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1.
Morihiko Nakamura Naoyuki Itoh Kenki Hashimoto Tokuzo Tsujimoto Toshiyuki Suzuki 《Metallurgical and Materials Transactions A》1994,25(2):321-330
Isothermally forged TiAl-base alloys (Al-rich, Mn-containing, and Cr-containing TiAl) were heat-treated in various conditions,
and equiaxed grain structures consisting of γ and α2 or Β phases were obtained. The heat-treated alloys were tensile tested in vacuum and air at room temperature, and the environmental
effect on tensile elongation was studied. The ductility of the alloys consisting of equiaxed γ grains and a large amount of
α2 grains was not largely affected by laboratory air, and a decrease in the amount of α2 grains resulted in a large reduction of ductility in air. The Β phase in the Cr-containing alloy improved the ductility in
vacuum, but it resulted in a large reduction of ductility in air.
Formerly with Kougakuin University, Shinjyuku-ku, Tokyo,
Formerly with National Research Institute for Metals, Meguro-ku, Tokyo, 相似文献
2.
Sandeep Sangal Naresh C. Goel Kris Tangri 《Metallurgical and Materials Transactions A》1985,16(11):2023-2029
The role of metastable retained austenite(γ
R), its volume fraction, and mechanical stability on the flow characteristics of a dual phase steel containing 20 vol pct of
‘as quenched’ martensite in a ferrite matrix has been examined in this paper employing the flow curve expressions derived
in Part I of this paper. It has been found that for a given volume fraction ofγ
R, its mechanical stability plays a crucial role in enhancing the ductility. Whereas highly stableγ
R does not contribute either to strength or ductility of the steel, highly unstableγ
R which causes an increase in the strength is detrimental to ductility. Aγ
R which is moderately stable and undergoesγ
R → α′ transformation over a larger strain range is beneficial to enhanced ductility. Increasing amounts of moderately stableγ
R significantly increase both the strength and the ductility of dual-phase steels through a sustained work-hardening due toγ
R → α′ transformation. Load transfer which is determined by a parameterq has a significant contribution to work-hardening. A value of ∣|q∣|= 4500 MPa has been found to partition realistically the stress and strain in these steels. 相似文献
3.
Room-temperature deformation behavior of directionally solidified multiphase Ni-Fe-Al alloys 总被引:1,自引:0,他引:1
Directionally solidified (DS) β + (γ + γ′) Ni-Fe-Al alloys have been used to investigate the effect of a ductile second phase on the room-temperature mechanical behavior
of a brittle 〈001〉-oriented β (B2) phase. The ductile phase in the composite consisted of a fine distribution of ordered γ′ precipitates in a γ (fcc) matrix. Three microstructures were studied: 100 pct lamellar/rod, lamellar + proeutectic β, and discontinuous γ. The β matrix in the latter two microstructures contained fine-scale bcc precipitates formed due to spinodal decomposition. Room-temperature
tensile ductilities as high as 12 pct and fracture toughness (K
Q
) of 30.4 MPa √m were observed in the 100 pct lamellar/rod microstructure. Observations of slip traces and dislocation substructures
indicated that a substantial portion of the ductility was a result of slip transfer from the ductile phase to the brittle
matrix. This slip transfer was facilitated by the Kurdjumov-Sachs (KS) orientation relationship between the two phases and
the strong interphase interface which showed no decohesion during deformation. In microstructures which show higher values
of tensile ductility and fracture toughness, 〈100〉 slip was seen in the β phase, whereas 〈111〉 slip was seen in the β phase in the microstructure which showed limited ductility. The high ductility and toughness are explained in terms of increased
mobile dislocation density afforded by interface constraint. The effect of extrinsic toughening mechanisms on enhancing the
ductility or toughness is secondary to that of slip transfer. 相似文献
4.
The effect of a ductile γ′-Ni3Al phase on the room-temperature ductility, temperature-dependent yield strength, and creep resistance of β-NiAl was investigated. Room-temperature tensile ductility of up to 9 pct was observed in directionally solidified β/γ′ Ni-30 at. pct Al alloys, whereas the ductility of directionally solidified (DS), single-phase [001] β-NiAl was negligible. The enhancement in ductility was attributed to a combination of slip transfer from the ductile γ′ to the brittle β phase and extrinsic toughening mechanisms such as crack blunting, deflection, and bridging. As in single-phase Ni3Al, the temperature-dependent yield strength of these two-phase alloys increased with temperature with a peak at approximately
850 K. The creep strength of the β/γ′ alloys in the temperature range 1000 to 1200 K was found to be comparable to that of monolithic β-NiAl. A creep strengthening phase needs to be incorporated in the β/γ′ microstructure to enhance the elevated temperature mechanical properties. 相似文献
5.
Stress-induced martensitic transformations have been studied in Β1 Cu Al Ni single crystals in which two martensite crystal structures can form, Β
i
′
and γ′. By straining specimens at one temperature and releasing the strain at either the same temperature or a different
temperature, stresses corresponding to the transitions Β1
⇌ Β
i
′
,Β
1 ⇌ γ′,Β
i
′
⇌ γ′ could all be measured. This enabled a quantitative stress-temperature diagram to be drawn, giving the stability ranges
of the Β1,Β
i
′
and γ′ phases. The slope of the stress-temperature lines separating the different phases enabled the value of the entropy
changes for the transformations to be calculated. This was very small for theΒ
i
′
→ γ′ transformation (0.08 J/mole K) and much larger for the Β1 →Β
i
′
and Β1 → γ′ transformations (-1.21 and -1.4 J/mole K, respectively). The hysteresis between the forward and reverse transformations
enabled evaluation of the critical free energy for transformation. This was small for the Β1
→ Β
i
′
transformation (-2.9 J/mole), and large for the Β1 → γ′ and Β
i
′
→ γ′ transformations (-28 and -29 J/mole respectively).
Formerly Post Doctoral Fellow, Department of Metallurgy, University of British Columbia 相似文献
6.
Loss of hot ductility at the straightening stage of the continuous casting of high-strength low-alloy steel is attributed
to different microalloying elements, in particular, Nb. However, such elements are essential for the desired mechanical characteristics
of the final product. Since the chemistry cannot be altered to alleviate the problem, thermomechanical processing was studied
in order to improve the hot ductility. Two Nb-microalloyed steels, one also containing B, were examined. The thermal history
occurring in the continuous casting process was taken into account as well. First, it was noticed that the steel with B has
a higher hot ductility than the other after being subjected to in-situ melting followed by the thermal schedule. Grain boundary sliding was recognized as the failure mechanism. Then, the effect
of deformation applied in the vicinity of the δ→γ transformation, while the thermal schedule was being executed, was investigated. Such deformation appeared to improve the
hot ductility remarkably. Finally, the mechanism of such improvement in the hot ductility was elaborated. 相似文献
7.
Phase equilibria between γ (Al), γ′ (Ll2), and β (B2) phases in the Ni-rich portions of the Ni-Al-X ternary systems were investigated over a temperature range of
800 °C to 1300 °C. The tie lines and phase boundaries were accurately determined by the diffusion couple technique. It was
established that Co, Cu, Mn, Fe, and Cr concentrated more into the γ phase than into the γ′ phase, while Ta, Nb, Ti, V, and
Si mostly partitioned to the γ′ phase. The partition coefficients for alloying elements between γ and γ′ phases varied as
a function of temperature for most of the elements and also as a function of concentration for some of the elements, such
as Mo, W, and V. In the equilibrium between γ′ and β phases, Mn, Fe, Co, and Cu partitioned to the β phase rather than to
the γ′ phase, whereas Nb, Mo, Ta, Ti, W, V, Cu, and Si concentrated into the γ′ phase. The partition of alloying elements
in the metastable equilibrium between γ and β phases, in the Ni-Al binary system, was estimated from the data on γ/γ′ and
γ′/β equilibria. Based on these data, the relative stabilizing effects of alloying elements on γ, γ′, and β phases are also
discussed. 相似文献
8.
The stacking-fault energy (γ
sf
) of iron-based shape-memory alloys was calculated by the extended dislocation-node method. The results show that Ni and Mn
increase the γ
sf
of the alloys with an austenite structure, while Cr and Si decrease it. An expression relating the alloying elements of Ni,
Cr, Mn, and Si to the γ
sf
of the alloys is established. Moreover, in terms of the γ
sf
values of the alloys and the Schaeffler diagram, the alloy design of iron-based shape-memory alloys is carried out. It is
found that the alloy having the lowest γ
sf
has the best shape-memory effect (SME), with a martensite transition temperature (M
s
) being a little lower than ambient temperature. The corresponding composition of the alloy is located in the γ-phase zone near the phase line between the γ and γ+ε phase zone and the triple point of α+γ+ε in the Schaeffler diagram. 相似文献
9.
A melting and solidification study of alloy 625 总被引:1,自引:0,他引:1
M. J. Cieslak T. J. Headley A. D. Romig T. Kollie 《Metallurgical and Materials Transactions A》1988,19(9):2319-2331
The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron
microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed
that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range.
Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc
welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ)
matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M6C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense
of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation
of the γ/M6C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present,
the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and
solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying
variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m
L
, m
S
) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution
coefficients (k) for these same elements in Alloy 625. 相似文献
10.
M. Lamirand J. -L. Bonnentien S. Guérin G. Ferrière J. -P. Chevalier 《Metallurgical and Materials Transactions A》2006,37(8):2369-2378
The effect of added oxygen in the range of 1000 to 4000 wt ppm on the microstructures of a Ti-48Al-2Cr-2Nb alloy has been
investigated and compared to the microstructures for a high-purity alloy. For specimens cooled from theα phase, interstitial oxygen stabilizes the lamellar microstructure with respect toγ grains, with higher than equilibrium values for theα
2 volume fraction. For specimens cooled from theα +γ phase field, the lamellar microstructure still tends to be favored by oxygen, but it is found that the phase volume fractions
are not significantly different from equilibrium values. This suggests that interstitial O essentially reduces the kinetics
of theα toα +γ transformation. Thus, interstitial oxygen will have a strong effect on microstructures obtained by continuous cooling fromα, but significantly less on those, such as the duplex microstructure, obtained by long treatment in theα +γ phase field. In general, increased O content is well correlated with reduced ductility. Finally, the role of interstitial
oxygen on this phase transformation is discussed. 相似文献
11.
Muratahan Aykol Amdulla O. Mekhrabov M. Vedat Akdeniz 《Metallurgical and Materials Transactions A》2010,41(2):267-274
Site selection and pseudo-clustering behaviors of the various M alloying elements in Al-lean Ti50Al50–X
M
X
(X = 1, 2, 3, 4, and 5 at. pct) intermetallics have been investigated by means of the ordering energy-dependent and long-range-order
forced fast Monte Carlo simulation method. The ordering energies have been calculated via pseudopotential approximation in the electronic theory of alloys up to the third coordination sphere (CS) taking the anisotropic
nature of tetragonal L10-type structure of γ-TiAl into account. It was shown that the site occupation characteristics of the M alloying element atoms in γ-TiAl intermetallics are governed by the relative magnitude of partial ordering energies between Ti-M and Al-M atomic pairs.
However, the sign of partial ordering energies of these atomic pairs at the first CS becomes important in determining the
clustering behavior and controls the dissolution modes of alloying element atoms in the γ-TiAl matrix. The pseudo-clustering behavior of alloying elements reveals three dissolution modes, namely, random dissolution
(mode I), planar clustering in two dimensions (mode II), and three-dimensional (3-D) clustering (mode III) of the M occupant
atoms. 相似文献
12.
Shi Ming Hao T. Takayama K. Ishida T. Nishizawa 《Metallurgical and Materials Transactions A》1984,15(10):1819-1828
The phase equilibria in Fe-Ni-Al and Fe-Ni-Al-Co systems have been investigated by the diffusion couple technique, and the
miscibility gap that separates α1 (Fe-rich disordered bcc phase) from α2 (NiAl-rich ordered bcc phase) has been determined. It has been ascertained that the three-dimensional shape of the miscibility
gap is not simple helmetlike, but has a peculiar ridge at the order-disorder transition temperature. The miscibility gap is
narrowed and shifted to the Fe-rich corner by the addition of Co, and Co atoms are distributed to the α2 rather than the α1 Furthermore, by alloying Co, the fcc γ phase is stabilized and the miscibility gap between α1 and α2 is hidden to some extent by the phase regions concerned with the γ phase. 相似文献
13.
Alloying mechanism of beta stabilizers in a TiAl alloy 总被引:1,自引:0,他引:1
Fu-Sheng Sun Chun-Xiao Cao Ming-Gao Yan Seung-Eon Kim Yong-Tailee 《Metallurgical and Materials Transactions A》2001,32(7):1573-1589
The effects of beta stabilizers such as Fe, Cr, V, and Nb on the microstructures and phase constituents of Ti52Al48-xM (x=0, 1.0, 2.0, 4.0, or 6.0 at. pct and M=Fe, Cr, V, and Nb) alloys were studied. The dependence of the tensile properties and
creep resistance of TiAl on the alloying elements, especially the formation of B2 phase, was investigated. Fe is the strongest
B2 stabilizer, Cr is second, V is an intermediate stabilizer, and Nb is the weakest stabilizer. The composition partitioning
of Fe, Cr, V, and Nb in the γ phase is affected by the formation of B2 phase. The peaks of the tensile strengths and creep rupture life of Ti52Al48-xM generally occur at the maximum solid solution of these elements in the γ phase, which is just before the formation of B2 phase. Ti52Al48-0.5Fe shows an attractive elongation of 2.5 pct at room temperature, and the Ti52Al48-1V, Ti52Al48-Cr, and Ti52Al48-2Nb alloys have about 1.1 to 1.3 pct elongation at room temperature. The increase of tensile strengths and creep resistance
with increasing Fe, Cr, V, and Nb contents is chiefly attributed to the solid-solution strengthening of these elements in
the γ phase. The appearance of B2 phase deteriorates the creep resistance, room-temperature strengths, and ductility. With respect
to the maximum solid-solution strengthening, an empirical equation of the Cr equivalent [Cr] is suggested as follows: [Cr]=Cr+Mn+3/5V+3/8Nb+3/2
(W+Mo)+3Fe=1.5 to 3.0. The solid-solution strengthening mechanism of Fe, Cr, V, and Nb at room temperature arises from the
increase of the Ti 3s and Al 2s binding energies in Ti-Ti and Al-Al bonds, and the retention of the strength and creep resistance
at elevated temperatures in Ti52Al48-xM is mainly attributed to the increase of the Ti 3s and Al 2s binding energies in Ti-Al bonds in γ phase. The decrease of the Ti 3p and Al 2p binding energies in Ti-Ti, Ti-Al, and Al-Al bonds benefits the ductility of TiAl. 相似文献
14.
R. V. Miner 《Metallurgical and Materials Transactions A》1997,28(4):1011-1020
Prototypical single-crystal NiAlCrX superalloys were studied to examine the effects of the common major alloying elements,
Co, Mo, Nb, Ta, Ti, and W, on yielding behavior. The alloys contained about 10 at. pct Cr, 60 vol pct of the γ′ phase, and
about 3 at. pct of X in the γ′. The critical resolved shear stresses (CRSSs) for octahedral and primary cube slip were measured
at 760 °C, which is about the peak strength temperature. The CRSSOct and CRSScube are discussed in relation to those of Ni3 (Al, X) γ′ alloys taken from the literature and the γ′/γ lattice mismatch. The CRSSOct of the γ+γ′ alloys reflected a similar compositional dependence to that of both the CRSSOct of the γ′ phase and the γ′/γ lattice parameter mismatch. The CRSScube of the γ+γ′ alloys also reflected the compositional dependence of the γ′/γ mismatch, but bore no similarity to that of CRSScube for γ′ alloys since it is controlled by the γ matrix. The ratio of CRSScube/CRSSOct was decreased by all alloying elements except Co, which increased the ratio. The decrease in CRSScube/CRSSOct was related to the degree in which elements partition to the γ′ rather than the γ phase. 相似文献
15.
G. M. Janowski R. W. Heckel B. J. Pletka 《Metallurgical and Materials Transactions A》1986,17(11):1891-1905
Changes in the γ/γ'/carbide microstructure as a function of Ta content were studied in conventionally cast B-1900 + Hf and
both conventionally cast and directionally solidified MAR-M247.* The effects of tantalum on the microstructure were found
to be similar in both nickel-base superalloys. In particular, the γ' and carbide volume fractions increased approximately
linearly with tantalum additions in both alloys. The γ' phase compositions did not change as tantalum additions were made
with the exception of an increase in the tantalum level. Bulk tantalum additions increased the tantalum, chromium, and cobalt
levels of the γ phase in both alloy series. The increase in the concentrations of the latter two elements was attributed to
a decrease in the γ phase fraction with increasing bulk tantalum level and nearly constant γ' /γ partitioning ratios. It was
demonstrated that the large increase in the γ ' volume fraction was a result of tantalum not affecting the partitioning ratios
of the other alloying elements. The addition of tantalum led to a partial replacement of the hafnium in the MC carbides, although
the degree of replacement was reduced by the solutionizing and aging heat treat-ment. In addition, chromium-rich M23C6 carbides formed as a result of MC carbide decomposition during heat treatment. 相似文献
16.
The effect of iron addition on the precipitation behavior of Co-Ni-Cr-Nb alloys is discussed. Iron addition changes the main
precipitate from orthorhombic β-Ni3Nb (Ni3Cb) to BCTγ″-phase which is disc shaped and precipitates on {100} matrix planes. The growth ofγ″-precipitate follows the Lifshitz-Wagner theory of diffusion controlled growth. An attempt has been made to analyze the structure
ofγ″ using the electronic considerations of the Engel-Brewer theory. On continued aging, metastableγ″ transforms to a stable β-phase on {111} matrix planes. The orientation relationship of this phase is similar to that observed
in other alloys. In addition to intragranular precipitation, β=phase also precipitates at the grain boundaries and grows into
the grains. The transformation ofγ″ into β-phase does not affect the hardness to any significant extent. 相似文献
17.
T. Amadou A. Ben Rhouma H. Sidhom C. Braham J. Ledion 《Metallurgical and Materials Transactions A》2000,31(8):2015-2024
The annealing of large cast pieces in duplex stainless steel (SS) and the different heat cycles resulting from repairs involve
significant structural changes characterized by carbide and intermetallic phase precipitation. This yields to lower local
corrosion resistance in sea water due to changes in the local content of alloying elements. The precipitation of chromium
carbide affects the resistance to the intergranular corrosion and the repassivation behavior. The eutectoidal decomposition
of ferritic phase into regenerated austenite and in sigma phase (α → γ
r
+ σ) results in weakening the resistance to pit nucleation in synthetic sea water. In contrast, such precipitation will not have
any significant effect when the treatment temperature is high enough to involve a rapid rehomogenization of depleted zones
and ensure a self-healing. 相似文献
18.
Directionally solidified (DS) β+(γ+γ′) Ni−Fe−Al alloys have been used to investigate the effect of a ductile second phase
on the room-temperature mechanical behavior of a brittle 〈001〉-oriented β (B2) phase. The ductile phase in the composite consisted
of a fine distribution of ordered γ′ precipitates in a γ (fcc) matrix. Three microstructures were studied: 100 pct lamellar/rod,
lamellar+proeutectic β, and discontinuous γ. The β matrix in the latter two microstructures contained fine-scale bcc precipitates
formed due to spinodal decomposition. Room-temperature tensile ductilities as high as 12 pct and fracture toughness (K
Q) of 30.4 MPa
were observed in the 100 pct lamellar/rod microstructure. Observations of slip traces and dislocation substructures indicated
that a substantial portion of the ductility was a result of slip transfer from the ductile phase to the brittle matrix. This
slip transfer was facilitated by the Kurdjumov-Sachs (KS) orientation relationship between the two phases and the strong interphase
interface which showed no decohesion during deformation. In microstructures which show higher values of tensile ductility
and fracture toughness, 〈100〉 slip was seen in the β phase, whereas 〈111〉 slip was seen in the β phase in the microstructure
which showed limited ductility. The high ductility and toughness are explained in terms of increased mobile dislocation density
afforded by interface constraint. The effect of extrinsic toughening mechanisms on enhancing the ductility or toughness is
secondary to that of slip transfer.
A. MISRA, formerly Graduate Student, Department of Materials Science and Engineering, University of Michigan is Research Associate 相似文献
19.
M. L. Öveçoglu O. N. Senkov F. H. Froes N. Srisukhumbowornchai 《Metallurgical and Materials Transactions A》1999,30(3):751-761
Prealloyed, gas-atomized (GA) Ti-47Al-3Cr alloy powder, containing about 70 pct of the α
2 (Ti3Al) phase and 30 pct of the γ (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600 °C, but decomposed
at higher temperatures, with an exothermic reaction peak at 624 °C as the material transformed to a mixture of α
2 and γ and then to a fully γ structure at 722 °C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP’ing)
of the amorphous powder at 725 °C. Isothermal annealing experiments were conducted in the two-phase α+γ field, at 1200 °C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and
analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only
the presence of the γ grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours,
with a saturation grain size of about 1 μm. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning
force is taken into account. Twins formed in the γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The
minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 μm, which suggests that twinning is energetically unfavorable in the nanometer-sized grains. 相似文献
20.
Raghvendra Tewari Hyo-Jin Song Vijay K. Vasudevan Amit Chatterjee 《Metallurgical and Materials Transactions A》2006,37(9):2669-2682
Recently, alloys based on the Nb-Ti-Si system have become of interest for high-temperature structural applications. In the
present work, the microstructure of multicomponent Nb-30Ti-8Si-10Cr-10Al-X (in at. pct) alloys in the as-cast and heat-treated
conditions was studied using X-ray diffraction, electron probe microanalysis, scanning electron microscopy and transmission
electron microscopy. The effect of temperature and time on phase evolution was examined in detail. The as-cast microstructure
was found to be composed of three phases:β (bcc), silicides (M5Si3 type), and a Cr-rich Laves phase. Theβ phase was found to display B2-type ordering. The silicides in these alloys were generally quite stable during heat treatment,
whereas the Cr-rich Laves phase was observed to dissolve on solutionization at temperatures above 1300 °C. Aging of the solutionized
materials between 900 °C and 1100 °C led to the precipitation of fine particles of another Laves phase in theβ matrix. In addition, theβ matrix revealed a tendency toward phase separation into Nb-rich (β
1) and Ti-rich (β
2) regions. The volume percentage and chemical composition of each phase has been determined as a function of time and temperature
and the changes in microstructure have been rationalized in terms of the distribution of elements in various phases. The role
of different alloying elements on the formation of these phases has also been critically examined. 相似文献