333-337
Bulk samples of Ti4AIN3 were fabricated by reactive hot isostatic pressing (hipping) of TiH2, AlN, and TiN powders at 1275 °C for 24 hours under 70 MPa. Further annealing at 1325 °C for 168 hours under Ar resulted
in dense, predominantly single-phase samples, with <1 vol pct of TiN as a secondary phase. This ternary nitride, with a grain
size of ≈20 μm on average, is relatively soft (Vickers hardness 2.5 GPa), lightweight (4.6 g/cm3), and machinable. Its Young’s and shear moduli are 310 and 127 GPa, respectively. The compressive and flexural strengths
at room temperature are 475 and 350 MPa, respectively. At 1000 °C, the deformation is plastic, with a maximum compressive
stress of ≈450 MPa. Ti4AlN3 thermal shocks gradually, whereby the largest strength loss (50 pct) is seen at a ΔT of 1000 °C. Further increases in quench temperature, however, increase the retained strength before it ultimately decreases
once again. This material is also damage tolerant; a 100 N-load diamond indentation, which produced an ≈0.4 mm defect, reduces
the flexural strength by only ≈12 pct. The thermal-expansion coefficient in the 25 °C to 1100 °C temperature range is 9.7±0.2
× 10−6 °C−1. The room-temperature electrical conductivity is 0.5 × 106 (Θ · m)−1. The resistivity increases linearly with increasing temperature. Ti4AlN3 is stable up to 1500 °C in Ar, but decomposes in air to form TiN at ≈1400 °C.
graduated from the Department in June of 1999 with an MS thesis. 相似文献
17.
Yoshihiro Terada Rie Sota Naoya Ishimatsu Tatsuo Sato Koichi Ohori 《Metallurgical and Materials Transactions A》2004,35(9):3029-3032
The effect of calcium addition on the microstructure and creep strength of the die-cast AM50 magnesium alloy was investigated.
The α-Mg grains with the diameter of 4.9 μm are surrounded by the eutectic phases for the AM50-1.72 mass pct Ca alloy, while the β(Mg17Al12) particles are located mainly on the grain boundaries of the α grains for the AM50 alloy. The minimum creep rates of the AM50-1.72 mass pct Ca alloy are three orders of magnitude lower
than those of the AM50 alloy at 423 K typically below 120 MPa. The thousandfold creep strengthening by the Ca addition is
ascribed to the thermally stable eutectic phases appearing in the AM50-1.72 mass pct Ca alloy, which is expected to yield
effective grain boundary strengthening or to resist the plastic flow of the α-Mg grains. 相似文献
18.
Douglas T. Queheillalt Haydn N. G. Wadley Bill W. Choi Daniel S. Schwartz 《Metallurgical and Materials Transactions A》2000,31(1):261-273
Low-density titanium alloy sandwich structures consisting of a porous core and fully dense face sheets can be produced by
consolidating argon gas charged powder compacts followed by not rolling and annealing to expand the gas-filled pores. Little
is known about the rate of pore expansion, its dependence upon temperature, and the morphological evolution of the pore shape
during expansion. In situ eddy current and laser ultrasonic sensors have been combined with metallographic and texture measurements to measure the
relative density, the elastic moduli, and the microstructural evolution of Ti-6Al-4V sandwich panels during the annealing
stage of low-density core (LDC) processing. The eddy current data indicated that expansion began during, the heating phase,
reached a maximum expansion rate (Δ) of 2 × 10−5 s−1 at approximately 685 °C, and had almost ceased (Δ < 1 × 10−6 s−1) after annealing for 4 hours at 920 °C. The elastic moduli were found to decrease with increasing temperature and volume
fraction of porosity. The initial (as-rolled) microstructure consisted of a lamellar α + β microstructure with an α-phase lath thickness of 2.0 μm and contained a distribution of oblate-shaped pores with aspect ratios of up to 10. During the expansion process, it recrystallized
into an equiaxed α + β structure with an α-phase grain diameter of 7.5 μm with spheroidal pores with aspect ratios of up to 3. The combination of the two sensors was found to enable the in situ determination of both the porous cores relative density and its elastic properties. These are the two material indices that
govern the elastic response of a sandwich structure. 相似文献
19.
Toshikazu Akahori Mitsuo Niinomi Kei-Ichi Fukunaga Ikuhiro Inagaki 《Metallurgical and Materials Transactions A》2000,31(8):1949-1958
This article presents the results of a study of the effects of microstructure on the fatigue strength and the short fatigue
crack initiation and propagation characteristics of a biomedical α/β titanium alloy, Ti-6Al-7Nb. The results are compared to those obtained from a Ti-6Al-4V extra-low interstitial (ELI) alloy.
Fatigue crack initiation occurs mainly at primary α grain boundaries in an equiaxed α structure, whereas, in a Widmanst?tten α structure, initiation occurs within the α colonies and prior β grains, where α plates are inclined at around 45 deg to the stress-axis direction. In an equiaxed α structure, the short fatigue crack initiation and propagation life, where the length of the crack (a) is in a microstructurally short fatigue-crack regime (2a < 50 μm), occupies around 50 pct of the total fatigue life. On the other hand, the fatigue crack in a Widmanst?tten α structure initiates at very early stages of fatigue, and, therefore, the fatigue crack-initiation life occupies a few percentages
of the total fatigue life in an α structure. Then, the short fatigue crack propagates rapidly and is arrested at the grain boundaries of α colonies or prior β grains for a relatively long period, until the short crack passes through the boundaries to specimen failure. Therefore,
the short fatigue crack-arrest life occupies more than 90 pct of the total fatigue life in a Widmanst?tten α structure. These trends are similar between the Ti-6Al-7Nb and Ti-6Al-4V ELI alloys and biomedical α/β titanium alloys. The total fatigue life for the Ti-6Al-7Nb alloy with an equiaxed α structure is changed by the volume fraction of primary α phase and the cooling rate after solution treatment. By increasing the volume fraction of the primary α phase from 0 to 70 pct, the fatigue limit of the Ti-6Al-7Nb alloy is raised. Changing the cooling rate after solution treatment
by switching from air cooling to water quenching improves the fatigue limit of the Ti-6Al-7Nb alloy significantly. 相似文献
20.
Dense crack-free coatings of Fe-Cr-Al-Y quaternary alloy were produced on stainless steel 316L substrates using a continuous
wave Nd-YAG solid-state laser coupled with a fiber optic beam delivery system. Experiments were performed at a laser power
between 0.6 and 2.4 kW, process speed in the range 0.053 to 0.423 cm/s, powder feed rate fixed at 0.083 g/s, and focused multimode
laser beam with a diameter of 0.5 cm. Various microanalysis techniques demonstrated that the coatings were metallurgically
bonded to the substrate and possessed thicknesses between 0.35 and 4.64 mm, refined columnar microstructures with grain sizes
of 15 to 150 μxm, increased concentration of key alloying elements, and appreciably high microhardness up to 409 kg/mm2. The laser-processed microstruc-tures comprised a body-centered cubic (bcc) ferrite(α phase) crystal structure with a relatively large lattice parameter compared to α-Fe due to the enhanced dissolution of varying
amounts of Cr, Al, Ni, and Y, depending on the dilution from the substrate material. Oxidation tests conducted in air at temperatures
of 1100 ° to 1200 ° for 95 hours revealed the formation of an approximately 5-μm-thick dense α-Al2O3 oxide scale of a rhombohedral (hexagonal) crystal structure. The α-Al2O3 scale exhibited remarkable high-temperature resistance and strong adherence to the coating surface. Extensive oxidation of
the uncoated stainless steel substrate produced a porous and heavily spalled alloy oxide scale about 60-μm thick consisting
of FeCr2O4 and NiCr2O4 with cubic and tetragonal crystal structures, respectively. The retention of the bcc α phase and the insignificant grain
growth after oxidation are indicative of the thermal stability of the laser-processed coating microstructures. The obtained
results demonstrate that Fe-Cr-Al-Y alloy coatings exhibiting fine-grained hard mi-crostructures, high-temperature oxidation
resistance, and strong adherence to stainless steel can be developed by means of laser processing. 相似文献