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1.
Superplastic behavior of two-phase titanium aluminides 总被引:1,自引:0,他引:1
S. C. Cheng J. Wolfenstine O. D. Sherby 《Metallurgical and Materials Transactions A》1992,23(5):1509-1513
A two-phase Ti(57 at. pct)-Al(43 at. pct) alloy with an initial lamellar microstructure was thermomechanically processed to
form an equiaxed fine-grained structure. The fine-grained (- L = 5 μm) material was superplastic in the temperature range 1000 °C to 1100 °C, exhibiting a stress exponent of about 2 with
a tensile ductility of 275 pct. The rate-controlling deformation mechanism is proposed to be grain boundary sliding accommodated
by slip controlled by lattice diffusion in TiAl. At room temperature, the lamellar and fine-grained materials exhibit the
same compressive yield stress. The compressive strain to failure, however, for the fine-grained material was about 28 pct
compared to 6 pct for the lamellar material. 相似文献
2.
Brian D. Worth J. Wayne Jones John E. Allison 《Metallurgical and Materials Transactions A》1995,26(11):2961-2972
The influence of interstitial strengthening and microstructure on creep deformation has been examined in the near-γ TiAl alloy
Ti-48Al-lV-0.3C. Creep studies were conducted under constant load in air at 815 °C in the stress range of 50 to 200 MPa. Significant
improvement in creep resistance was observed in this alloy compared with a similar alloy (Ti-49Al-lV) containing low levels
of carbon (0.07 at. pct). The degree of strengthening resulting from the addition of carbon was found to be dependent on microstructure.
At 815 °C and 150 MPa, the addition of carbon reduced the minimum creep rate by a factor of approximately 20 in the equiaxedy and duplex microstructures and by a factor of 3 in the fully lamellar microstructures. Carbide precipitation occurred in
this alloy when aged in the temperature range of 700 °C to 950 °C. The addition of carbon leads to a decrease in the stress
exponent from 4 to 3 in the duplex and equiaxedy microstructures and the inhibition of sub-boundary formation in the duplex microstructure. This suggests that solute/dislocation
interaction mechanisms, rather than a direct effect of carbide precipitates, are responsible for the significant increase
in creep resistance observed in this alloy.
Brian D. Worth, formerly with the Department of Materials Science and Engineering, The University of Michigan. 相似文献
3.
《Acta Metallurgica Materialia》1992,40(9):2167-2175
The steady-state creep deformation behavior of a cast two phase gamma TiAl alloy having the composition Ti48Al1Nb (at.%) has been studied. Tension creep tests using the stress increment technique (θθ2θ3) were conducted over the temperature range of 704–850°C at constant initial applied stress level of 103.4–241.3 MPa. The activation energy for creep over the temperature and stress regime of this study varied 317.5 kJ/mol (137.8 MPa) up to 341.0 kJ/mol (206.8 MPa) with an average value of 326.4 kJ/mol. This is well within the range of values previously measured for gamma TiAl alloys where creep controlled by volume diffusion has been suggested as rate controlling. The stress exponents meaured were 5.0 at 704°C, 4.9 at 750°C, 4.7 at 800°C and 4.46 at 850°C. Using the activation energy of 326.4 kJ/mol, the temperature compensated steady-state creep rate was plotted against long stress with all temperatures collapsing onto a single line having a slope equal to 4.95. Using conventional creep analysis, this value of the stress exponent can be taken as suggestive of dislocation climb controlled power law creep as the operative deformation mechanism within the stress and temperature regime of the present study. The boundary separating the lamellar grains in two phase gamma TiAl alloys having the duplex microstructure may be a very important aspect of this microstructure with respect to creep deformation resistance. The interlocking γ/α2 laths making up these boundaries are expected to be very resistant to grain boundary sliding which may contribute to creep deformation in the dislocation creep regime. Finally, some previous observations along with a comparison of the creep behavior of the Ti48Al1Nb alloy to that of a Tiz.sbnd;50.3Al binary have been discussed in terms of the pre-exponential constant A in the power law creep equation. TiAl alloys having similar stress and temperature dependencies but differing steady-state strain rates over comparable stress-temperature regimes may be rationalized on the basis of differing power law creep constants which may reflect differences in stacking fault energies. 相似文献
4.
Brian D. Worth J. Wayne Jones John E. Allison 《Metallurgical and Materials Transactions A》1995,26(11):2947-2959
The influence of microstructure on creep deformation was examined in the near-y TiAl alloy Ti-49A1-1V. Specifically, microstructures
with varying volume fractions of lamellar constituent were produced through thermomechanical processing. Creep studies were
conducted on these various microstructures under constant load in air at temperatures between 760 °C and 870 °C and at stresses
ranging from 50 to 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar
microstructure yielding the highest creep resistance of the microstructures examined. Creep resistance is dependent on the
volume fraction of lamellar constituent, with the lowest creep resistance observed at intermediate lamellar volume fractions.
Examination of the creep deformation structure revealed planar slip of dislocations in the equiaxed y microstructure, while
subboundary formation was observed in the duplex microstructure. The decrease in creep resistance of the duplex microstructure,
compared with the equiaxed y microstructure, is attributed to an increase in dislocation mobility within the equiaxedy constituent, that results from partitioning of oxygen from the γ phase to the α2 phase. Dislocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence
of shearing of γ/γ or γ/α2 interfaces. This suggests that the high creep resistance of the fully lamellar microstructure is a result of the fine spacing
of the lamellar structure, which results in a decreased effective slip length for dislocation motion over that found in the
duplex and equiaxed y microstructures.
BRIAN D. WORTH, formerly with the Department of Materials Science and Engineering, The University of Michigan 相似文献
5.
Michael F. Bartholomeusz John A. Wert 《Metallurgical and Materials Transactions A》1994,25(10):2161-2171
A two-phase TiAl/Ti3Al alloy with a lamellar microstructure has been previously shown to exhibit a lower minimum creep rate than the minimum creep
rates of the constituent TiAl and Ti3Al single-phase alloys. Fiducial-line experiments described in the present article demonstrate that the creep rates of the
constituent phases within the two-phase TiAl/Ti3Al lamellar alloy tested in compression are more than an order of magnitude lower than the creep rates of single-phase TiAl
and Ti3Al alloys tested in compression at the same stress and temperature. Additionally, the fiducial-line experiments show that
no interfacial sliding of the phases in the TiAl/Ti3Al lamellar alloy occurs during creep. The lower creep rate of the lamellar alloy is attributed to enhanced hardening of the
constituent phases within the lamellar microstructure. A composite-strength model has been formulated to predict the creep
rate of the lamellar alloy, taking into account the lower creep rates of the constituent phases within the lamellar micro-structure.
Application of the model yields a very good correlation between predicted and experimentally observed minimum creep rates
over moderate stress and temperature ranges.
Formerly with the Department of Materials Science and Engineering, University of Virginia 相似文献
6.
通过高温压缩模拟试验结果建立TiAl基合金的热加工图,结合扫描电镜、透射电镜等试验手段,研究铸造TiAl基合金在温度为1 000~1 150℃、应变速率为0.001~1 s 1范围内的热变形行为。结果表明:铸造TiAl基合金是温度、应变速率敏感材料,其流变应力随温度升高和应变速率降低而降低。铸造TiAl基合金的高温变形机制以层片晶团的扭折、弯曲及动态再结晶过程为主。在高温(1 150℃),低应变速率(≤0.01 s 1)下变形后,铸态组织中β相含量明显减少直至消除。在变形温度1 150℃、应变速率0.001 s 1下变形时,铸造TiAl基合金未发生超塑性变形;此时由于动态再结晶晶粒异常长大导致加工图上该区域功率耗散值未达到最大,而是有减小的趋势。 相似文献
7.
The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing
tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully
lamellar microstructure with approximately 150-μm grain size and 1-μm lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile
transition temperature (BDTT), while fracture and fatigue tests were performed at 25 °C and 815 °C. Fracture toughness tests
were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using
ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and
by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified
and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture
toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness,
and fatigue resistance were identified and related to pertinent microstructural variables. 相似文献
8.
H. S. Park S. K. Hwang C. M. Lee Y. C. Yoo S. W. Nam N. J. Kim 《Metallurgical and Materials Transactions A》2001,32(2):251-259
Strengthening of a gamma TiAl alloy was sought by a chemical modification of the composition with carbon. Up to 0.6 at. pct
of carbon was added to the Ti-46.6Al-1.4Mn-2Mo alloy processed by elemental powder metallurgy. Carbon addition resulted in
considerable microstructural changes such as refinement, by a factor of about 2, of the lamellar microstructure and carbide
precipitation. The cause of the lamellar structure refinement is twofold, increased heterogeneous nucleation rate and decreased
γ platelet growth rate, the net result of which was a retarded diffusional transformation kinetics of α to α/γ lamellae. As a consequence of the microstructural changes, the high-temperature tensile properties and the creep properties
of the alloy were significantly improved. Anomalous hardening was also observed at 800 °C, resulting in a tensile yield strength
of 700 MPa. The strengthening effect of carbon was realized by the microstructural refinement and by precipitation hardening
of intergranular as well as interlamellar Ti3AlC. In terms of the tensile properties and the creep properties, the optimum amount of carbon addition was 0.3 at. pct. 相似文献
9.
Influence of microstructure on crack-tip micromechanics and fracture behaviors of a two-phase TiAl alloy 总被引:4,自引:0,他引:4
The tensile deformation, crack-tip micromechanics, and fracture behaviors of a two-phase (γ + α2) gamma titanium aluminide alloy, Ti-47Al-2.6Nb-2(Cr+V), heat-treated for the microstructure of either fine duplex (gamma
+ lamellar) or predominantly lamellar microstructure were studied in the 25 °C to 800 °C range.In situ tensile and fracture toughness tests were performed in vacuum using a high-temperature loading stage in a scanning electron
microscope (SEM), while conventional tensile tests were performed in air. The results revealed strong influences of microstructure
on the crack-tip deformation, quasi-static crack growth, and the fracture initiation behaviors in the alloy. Intergranular
fracture and cleavage were the dominant fracture mechanisms in the duplex microstructure material, whose fracture remained
brittle at temperatures up to 600 °C. In contrast, the nearly fully lamellar microstructure resulted in a relatively high
crack growth resistance in the 25 °C to 800 °C range, with interface delamination, translamellar fracture, and decohesion
of colony boundaries being the main fracture processes. The higher fracture resistance exhibited by the lamellar microstructure
can be attributed, at least partly, to toughening by shear ligaments formed as the result of mismatched crack planes in the
process zone. 相似文献
10.
11.
H. Huang S. P. Wen K. Y. Gao W. Wang Z. R. Nie 《Metallurgical and Materials Transactions A》2013,44(6):2849-2856
The age hardening and the microstructure of dilute Al-Er-Zr alloys were investigated by microhardness tests and TEM. The Al-0.04Er alloy shows a conventional age hardening behavior and obtains a maximum hardness of 410 MPa after aging for 2 h at 523 K (250 °C) due to precipitation of Al3Er. The addition of Zr to Al-Er alloy can slow down the growth of the precipitates and make the age hardening effect remain for a long time in Al-0.04Er-0.04Zr alloy. Addition of Zr retards the decomposition of Al-Er and the Al-0.04Er-0.08Zr alloy can reach higher peak hardness than that of Al-0.04Er after aging for long time at elevated temperature. The precipitation behavior of Al-Er-Zr system is likely to be a new commercial way to developing creep-resistant aluminum alloy. 相似文献
12.
Compression testing of a Ti-44Al-llNb alloy was carried out at various temperatures and for different microstructures. Annealing
was done at temperatures from 1000 °C to 1500 °C to produce the unrecrystallized, duplex (gamma grains plus lamellar colonies)
and the fully lamellar microstructures. Samples of each of these microstructures were then tested in air at room temperature
and at various temperatures from 1000 °C to 1300 °C. Results indicate that successively higher temperature anneals produce
increasing grain or colony sizes from 138 jam in the unrecrystallized microstructure to 1017 ώm in the fully lamellar microstructure.
A sequentially lower yield stress was produced on samples tested at increasingly higher temperatures for a given microstructure.
In addition, a minimum yield stress on each yield stressvs temperature curve was recorded for the duplex microstructure with a colony size of 154ώm. One promising result was a sample of this microstructure tested at room temperature, where a yield stress of better than
800 MPa and a compressive strain at the cessation of testing of better than 14 pct were obtained. 相似文献
13.
The hot deformation behavior, microstructure development, and fracture characteristics of a wrought two-phase γ-titanium aluminide
alloy Ti-45.5Al-2Nb-2Cr containing a fine, equiaxed microstructure were investigated with special reference to the influence
of temperature transients immediately pre-ceding plastic deformation. Specimens were soaked at 1321 °C or 1260 °C, cooled
directly to test temperatures of 1177 °C and 1093 °C, and upset under conditions of constant strain rate and tem-perature.
Plastic flow behavior and microstructure evolution occurring in tests involving prior tem-perature transients were compared
with those occurring in specimens which were directly heated to the test temperature and upset under identical deformation
conditions. Flow curves associated with prior exposure at 1321 °C exhibited very sharp peaks and strong flow softening trends
compared to those obtained under isothermal conditions,i.e., involving no temperature transients. During cooling from 1321 °C, the metastable α phase undergoes limited or complete decomposition
into α/α2 + γ lamellae, depending on the final temperature (1177 °C/1093 °C). Subsequent hot deformation leads to partial globularization
of the lamellae together with extensive kinking and reorientation of lamellae. In contrast, isothermal deformation at 1177
°C/1093 °C preserves the fine, equiaxed microstructure, through dynamic recrystallization of the γ grains. Cracking observed
in specimens deformed at 1093 °C and 1.0 s−1 after exposure at 1321 °C has been attributed to the low rate of globularization as well as the occurrence of shear localization.
Plastic flow behavior observed in this work is compared with that observed in several single-phase and two-phase gamma titanium
aluminide alloys in order to identify mechanism(s) responsible for flow softening. 相似文献
14.
A relatively new titanium alloy, TIMETAL 21S (Ti-15Mo-2.7Nb-3Al-0.2Si-0.15O (in wt pct)), is a potential matrix material for
advanced titanium matrix composites for elevated temperature use. In order to develop a perspective on the microstructural
stability of this alloy, the influence of several commonly used heat treatments on the microstructure of TIMETAL 21S was studied
using optical and transmission electron microscopy (TEM). Depending on the specific thermal treatment, a number of phases,
includingα,ω- type, and silicide, can form in this alloy. It was found that both recrystallized and nonrecrystallized areas could be present
in the microstructure of an annealed bulk alloy, but the microstructure of annealed sheet alloy was fully recrystallized.
The mixed structure of the bulk alloy, developed as a result of inhomogeneous deformation, could not be removed by heat treatment
alone at 900 °C. Athermalω-type phase formed in this alloy upon quenching from the solution treatment temperature (900 °C). Silicide precipitates were
also found in the quenched sample. Thermal analysis was used to determine theβ transus and silicide solvus as close to 815 °C and 1025 °C, respectively. In solution-treated and quenched samples, a high-temperature
aging at 600 °C resulted in the precipitation ofα phase. The precipitation reaction was slower in the recrystallized regions compared to the nonrecrystallized regions. During
low-temperature aging (350 °C), the ellipsoidalω-type phase persisted in the recrystallized areas even after 100 hours, whereas a high density ofα precipitates developed in the nonrecrystallized areas within only 3 hours. The observed behavior in precipitation may be
related to the influence of substructure in the nonrecrystallized areas, providing for an enhanced kinetics during aging.
Theα precipitates (formed during continuous cooling from the solution treatment temperature, low-temperature aging, and high-temperature
aging) always obeyed the Burgers orientation relationship. With respect to the microstructure, TIMETAL 21S is similar to other
solute-lean, metastableβ titanium alloys. 相似文献
15.
S. A. Nikulin S. O. Rogachev A. B. Rozhnov V. M. Khatkevich T. A. Nechaikina M. V. Morozov 《Russian Metallurgy (Metally)》2016,2016(4):375-379
The microstructure and hardness of a layered steel 08Kh17T/V–10Ti–5Cr/steel 08Kh17T composite, which was prepared by torsion under a high hydrostatic pressure at temperatures of 20, 200, and 400°C, have been studied. Severe plastic deformation under used conditions is shown to provide good joining of layers, which is accompanied by their substantial hardening (from 2.0 to 3.5 times). During deformation at temperatures of 20 and 200°C, fragmentation of the vanadium alloy layer into thinner layers is observed; at 400°C, mainly a plane interface between the vanadium alloy and the steel layers is formed. 相似文献
16.
Dual-phase structures are produced in the three experimental steels, namely A1, A2 and A3, a) by air-cooling from the austenitising temperature (910°C) and then intercritically annealing the ferrite-pearlite structure at 750°C and 810°C followed by water quenching, and b) by water-quenching from the same austenitising temperature and then intercritically annealing the martensitic structure again at 750°C and 810°C followed by water quenching. The ferrite phases present in the alloys A1 and A2 have formed in two different ways: i) before and/or during intercritical annealing (old ferrite) and ii) during cooling of the alloys from the intercritical annealing temperature (new ferrite). The amount of new ferrite has been found to be larger in alloy A1 as compared to alloy A2. Alloy A3 did not show any measurable amount of new ferrite. TEM studies did not reveal any significant difference in microstructure in any of the alloys as a result of the initial heat treatment. The volume percent of martensite is maximum in alloy A2 and minimum in alloy A1, with alloy A3 coming in between. Although the amount of martensite in alloy A1 is somewhat lower than that in alloy A3, the overall strength of alloy A1 is higher than that of alloy A3 due possibly to the significant solid solution hardening of the ferritic matrix caused by silicon. Alloy A2 has been found to have the highest strength amongst the three alloys. 相似文献
17.
M. Doner J. C. Diprimio E. I. Salkovitz 《Metallurgical and Materials Transactions B》1974,5(2):433-439
Low cycle fatigue properties of unidirectionally solidified lamellar eutectic Ni-51 Cr alloy were determined and compared
with those of the cast microstructure in the temperature range of 300° to 760°C. Both materials exhibited an initial cyclic
strain hardening followed by saturation over most of the temperature range. The rate and the amount of cyclic work-hardening
decreased with temperature above 600°C. Rapid softening due to macro-crack propagation occurred at later stages of the fatigue
process, which occupied an increasing portion of the fatigue life in the lamellar material as the strain amplitude was decreased.
At Δ∈T = 0.0190, the lamellar material exhibited longer fatigue life over the entire temperature range which has been related to
the ability of Cr-rich lamellae to deflect fatigue cracks. At 625°C, the fatigue life (Nf) of both materials was related to the plastic strain range ( Δ∈P) through the relationship (Δ∈P/2 =K(2Nf)c wherec andK are -0.39 and 0.068 for the lamellar, and -0.45 and 0.074 for the cast structure, respectively. At this temperature with
decreasing strain amplitude lamellar material became more resistant to fatigue than as-cast structure, which has been related
to the more efficient deflection of fatigue cracks by Cr-rich lamellae at lower strain amplitudes .
Formerly with the Dept. of Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, Pa.
Formerly Visiting Scientist, Department of Metallurgical and Materials Engineering, University of Pittsburgh
Formerly Professor and Chairman, Department of Metallurgical and Materials Engineering, University of Pittsburgh 相似文献
18.
N. Masahashi S. Watanabe S. Hanada K. Komatsu G. Kimura 《Metallurgical and Materials Transactions A》2006,37(5):1665-1673
Laminates of an iron-aluminum alloy (20Al) and three types of steel—chromium molybdenum (CrMo), high carbon (FeCMn), and precipitation
hardening steel with niobium carbide (FeCNb)—were fabricated at 600 °C and 1000 °C by clad rolling based on the compression
stress ratio of 20Al to steel. The laminates fabricated at 600 °C exhibit a deformation microstructure with partial recrystallization,
while those at 1000 °C reveal a refined microstructure. The 20Al layer of all the laminates exhibit a {001}〈110〉 texture,
and the intensity of the texture increases with a decrease in the fabrication temperature and an increase in the reduction.
The bending deformability of a laminate increases with a decrease in the compression stress ratio and by a reduction in the
intensity of the {001}〈110〉 texture. The clad plate is further rolled at room temperature to a thickness of approximately
150 μm, which enables winding without damage. It is concluded that a high-strength steel at high temperatures and a high Al
content in the Fe-Al alloy is beneficial for the fabrication of deformable laminates. 相似文献
19.
K. Bhanu Sankara Rao M. G. Castelli G. P. Allen John R. Ellis 《Metallurgical and Materials Transactions A》1997,28(2):347-361
The low-cycle fatigue (LCF) behavior of a wrought cobalt-base superalloy, Haynes 188, has been investigated over a range of
temperatures between 25 °C and 1000 °C employing a triangular waveform and a constant strain amplitude of ±0.4 pct. Correlations
between macroscopic cyclic deformation and fatigue life with the various microstructural phenomena were enabled through scanning
electron microscopy (SEM) and transmission electron microscopy (TEM), detailing the crack initiation and propagation modes,
deformation substructure, and carbide precipitation. Cyclic stress response varied as a complex function of temperature. Dynamic
strain aging (DSA) was found to occur over a wide temperature range between 300 °C and 750 °C. In the DSA domain, the alloy
exhibited marked cyclic hardening with a pronounced maximum at 650 °C. Dynamic strain aging has been documented through the
occurrence of serrated yielding, inverse temperature dependence of maximum cyclic stress, and cyclic inelastic strain developed
at half of the fatigue life. Additionally, the alloy also displayed a negative strain rate sensitivity of cyclic stress in
the DSA regime. These macroscopic features in the DSA domain were accompanied by the substructure comprised of coplanar distribution
of dislocations associated with the formation of pileups, stacking faults, and very high dislocation density. Toward the end
of the DSA domain, dislocation pinning by M23C6 precipitates occurred predominantly. The deformation behavior below and above the DSA domain has also been investigated in
detail. The temperature dependence of LCF life showed a maximum at ≈300 °C. The drastic reduction in life between 300 °C and
850 °C has been ascribed primarily to the deleterious effects of DSA on crack initiation and propagation, while the lower
life at temperatures less than 200 °C has been attributed to the combined influence of low ductility and larger cyclic response
stress. 相似文献
20.
Fatigue crack growth experiments were carried out on Ti6242 alloy with large colony size. The alloy was heat treated to provide
three different lamella size; fine, coarse, and extra coarse. Tests were conducted at two temperatures, 520 °C and 595 °C,
using two loading frequencies, 10 and 0.05 Hz. The latter frequency was examined with and without a 300-second hold time.
All tests were performed in air environment and at a stress ratio of 0.1. This study shows that at 520 °C, the Fatigue crack
growth rate (FCGR) is not significantly influenced by changes in the microstructure. For 0.05 Hz/low ΔK, however, the FCGR is higher in the fine lamellar microstructure and is accompanied by- the appearance of a plateau, which
disappears in the extra large lamella microstructure. Furthermore, the addition of a 300-second hold time does not alter the
crack growth rate. At 595 °C, while the general level of the FCGR is higher than that at 520 °C, the effects of loading frequency
and hold time remain similar to those reported at the lower temperature. Unlike the results at 520 °C, however, the FCGR at
low δK is not influenced by variations in lamellar microstructure. Under all test conditions, the fatigue process is predominantly
controlled by one single mechanism associated with transcolony fracture and formation of quasi-cleavage facets. The fatigue
crack growth results and the associated fracture behavior as obtained in this study are correlated to the crack-tip shear
activity and transmission at the α/β interfaces. A general hypothesis accounting for the role of loading frequency, temperature, and microstructure on the observed
cracking mechanisms is presented. 相似文献