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1.
J. Daniel Whittenberger Thomas J. Moore 《Metallurgical and Materials Transactions A》1979,10(11):1597-1605
A study of the flow strength, creep resistance and diffusion welding characteristics of the titanium alloy Ti-6Al-2Nb-1Ta-0.8Mo
has been conducted. Two mill-processed forms of this alloy were examined. The forged material had been essentially processed
above the beta transus (∼1275 K) while the rolled form had been subjected to work below the beta transus. Between 1150 and
1250 K, the forged material was stronger and more creep resistant than the rolled alloy. Both forms exhibit superplastic characteristics
in this temperature range. Strain measurements during diffusion welding experiments at 1200 K reveal that weld interfaces
have no measurable effect on the overall creep deformation. Significant deformation appears to be necessary to produce a quality
diffusion weld between superplastic materials. A “soft” interlayer inserted between faying surfaces would seemingly allow
manufacture of quality diffusion welds with little overall deformation. 相似文献
2.
The effect of microstructure and deformation behavior on the hot ductility of Ti-6Al-2Nb-1Ta-0.8Mo 总被引:1,自引:0,他引:1
Hot ductility of the alloy Ti-6Al-2Nb-lTa-0.8Mo has been correlated with microstructure and fracture behavior. Low hot ductility was found to be associated with strain localization within the grain boundary alpha phase, producing void formation along the prior-beta grain boundaries and inter-granular fracture. Microstructural features that appear to be critical to the strain localization process are beta grain shape and alpha phase morphology. For the case of Widmanstätten + grain boundary alpha phase morphologies, equiaxed prior-beta grains formed by annealing above the beta transus are required to produce significant strain localization. For the beta processed structure with elongated beta grains due to working above the beta transus temperature, the orientation of the grain boundary alpha phase limits strain localization due to low resolved shear stress. The martensitic Widmanst?ten alpha prime structure formed by quenching from above the beta transus temperature rapidly forms grain boundary alpha upon reheating to temperatures high in the alpha + beta phase field. This results in strain localization in the grain boundary regions in an apparently similar manner to that observed in the Widmanstätten + grain boundary alpha phase morphologies with equiaxed prior-beta grains. 相似文献
3.
FU-Shiong Lin E. A. Starke A. Gysler 《Metallurgical and Materials Transactions A》1984,15(10):1873-1881
The Ti-6Al-2Nb-lTa-0.8Mo alloy was processed to develop both near-basal and transverse textures. Samples were annealed at different temperatures to vary the equiaxed alpha grain size and the thick-ness of the grain boundary beta, and subsequently quenched in order to transform the beta phase to either martensite, tempered martensite, or Widmanstätten alpha + beta. The effect of microstructure and texture on tensile properties and on fracture toughness was investigated. In addition, yield locus diagrams were constructed in order to study the texture strengthening effect. The yield strength was found to be strongly dependent on the thickness and Burgers relationship of the transformed beta phase surrounding the alpha grains. A texture hardening effect as large as 60 pct was found for the basal-texture material but only 15 pct for the transverse texture material. These variations are asso-ciated with differences in deformation behavior. 相似文献
4.
J. W. Zhang C. S. Lee J. K. L. Lai D. X. Zou S. Q. Li 《Metallurgical and Materials Transactions A》1998,29(2):559-564
Microstructural evolution during three heat-treatment schedules and the terminal microstructures in an orthorhombic alloy
of Ti-25Al-17Nb-1Mo were observed and analyzed with optical microscopy, transmission electron microscopy (TEM), scanning electron
microscopy (SEM), and X-ray diffraction (XRD). The creep behavior of the alloy with three different microstructures (a coarse-lath,
fine-lath, and fine equiaxed microstructure) was studied over a temperature range of 600 °C to 750 °C and over a stress range
of 150 to 400 MPa in air. The steady-state creep rates, apparent stress exponents, and apparent creep activation energies
of the various samples have been determined. The results show that creep behaviors in the alloy are strongly influenced by
microstructure. The effect on creep by some of the microstructural features, such as the multivariants within the coarse laths
and the interfaces of the laths and the equiaxed grains, is also discussed. 相似文献
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F. S. Lin E. A. Starke S. B. Chakrabortty A. Gysler 《Metallurgical and Materials Transactions A》1984,15(6):1229-1246
An alpha + beta Ti-6Al-2Nb-lTa-0.8Mo alloy with an initial Widmanstätten structure was thermally treated to produce a wide range of microstructures. The effects of individual microstructural parameters on deformation behavior and mechanical properties were investigated. The results show that the Widmanstätten colony boundaries are major barriers to slip. However, the slip distance can be decreased to a distance equal to the thickness of acicular alpha by transforming the beta phase in the Widmanstätten structure to martensite by quenching from 950°C. The decrease in slip distance is accompanied by a 25 pct increase in yield strength with no loss in ductility. A large decrease in ductility occurs after excursions above the beta-transus. The development of both equiaxed beta grains during heating in the beta phase field and continuous grain boundary alpha during cooling in the alpha + beta phase field leads to strain localization along prior beta grain boundaries. 相似文献
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Wonsuk Cho Anthony W. Thompson James C. Williams 《Metallurgical and Materials Transactions A》1990,21(2):641-651
A study has been made of the role of microstracture in room-temperature tensile properties as well as elevated-temperature
creep behavior of an advanced Ti3Al-base alloy, Ti-25Al-10Nb-3V-lMo (atomic percent). Creep studies have been performed on this alloy as a function of stress
and temperature between 650 °C and 870 °C, since the use of conventional titanium alloys has generally been restricted to
temperatures below 600 °C. A pronounced influence of microstructure on creep resistance was found. Generally, the β solution-treated
colony-type (slow-cooled or SC) microstructure showed superior creep resistance. This improved creep resistance in β/SC is
accompanied by lower room-temperature tensile strength and ductility. Study of the stress dependence of steady-state creep
rate indicates that increasing temperature caused a gradual decrease in the stress exponentn and a transition in creep mechanism at 870 °C, depending on applied stress level. Transmission electron microscopy observations
of deformed dislocation structures developed during steady-state creep and room-temperature tensile tests, as well as the
corresponding fracture modes, were used to interpret properties as a function of temperature. Finally, creep behavior of the
present Ti3Al alloy was found to be superior to that of conventional near-α titanium alloys.
WONSUK CHO, formerly with Carnegie Mellon University, is Senior Research Staff Member, Kia Technical Center, Yeoeuido, P.O.
Box 560, Seoul, Korea.
JAMES WILLIAMS, formerly Dean of Engineering, Carnegie Mellon University. 相似文献
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12.
R. W. Hayes P. B. Berbon R. S. Mishra 《Metallurgical and Materials Transactions A》2004,35(12):3855-3861
The creep behavior of a cryomilled Al-10Ti-2Cu nanocomposite has been studied at temperatures of 533, 588, and 644 K at initial
applied stresses ranging from 55 to 117 MPa. Although the strain rates fall within the 10−10 to 10−9 S−1 regime, we observe no evidence of threshold-type creep behavior in this material. We attribute this to the unique microstructure
of the present material combined with the mechanism of dislocation slip in ultrafine grain size materials. In particular,
the very fine AIN precipitates present within the microstructure are ineffective as obstacles to dislocations during high-temperature
deformation. The coherent nature of these fine particles along with their extremely small size prevents a strong dislocation-particle
attraction. The inability of the activation energy for self-diffusion in Al to successfully collapse the present creep data
onto a single slope combined with the fact that the true activation energy for creep exceeds the value for lattice self-diffusion
are both features found in materials containing second-phase particles, which deform simultaneously with the matrix during
high-temperature deformation. In the present case, these particles are likely to be Al3Ti. 相似文献
13.
C. H. Ward A. W. Thompson J. C. Williams 《Metallurgical and Materials Transactions A》1995,26(3):703-720
The effects of microstructure and temperature on tensile and fracture behavior were explored for the titanium aluminide alloy
Ti-25Al-10Nb-3V-lMo (atomic percent). Three microstructures were selected for study in an attempt to determine the role of
the individual microstructural constituents in this α2 + B2 alloy. Tensile testing of both round and flat specimens in vacuum indicated a change in deformation behavior from 25
°C to 450 °C. Observations suggested that this change in deformation behavior occurred within the α2 phase. Failure initiation at 450 °C and above was by a ductile process and was associated with the B2 phase. Above 600 °C
and at high strains, plastic deformation occurred predominantly in the B2 phase. Strain localization was observed above 600
°C and found to be due to the lower work-hardening rate of the B2 phase. Strain localization at slip band intersections with
prior β grain boundaries resulted in rapid strain accumulation in the B2 phase. Alignment of secondary α2 laths with the tensile axis at high deformation levels appeared to inhibit shear band localization between voids due to a
lack of participation of the α2 phase in deformation.
Formerly Materials Scientist, Materials Directorate, Wright Laboratory, Wright-Patterson AFB, Dayton, OH 45433, is Program
Manager, Air Force Office of Scientific Research, Boiling AFB, Washington, DC 20332.
Formerly Professor, Carnegie Mellon University, Department of Materials Science and Engineering, Pittsburgh, PA 15213, is
Scientist, Lawrence Berkeley Laboratory, Berkeley, CA 94720. 相似文献
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Creep tests were conducted at 295 K on Ti-6 Al-4 V in the solution treated and aged (4 h at 815 K) condition, and in the as-welded
condition. Some aged specimens were tested after pre-straining. Creep stresses ranged from 40 to 90 pct of the aged material
yield strength. Results showed that creep was of the primary or transient kind in all cases, and was much greater in welded
than in aged material. In general, pre-strains reduced creep, although a strain larger than 10-3 was needed to do this at the highest creep stress. Activation areas A* were between 10 and 20 b2, and thus were similar to tensile results on titanium and its alloys. The microstructural rationale applied to Ti-5 Al-2.5
Sn in earlier work, based on the character of dislocation sources, proved successful in understanding the effects of prestrain
in this work.
Formerly with Sandia Laboratories, Livermore, Calif. 相似文献
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D. Y. Seo T. R. Bieler S. U. An D. E. Larsen 《Metallurgical and Materials Transactions A》1998,29(1):89-98
Cast gamma titanium aluminides are gaining acceptance as potential replacements for superalloy and steel components in many
applications. One particular alloy with W, Mo, and Si additions has shown exceptional primary creep resistance. Quantitative
microscopic comparisons were made between microstructures in undeformed and deformed regions in creep specimens deformed to
strains between 0.1 and 1.5 pct strain, using optical microscope, scanning electron microscope (SEM), and transmission electron
microscope (TEM) techniques. As-hot isostatically pressed (“hipped”) and heat-treated (1010 °C for 50 hours) conditions were
compared. The as-hipped specimen had a higher lamellar volume fraction, and it crept more than 100 times faster. The lamellar
spacing in the lamellar grains systematically decreased by 15 to 35 pct, with increasing stress, during the first 0.1 to 2
pct strain. Precipitates containing W, Mo, and/or Si were observed in the deformed gage and undeformed grip sections of the
heat-treated specimens. Precipitation is nucleated by heat treatment, but, during creep deformation, a more homogeneous and
faster growth process occurs in the gage section than in the aged but undeformed grip section. The gage section had a 35 pct
higher precipitate volume fraction, but their average size was smaller. A lower volume fraction of lamellar grains and the
presence of precipitates account for the excellent creep resistance in the heat-treated alloy.
This article is based on a presentation made in the symposium “Fundamentals of Gamma Titanium Aluminides,” presented at the
TMS Annual Meeting, February 10–12, 1997, Orlando, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations
Committees. 相似文献
18.
Kevin S. McReynolds Seshacharyulu Tamirisakandala 《Metallurgical and Materials Transactions A》2011,42(7):1732-1736
Isothermal oxidation experiments in air were performed on Ti-6Al-2Sn-4Zr-2Mo (Ti-6242) with a bimodal microstructure in the
temperature range 811 K to 922 K (538 °C to 649 °C) for up to 500 hours, and α-case depths were quantified using metallography. Alpha-case depth followed a parabolic variation with time. Alpha-case depths
in excess of 10 μm formed above 811 K (538 °C) and 100-hour exposures. An activation energy of 244 kJ/mol was estimated for diffusion of oxygen
in the α phase of Ti-6242. 相似文献
19.
The alloy Ti-6-Al-4V deforms superplastically in the temperature range 750 to 950° The most important factor which is responsible
for superplastic behavior was found to be the very fine grain size. Strain rate has no direct effect on superplasticity, however
when the strain rate is very low (approximately 2 × 10 s), prolonged exposure to high temperature causes grain growth and
early failure. The strain rate sensitivity factorm = 0.5 and the apparent activation energyAH = 45,000 cal/mole, which is approximately the same as the activation energy for grain boundary self diffusion of titanium.
Both values are independent of strain rate within the range 10 - 2.5 × 10 s. All the experimental points fall in a straight
line when plotted as log (εkTd*
2/DgbGb3) vs log (σ/G) with a slopen = l/m = 2. This is in excellent agreement with the theory of grain boundary sliding accommodated by dislocation motion. 相似文献