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1.
Tensile properties up to 1100°F and the creep resistance at 1000°F were correlated with composition for twelve complex developmental
titanium alloys with additions of Al, Ga, Sn, Mo, Zr, and Si. Creep resistance for these alloys in the β heat-treated condition
was found to be strongly dependent on the totalα stabilizer content and the silicon concentration. The creep activation energy for a Ti-4.5 Al-2 Sn-3 Zr-3 Ga-1 Mo-0.5 Si
alloy, established over the 900° to 1100°F temperature range, was about 100 kcal per g-mole. This high creep activation energy
is hypothesized to result from dispersion strengthening within theα matrix by the Ti3
X
(X = Al, Ga, Sn) phase and pinning of the interplatelet and priorβ grain boundaries by the Zr5Si3 phase. Both phases were identified by transmission electron microscopy in these respective locations. Metallurgical instability,
as evidenced by decreased fracture toughness, is also shown to be relatable to the totalα stabilizer content. The activation energy for the embrittlement process is about 45 kcal per g-mole. which approximates that
for interdiffusion of gallium inα titanium. 相似文献
2.
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
3.
M. Ghosh S. Kundu S. Chatterjee B. Mishra 《Metallurgical and Materials Transactions A》2005,36(7):1891-1899
Solid-state diffusion bonding of Ti-6Al-4V and type 304 SS was investigated in the temperature range of 750 °C to 950 °C,
under a uniaxial load for 5.4 ks in vacuum. The diffusion bonds were characterized using light and scanning electron microscopy.
The scanning electron microscopic images in backscattered mode show the existence of different reaction layers in the diffusion
zone. The composition of these layers was determined by energy-dispersive X-ray spectroscopy (EDS) to contain the α-Fe, χ, λ, FeTi, β-Ti, and Fe2Ti4O phases. The presence of these intermetallics was confirmed by X-ray diffraction. The bond strength was evaluated, and the
maximum tensile strength of ∼342 MPa and the maximum shear strength of ∼237 MPa were obtained for the diffusion couple processed
at 800 °C due to the finer width of the brittle intermetallic layers. With a rise in joining temperature, the bond strength
drops owing to an increase in the width of the reaction layers. The activation energy and growth constant were calculated
in the temperature range of 750 °C to 950 °C for the reaction products. The χ phase showed the fastest growth rate. A fracture-surface observation in a scanning electron microscope (SEM) using EDS demonstrates
that failure takes place mainly through the β-Ti phase for the diffusion couples processed in the aforementioned temperature range. 相似文献
4.
Processing of two-phase γ-TiAl alloys (Ti-47Al-2Cr-2Nb, or minor modifications thereof) above the α-transus temperature (T
α
) produced unique refined-colony/ultrafine lamellar structures in both powder-and ingot-metallurgy (PM and IM, respectively)
alloys. These ultrafine lamellar structures consist of fine laths of the γ and α
2 phases, with average interlamellar spacings (λ
L
) of 100 to 200 nm and α
2-α
2 spacings (λ
α
) of 200 to 500 nm, and are dominated by γ/α
2 interfaces. This characteristic microstructure forms by extruding PM Ti-47Al-2Cr-2Nb alloys at 1400 °C and also forms with
finer colony size but slightly coarser, fully lamellar structures by hot-extruding similar IM alloys. Alloying additions of
B and W refine λ
L
and λ
α
in both IM Ti-47Al (cast and heat treated at 1400 °C) and IM Ti-47Al-2Cr-2Nb alloys (extruded at 1400 °C). The ultrafine
lamellar structure in the PM alloy remains stable during heat treatment at 900 °C for 2 hours but becomes unstable after 4
hours at 982 °C; the ultrafine lamellar structure remains relatively stable after aging for >5000 hours at 800 °C. Additions
of B+W dramatically improve the coarsening resistance of λ
L
and λ
α
in the IM Ti-47Al alloys aged for 168 hours at 1000 °C. In both the PM and IM Ti-47Al-2Cr-2Nb alloys, these refined-colony/ultrafine
lamellar structures correlate with high strength and good ductility at room temperature, and very good strength at high temperatures.
While refining the colony size improves the room-temperature ductility, alloys with finer λ
L
are stronger at both room and high temperatures. Additions of B + W produce finer as-processed λ
L
and λ
α
in IM TiAl alloys and stabilize such structures during heat treatment or aging.
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
5.
Jeoung Han Kim S. L. Semiatin You Hwan Lee Chong Soo Lee 《Metallurgical and Materials Transactions A》2011,42(7):1805-1814
The flow behavior of the α and β phases in Ti-6Al-4V was interpreted in the context of a self-consistent modeling formalism. For this purpose, high-temperature
compression tests were conducted at various temperatures for a single-phase α alloy (Ti-7Al-1.5V), a variety of near-β alloys, and the two-phase alloy Ti-6Al-4V, each with an equiaxed microstructure. The flow behavior of the α phase in Ti-6Al-4V was deduced from the experimental results of the single-phase α alloy. The flow behavior of the β phase, which was predicted by using the self-consistent approach and the measured flow behaviors of Ti-6Al-4V and Ti-7Al-1.5V,
showed good agreement with direct measurements of the various near-β alloys. From these results, it was shown that the strength of the α phase is approximately three times higher than that of the β phase at temperatures between 1088 K and 1223 K (815 °C and 950 °C). It was also concluded that the relative strain rates
in the two phases varies significantly with temperature. The usefulness of the approach was confirmed by comparing the predicted
and measured flow stresses for other Ti-6Al-4V and near-α alloys. 相似文献
6.
G. Sreenivasulu N. K. Mukhopadhyay G. V. S. Sastry A. K. Singh 《Metallurgical and Materials Transactions A》2005,36(10):2601-2613
The stability of a lamellar structure consisting of α
2 and γ phases in alloys Ti-48Al, Ti-48Al-2Mo, Ti-48Al-4Nb, and Ti-48Al-1Mo-4Nb has been studied as a function of aging time and
temperature. The alloys were solution treated (1400 °C, 30 min, and air-cooled (AC)) and aged at 1000 °C and 1100 °C for 1,
4, and 16 hours, respectively. The results indicate that the kinetics of lamellae to equiaxed transformation depends on alloy
chemistry, aging time, and temperature. The Nb decreases and Mo increases the kinetics of transformation. The combined effect
of Nb and Mo results in the highest volume fraction of equiaxed microstructure at a given aging time and temperature. The
results have been discussed in relation to microstructural features and have been compared with those reported in other α
2+γ alloys. 相似文献
7.
T. Hamajima G. Luetjering S. Weissmann 《Metallurgical and Materials Transactions B》1972,3(11):2805-2810
The influence of aluminum additions to a Ti-7 at. pet Mo alloy on the phase equilibria was investigated. The microstructures
of the alloys, Ti-7 pct Mo-7 pct Al and Ti-7 pct Mo-16 pct Al, were determined by light and electron microscopy. It was found
that with increasing aluminum concentration the formation of the metastable w phase was suppressed. In the Ti-7 pct Mo-16
pct Al alloy the β phase decomposed upon quenching by precipitating coherent, ordered particles having a B2 type of crystal
structure (β2). At low temperatures the equilibrium phases for this alloy were β + α+ β
2, whereas at high temperature (850° to 950°C) the Ti3Al phase was in two-phase equilibrium with the β phase. The four-phase equilibrium which exists at a temperature of about
550°C involves the reaction β + Ti3Al ⇌ α + β2.
G. LUETJERING, formerly Staff Member Materials Research Center, Allied Chemical Corp., Morristown, N. J., 相似文献
8.
9.
Kwai S. Chan 《Metallurgical and Materials Transactions A》1992,23(2):497-507
The feasibility of developing hydrogen-tolerant microstructures for α2 titanium aluminide alloys by heat treatment has been investigated. In particular, a variety of microstructures for the Ti-24Al-11Nb
(in atomic percent) alloy was developed by manipulating the heat-treatment conditions. After screening by the Vicker hardness
tests, three microstructures were evaluated for their resistance to hydrogen embrittlement by performing sustained load creep
tests in a gaseous hydrogen environment at an elevated temperature, followed by post-creep, slow-rate tensile tests at room
temperature. Tensile tests of hydrogen-exposed specimens without prior creep exposure were also performed. The results indicate
that one particular microcstructure of the Ti-24Al-11Nb alloy is resistant to hydrogen embrittlement under the test conditions
and hydrogen contents investigated, providing evidence that heat-treatment techniques can be used to develop hydrogentolerant
microstructures for α2 titanium aluminide alloys. 相似文献
10.
The effect of deformation-induced transformation on the fracture toughness of commercial titanium alloys 总被引:2,自引:0,他引:2
M. Niinomi T. Kobayashi I. Inagaki A. W. Thompson 《Metallurgical and Materials Transactions A》1990,21(6):1733-1744
The effect of deformation-induced transformation of metastableβ phase on the ductility and toughness of four commercial titanium alloys was investigated. Tensile tests, Charpy impact tests,
and both static and dynamic fracture toughness tests were carried out at temperatures between 77 and 473 K on four titanium
alloys containing metastableβ phase. Deformation-inducedα″ (orthorhombic martensite) was observed in an (α + β)-type Ti-6Al-2Sn-4Zr-6Mo alloy. The dynamic fracture toughness of this
alloy increased considerably at 223 K compared to those at other temperatures. In another (α + β)-type Ti-6A1-4V alloy, the
static fracture toughness at 123 K and the dynamic fracture toughness at 223 K were increased considerably by the presence
of deformation-induced martensite compared to those at other temperatures. The strength increased as the temperature decreased
in this alloy. An abnormal elongation of aβ-type alloy, Ti-15V-3Al-3Sn-3Cr, at 123 K was attributed to the mechanical twinning of theβ phase. However, the effect of deformation-induced transformation on the fracture toughness of Ti-3Al-8V-6Cr-4Mo-4Zr alloy
was not observed.
Formerly Visiting Associate Professor, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University,
Pittsburgh, PA.
Formerly with the Department of Production Systems Engineering, Toyohashi University of Technology. 相似文献
11.
Structure and properties of a β solution treated,quenched, and aged si-bearing near-α titanium alloy
The microstructure, tensile properties, and fractographic features of a near-α titanium alloy, IMI 829(Ti-6.1 wt pct Al-3.2
wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-05 wt pct Mo-0.32 wt pct Si) have been studied after aging over a temperature range of
550°C to 950°C for 24 hours following solution treatment in the β phase field at 1050°C and water quenching. Transmission
electron microscopy studies revealed that aging at 625°C and above produced discrete silicides at α′ interplatelet boundaries.
However, aging at 900°C and above has also resulted in the precipitation of β phase along the lath boundaries of martensite.
The silicides have been found to have a hexagonal structure withc=0.36 nm anda=0.70 nm (designated as S2 by earlier workers). There is a significant improvement in yield and ultimate tensile strength after aging at 625°C, but
there is less improvement at higher aging temperatures. The tensile ductility is found to be drastically reduced. While the
fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting
of facets, featureless parallel bands, and extremely fine dimples. 相似文献
12.
K. E. Wiedemann R. N. Shenoy J. Unnam 《Metallurgical and Materials Transactions A》1987,18(8):1503-1510
Standards were prepared for calibrating microanalyses of dissolved oxygen in unalloyed α-Ti and Ti-6Al-2Sn-4Zr-2Mo. Foils
of both these materials were homogenized for 120 hours in vacuum at 871 °C following short exposures to the ambient atmosphere
at 854 °C that had partially oxidized the foils. The variation of Knoop microhardness with oxygen content was calibrated for
both materials using 15 g and 5 g indentor loads. The unit-cell lattice parameters were calibrated for the unalloyed α-Ti.
Example analyses demonstrate the usefulness of these calibrations and support an explanation of an anomaly in the lattice
parameter variation. The results of the calibrations have been tabulated and summarized using predictive equations. 相似文献
13.
Howard G. Nelson Dell P. Williams James E. Stein 《Metallurgical and Materials Transactions B》1972,3(2):473-479
Environmental hydrogen embrittlement of a Ti-6 Al-4 V alloy has been studied as a function of test displacement rate and of
variations inα- β microstructure. Embrittlement in low pres sure (∼1 atm) gaseous hydrogen was inversely dependent on test displacement rate
and strongly dependent on microstructure. At a given displacement rate, microstructures having a continuous α-phase matrix
were less severely embrittled than those having a continuous β-phase matrix. Further, brittle fracture occurred in the former
microstructures by transgranular cleavage and in the latter microstructures by intergranular separation. These observations
are consistent with previous studies made on slow strain-rate embrittlement of hydrogen-charged titanium alloys and are explained
in terms of relative hydrogen transport rates within the α-phase and β-phase titanium. 相似文献
14.
The effect of alloying additions on the superplastic properties of Ti-6 pct Al-4 pct V 总被引:1,自引:0,他引:1
The superplastic deformation properties of Ti-6 pct Al-4 pct V and modified alloys containing 1/4 pct, 1/2 pct, 1 pct, and
2 pct of either cobalt or nickel have been investigated in the temperature range 950 to 750 °C. The results show that both
cobalt and nickel modified alloys have reduced flow stresses, in comparison with Ti-6 pct Al-4 pct V, the reductions being
particularly marked at the lower temperatures and lower strain rates. The results are shown to be consistent with an isostress
model for the deformation of (α + β) two-phase alloys in which the varying β volume fractions and differing diffusivities
of titanium, cobalt, or nickel in the β phase are taken into account. 相似文献
15.
C. J. Boehlert B. S. Majumdar V. Seetharaman D. B. Miracle 《Metallurgical and Materials Transactions A》1999,30(9):2305-2323
Phase transformations and the resulting microstructural evolution of near-Ti2AlNb and Ti-12Al-38Nb O+bcc orthorhombic alloys were investigated. For the near-Ti2AlNb alloys, the processing temperatures were below the bcc transus, while, for Ti-12Al-38Nb, the processing temperature was
supertransus. Phase evolution studies showed that these alloys contain several constituent phases, namely, bcc, O, and α
2; when present, the latter was in small quantities compared to the other phases. The transmission electron microscopy (TEM),
scanning electron microscopy (SEM), and X-ray investigations of samples that were solutionized and water quenched were used
to estimate the phase fields, and a pseudobinary diagram based on Ti=50 at. pct was modified. The aging-transformation behavior
was studied in detail. For solutionizing temperatures between 875 °C and the bcc transus, the phase composition and volume
fraction of the near-Ti2AlNb alloys adjusted through relative size changes of the equiaxed B2, O, and α
2 grains. The aging behavior followed three distinct transformation modes, dependent on the solutionizing and aging temperatures.
Widmanstatten formation was observed when a new phase evolved from a parent phase. Thus, Widmanstatten O phase precipitated
within the B2 phase for supertransus fully B2 microstructures, as well as for substransus α
2+B2 microstructures. Similarly, Widmanstatten B2 phase can form from a fully O microstructure, a transformation that has not
been observed before. In the case of equiaxed O+B2 solutionized and water-quenched microstructures, Widmanstatten O-phase
formation occurred only below 875 °C. For the subtransus-solutionized and water-quenched microstructures, a second aging transformation
mode, cellular precipitation, was dominant below 750 °C. This involved formation of coarse and lenticular O phase that grew
into the prior B2 grains from the grain boundaries. A third transformation mode involved composition-invariant transformation,
where the fully B2 supertransus-solutionized and water-quenched microstructure transformed to a fully O microstructure at
650 °C. This microstructure reprecipitated B2 phase out of the O phase with continued aging time. For Ti-12Al-38Nb, Widmanstatten
O precipitation remained the only transformation mode. It is shown that subtransus processing offers flexibility in controlling
microstructures through postprocessing heat treatments. 相似文献
16.
J. F. Nie S. Sridhara B. C. Muddle 《Metallurgical and Materials Transactions A》1992,23(Z1):3193-3205
The microstructures of Al-3Ti-lCe (wt pct) and Al-5Ti-5Ce alloys melt-spun under controlled He atmosphere have been characterized
using analytical electron microscopy. The rapidly so- lidified microstructures comprise uniform, fine-scale dispersions of
intermetallic phase in an aluminum matrix, and particular attention has been given to identification of the dispersed phases.
In the Al-3Ti-lCe alloy, the dispersed particles are polycrystalline with a complex twinned substructure and a diamond cubic
crystal structure (α
o
= 1.44 ± 0.01 nm) and composition consistent with the ternary compound Al20Ti2Ce (Al18Cr2Mg3 structure type, space group Fd3m). In the Al-5Ti-5Ce alloy, there is, in addition to the dispersed ternary phase, a separate
uniform array of fine-scale particles of the binary compound Al11Ce3. The majority of such particles have the body-centered orthorhombic structure of the low-temperature polymorph, α-Al11Ce3, but there is evidence to suggest that at least some particles developvia initial formation of the high-temperature body-centered tetragonal phase, β-Al11Ce3. The accumulated evidence sug- gests that both binary and ternary particles formed as primary phases directly from the melt
during rapid solidification, leaving only small concentrations of solute in aluminum matrix solid solution. Both phases are
observed to be resistant to coarsening for up to 240 hours at 400 °C.
Formerly Research Fellow, Department of Materials Engineering, 相似文献
17.
The solute distribution within and near the proeutectoid α plates which were isothermally formed from the β solid solution
in Ti-V, Cr, and Fe alloys was measured in a scanning transmission electron microscope (STEM) equipped with an energy dispersive
X-ray (EDX) analyzer. In the Ti-4.9 at. pct Cr and 5.1 at. pct Fe alloys in which all measurements were made above the calculated
To temperature (the maximum temperature at which the diffusionless β to α transformation can occur), the solute concentration
in the α plates was close to final equilibrium as long as the analyzed plates had grown to the sizes comparable to the STEM
spatial resolution (∼50 nm). In the Ti-5.4 at. pct V and 2.6 at. pct Fe alloys, the solute concentration in the α plates was
already close to equilibrium at very short reaction times at temperatures more than 100 °C below To. The ledgewise diffusion growth mechanism can roughly account for the thickening behavior of these plates, consistent with
the results of the composition analysis. In the Cr alloy, a microstructure resembling the lower bainite in steels, i.e., nonlamellar
distribution of TiCr2 particles in thick α plates, was formed by coalescence of solute-depleted thin α plates which were formed in aggregates (in
sheaves) at an earlier stage of growth.
This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World
Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations
Committee and the TMS Ferrous Metallurgy Committee. 相似文献
18.
Metallographic studies have been made of precipitation processes in Ti-50 pct Ni and Ti-52 pct Ni (at. pct) shape memory alloys.
The eutectoid and peritectoid reactions previously reported for near-equiatomic and Ni-rich TiNi alloys were not observed
for either composition. In the Ti-52Ni alloy, diffusional transformations take place, similar to those in supersaturated alloys.
The precipitation sequence can be written asβ
0 → Ti11Ni14 → Ti2Ni3 → TiNi3. The solidus line of the TiNi phase in the Ti-52Ni alloy lies at 812 ± 22 °C. Morphological characteristics of the various
precipitate phases are described in detail.
M. NISHIDA, formerly with the University of Illinois 相似文献
19.
Observations have been made of cracking which develops after small plastic strains in the Ti-6Al-2Sn-4Zr-6Mo and Ti-6A1-4V
alloys at α-@#@ β interfaces, and a long slip band in α. Hydrogen and surface stresses and heat treat condition,i.e., solution treated or solution treated and aged have been ruled out. Cracking is attributed to intense slip commencing at
the α-@#@ β interfaces, and progressing into the α, or at martensite-matrix interfaces. 相似文献
20.
R. H. Van Stone J. R. Low J. L. Shannon 《Metallurgical and Materials Transactions A》1978,9(4):539-552
Fractography and metallographic sectioning were used to investigate the influence of microstructure on the fracture mechanism
and fracture toughness (KIc) of normal interstitial and extra low interstitial (ELI) Ti-5Al-2.5Sn at 20 K (-423°F) and 77 K (-320°F). Plates of each
grade were mill annealed at 815°C (1500°F) followed by either air or furnace cooling. These variations in composition and
cooling rate resulted in differences in the volume fraction and internal structure of the dispersed β phase and in the ordering
of the α matrix. The ELI alloys were tougher than the normal interstitial plates. KIc of the furnace-cooled ELI plate was 25 pct lower than that of the air-cooled ELI material. Variations in cooling rate had
no influence of KIc of the normal interstitial alloys. Fractography showed that a large portion of the fracture surfaces were covered with elongated
dimples commonly called “flutes.” Metallographic sections of specimens deformed at 77 K showed that these features form at
the intersections of slip bands or deformation twins with grain or twin boundaries. Ordering and higher interstitial levels
increase the local strain in slip bands resulting in void nucleation at lower macroscopic strains and lower KIc values.
Formerly Graduate Student, Department of Metallurgy and Materials Science, Carnegie-Mellon University, Pittsburgh, PA. 相似文献