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1.
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
2.
The response of as-cast structures of 12 alloys in the Ti-Al-Mo system containing 44 to 50 at. pct Al and 2 to 6 at. pct Mo
to simple single step heat treatments in the temperature range 1373 to 1673 K is described. The microsegregation patterns
present in the cast structure persist to a large extent after heat treatment, especially below 1673 K. However, tentative
conclusions regarding phase equilibria in this temperature and composition range are drawn from the results. High-temperature
equilibria are dominated by the β, α+β, and α+γ phase fields, while the β+γ phase field dominates equilibrium below 1473 K. Three major types of transformation behavior are observed: a massive α to γ transformation, which occurs within the α phase on quenching from 1673 and 1573 K in alloys centered around the 48 pct Al composition; a eutectoid transformation from
α to B2+γ mixtures, which occurs at 1473 K and below in alloys centered around the 48Al-4Mo and 46Al-6Mo compositions; direct
γ precipitation in β, which occurs primarily in the 44Al-6Mo composition at 1273 K and below; and finally growth of γ lamellae in α+γ lamellar structures with B2 precipitation on lamellar interfaces, which occurs over a broad range of alloy compositions and
temperatures. 相似文献
3.
Phase transformation and microstructural evolution have been studied in Ti-44Al-4Nb-4Zr-0.2Si-0.1B alloys that were cooled
from theα +β phase region with various cooling rates. It has been shown that the cooling rates have different influence on the morphology
of the transformation products for the three phase transformations studied,α →α
2, B2 →ω, andα →γ. Under slow cooling, all three transformations can be fulfilled. Under rapid cooling, B2 →ω is partially detained and a diffuseω phase forms as metastable phase, butα →γ is almost completely suppressed, which supports that theγ lamellae formation is diffusion controlled. 相似文献
4.
Anirban Chakraborty James C. Earthman 《Metallurgical and Materials Transactions A》1997,28(4):979-989
Finite element simulations of the high-temperature behavior of single-phase γ, dual-phase α2+γ, and fully lamellar (FL) α2+γTiAl intermetallic alloy microstructures have been performed. Nonlinear viscous primary creep deformation is modeled in
each phase based on published creep data. Models were also developed that incorporate grain boundary and lath boundary sliding
in addition to the dislocation creep flow within each phase. Overall strain rates are compared to gain an understanding of
the relative influence each of these localized deformation mechanisms has on the creep strength of the microstructures considered.
Facet stress enhancement factors were also determined for the transverse grain facets in each model to examine the relative
susceptibility to creep damage. The results indicate that a mechanism for unrestricted sliding of γ lath boundaries theorized
by Hazzledine and co-workers leads to unrealistically high strain rates. However, the results also suggest that the greater
creep strength observed experimentally for the lamellar microstructure is primarily due to inhibited former grain boundary
sliding (GBS) in this microstructure compared to relatively unimpeded GBS in the equiaxed microstructures. The serrated nature
of the former grain boundaries generally observed for lamellar TiAl alloys is consistent with this finding. 相似文献
5.
6.
The deformation behavior during upset forging has been determined for Ti-6242 in both the (α + β) and β starting microstructures.
For (α + β), flow softening attributed to deformation heating was observed. Deformation heating accounted for only a fraction
of the extensive flow softening of the β microstructure. The dependence of log σ on 1/T was linear for (α + β) and bilinear
for the β microstructure, with an approximate transition temperature of 930 °C. The two temperature regimes for β corresponded
to distinct deformed microstructures which were manifestations of different softening mechanisms, all promoting the flow-induced
transformation of metastable β microstructure to the equilibrium (α + β) microstructure. Based on the experimental data, flow
stress equations for both microstructures, and empirical equations describing the flow softening behavior of β have been developed.
WithT and ġe as the only input variables, these equations can accurately predict the σ - ε relationships for process modeling of
this alloy. 相似文献
7.
The kinetics of dynamic spheroidization of the lamellar microstructure and the associated flow-softening behavior during isothermal,
constant-strain-rate deformation of a gamma titanium aluminide alloy were investigated, with special emphasis on the role
of the prior-alpha grain/colony size. For this purpose, fully lamellar microstructures with prior-alpha grain sizes between
80 and 900 μm were developed in a Ti-45.5Al-2Nb-2Cr alloy using a special forging and heat-treatment schedule. Isothermal hot compression
tests were conducted at 1093 °C and strain rates of 0.001, 0.1, and 1.0 s−1 on specimens with different grain sizes. The flow curves from these tests showed a very strong dependence of peak flow stress
and flow-softening rate on grain size; both parameters increased with alpha grain/colony size. Microstructures of the upset
test specimens revealed the presence of fine, equiaxed grains of γ + α
2 + β phases resulting from the dynamic spheroidization process that initiated at and proceeded inward from the prior-alpha grain/colony
boundaries. The grain interiors displayed evidence of microkinking of the lamellae. The frequency and severity of kinking
increased with strain, but were also strongly dependent on the local orientation of lamellae with respect to the compression
axis. The kinetics of dynamic spheroidization were found to increase as the strain rate decreased for a given alpha grain
size and to decrease with increasing alpha grain size at a given strain rate. The breakdown of the lamellar structure during
hot deformation occurred through a combination of events, including shear localization along grain/colony boundaries, microbuckling
of the lamellae, and the formation of equiaxed particles of γ + β
2 + α
2 on grain/colony boundaries and in zones of localized high deformation within the microbuckled regions. 相似文献
8.
T. R. Jervis T. G. Zocco K. M. Hubbard M. Nastasi 《Metallurgical and Materials Transactions A》1993,24(1):215-224
We have examined the effect of surface processing in air, using excimer laser light at 248 nm wavelength, on the oxygen content,
microstructure, and surface hardness of Ti-6Al-4V. Processing with a single pulse results in the transformation of theα +β material toα′ martensite. Multiple pulse processing results in rapid incorporation of oxygen in the material. Oxygen initially dissolves
in the material in the liquid phase. As the concentration exceeds the solid solubility limit during solidification, TiO particles
precipitate. In contrast to equilibrium oxidation processes in Ti, only TiO is observed as an oxidation product; further processing
results in increased oxygen incorporation and an increased volume fraction of TiO but no other oxides of Ti. The TiO particle
size is a function of the oxygen concentration and the number of pulses, with some grain growth occurring after many pulses.
The effects of solution hardening by dissolved oxygen and precipitation hardening by the TiO are identifiable as functions
of oxygen concentration and mean free path between particles, respectively. A maximum surface hardness almost twice that of
electropolished Ti-6Al-4V is observed. 相似文献
9.
The development of microstructure and its influence on creep properties have been studied for structures including equiaxed
γ, duplex, and other structures of varying α2 morphology in two Ti-48Al-2Cr-2Nb alloys. Heat treatment at 1125°C have been utilized to produce equiaxed γ microstructures
in alloys with or without Mo additions. The γ→α transformation produces α2 plates with several orientation variants with γ grains during subsequent annealing of the equiaxed γ microstructures below
the α transus. Formation of this α2 morphology results from rapid up-quenching (UQ), and this structure persists through annealing, cooling, and creep testing.
Differences in minimum creep rates for several microstructures, containing varying amounts of multi-or single variant γ/α2 grains are shown to be minimal. The presence of Mo has also resulted in improved creep resistance in equiaxed γ and γ + α2 + B2 structures, as compared to similar microstructures in the Ti-48Al-2Cr-2Nb alloy. Deformation during creep at 760 °C
at stresses between 200 and 400 MPa occurs by a combination of twinning and dislocation glide without recrystallization, resulting
in power-law stress exponents in the range of 6 to 9. Only minimal strain path dependence of the minimum creep rate is detected
in a comparison of creep rates in stress jump, stress drop, and single stress tests.
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, Orlandom, Florida, under the auspices of the ASM/MSD Flow & Fracture and Phase Transformations
Committees. 相似文献
10.
B. J. Shaiu H. T. Li H. Y. Lee Haydn Chen 《Metallurgical and Materials Transactions A》1990,21(4):1133-1141
Aside from its technological importance, the Al-Li alloy system also exhibits interesting phase transformations involving
both equilibrium and metastable states. Recent theoretical studies have shown that a supersaturated solid solution could take
different transformation paths when it is quenched into theα +γ′ field. Suggestions were made that a rapidly quenched solution phase should first undergo a congruent ordering transformation
before it decomposes into a two-phase mixture by either a secondary spinodal decomposition or the classical nucleation and
growth process. Moreover, a metastable miscibility gap was predicted at lower temperatures. The objective of this research
is to study the transformation paths and dynamics in Al-Li binary alloys of three compositions (5.2, 7.0, and 12.0 at. pct
Li). This investigation emphasizes thein situ small-angle X-ray scattering (SAXS) observations on specimens subjected to various aging conditions. Special attention is
paid to the early stages of the transformation in an attempt to characterize the various possible modes of phase separation
on one hand and to study the dynamics of the precipitation process on the other. The following results are obtained: the congruent
ordering precedes decomposition at low temperatures; the metastableγ′ solvus curve is reconfirmed; but the predicted metastable miscibility gap is not found. Guinier radii measurements of the
particles showed Ostwald ripening is quickly reached upon heating to the aging temperatures. Slowing down behavior is seen
at aging temperatures close to the solvus boundary. Activation energies for Li diffusion were obtained using the modified
Lifshitz, Slyozov, and Wagner (MLSW) model. A test of dynamical scaling behavior is carried out for the Al-12.0 at. pct Li
alloy.
Formerly Visiting Scientists at the University of Illinois 相似文献
11.
B. J. Shaiu H. T. Li H. Y. Lee Haydn Chen 《Metallurgical and Materials Transactions A》1990,21(5):1133-1141
Aside from its technological importance, the Al-Li alloy system also exhibits interesting phase transformations involving
both equilibrium and metastable states. Recent theoretical studies have shown that a supersaturated solid solution could take
different transformation paths when it is quenched into theα +γ′ field. Suggestions were made that a rapidly quenched solution phase should first undergo a congruent ordering transformation
before it decomposes into a two-phase mixture by either a secondary spinodal decomposition or the classical nucleation and
growth process. Moreover, a metastable miscibility gap was predicted at lower temperatures. The objective of this research
is to study the transformation paths and dynamics in Al-Li binary alloys of three compositions (5.2, 7.0, and 12.0 at. pct
Li). This investigation emphasizes thein situ small-angle X-ray scattering (SAXS) observations on specimens subjected to various aging conditions. Special attention is
paid to the early stages of the transformation in an attempt to characterize the various possible modes of phase separation
on one hand and to study the dynamics of the precipitation process on the other. The following results are obtained: the congruent
ordering precedes decomposition at low temperatures; the metastableγ′ solvus curve is reconfirmed; but the predicted metastable miscibility gap is not found. Guinier radii measurements of the
particles showed Ostwald ripening is quickly reached upon heating to the aging temperatures. Slowing down behavior is seen
at aging temperatures close to the solvus boundary. Activation energies for Li diffusion were obtained using the modified
Lifshitz, Slyozov, and Wagner (MLSW) model. A test of dynamical scaling behavior is carried out for the Al-12.0 at. pct Li
alloy.
Formerly Visiting Scientists at the University of Illinois 相似文献
12.
Hanliang Zhu K. Maruyama D. Y. Seo P. Au 《Metallurgical and Materials Transactions A》2005,36(5):1339-1351
XD TiAl alloys (Ti-45 and 47Al-2Nb-2Mn+0.8 vol pct TiB2) (at. pct) were oil quenched to produce fine-grained fully lamellar (FGFL) structures, and aging treatments at different
temperatures for different durations were carried out to stabilize the FGFL structures. Microstructural examinations show
that the aging treatments cause phase transformation of α
2 to γ, resulting in stabilization of the lamellar structure, as indicated by a significant decrease in α
2 volume fraction. However, several degradation processes are also introduced. After aging, within lamellar colonies, the α
2 lamellae become finer due to dissolution, whereas most of the γ lamellae coarsen. The dissolution of α
2 involves longitudinal dissolution and lateral dissolution. In addition, at lamellar colony boundaries, lamellar termination
migration, nucleation and growth of γ grains, and discontinuous coarsening occur. With the exception of longitudinal dissolution, all the other transformation
modes are considered as degradation processes as they result in a reduction in α
2/γ interfaces. Different phase transformation modes are present to varying degrees in the aged FGFL structures, depending on
aging conditions and Al content. A multiple step aging reduces the drive force for phase transformation at high temperature
by promoting phase transformation via longitudinal dissolution at low temperatures. As a result, this aging procedure effectively
stabilizes the lamellar structure and suppresses other degradation processes. Therefore, the multiple step aging is suggested
to be an optimal aging condition for stabilizing FGFL XD TiAl alloys. 相似文献
13.
The mechanism of formation of the fine duplex microstructure resulting from the α → γ transformation in water-quenched Ti-48Al-2Mn-2Nb
alloys was studied using transmission and analytical electron microscopy. As-cast Ti-48Al-2Mn-2Nb alloys were heat treated
in the α phase field and water quenched to room temperature. The resulting microstructure (referred to as a fine duplex microstructure)
consisted of equiaxed grains and abutting lath colonies. Both the colonies and the grains were composed of the γ phase, twinned
γ laths, and α2 laths. It was found that the transformation from α to γ in the fine duplex microstructure took place through long range diffusional
processes, and compctitive growth between the equiaxed and lath morphology occurred. Nucleation of they phase from the α matrix can occur through nucleation on stacking faults, followed by growth through the sympathetic nucleation
and growth of new γ laths on a substrate lath. The observed misorientations and the interfacial structures between the laths
were found to be consistent with such a mechanism. Compctition between such nucleation and growth mechanisms for the equiaxed
and lath morphologies of γ leads to the formation of lath colonies (of γ and α2) interspersed with equiaxed grains in these alloys.
Formerly Visiting Scientist, Metals and Ceramics Division, Oak Ridge National Laboratory
This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms
of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in
Rosemont, Illinois. 相似文献
14.
The development of microstructure and its influence on creep properties have been studied for structures including equiaxed
γ, duplex, and other structures of varying α
2 morphology in two Ti-48Al-2Cr-2Nb alloys. Heat treatments at 1125 °C have been utilized to produce equiaxed γ microstructures in alloys with or without Mo additions. The γ → α transformation produces α
2 plates with several orientation variants within γ grains during subsequent annealing of the equiaxed γ microstructures below the α transus. Formation of this α
2 morphology results from rapid up-quenching (UQ), and this structure persists through annealing, cooling, and creep testing.
Differences in minimum creep rates for several microstructures containing varying amounts of multi- or single variant γ/α
2 grains are shown to be minimal. The presence of Mo has also resulted in improved creep resistance in equiaxed γ and γ+α
2+B2 structures, as compared to similar microstructures in the Ti-48Al-2Cr-2Nb alloy. Deformation during creep at 760 °C at
stresses between 200 and 400 MPa occurs by a combination of twinning and dislocation glide without recrystallization, resulting
in power-law stress exponents in the range of 6 to 9. Only minimal strain path dependence of the minimum creep rate is detected
in a comparison of creep rates in stress jump, stress drop, and single stress tests.
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
15.
The α→γ phase transformation during rapid quenching and subsequent isothermal aging has been investigated in a Ti-48 at pct Al alloy.
The microstructure changes from a completely massively transformed γ-grain structure to a mixed microstructure of the massively transformed γ grains and the untransformed (meaning massively untransformed) fine α
2/γ lamellae with an increase in the cooling rate from the high-temperature α phase field. Fine γ grains are generated from these fine α
2/γ lamellae by subsequent again at 1323 K. The fine γ grains contain many defects, such as dislocations, microtwins (or stacking faults), domain boundaries, and variants, which
are frequently observed in the massive γ grains. This result suggests that the formation mechanism of the fine γ grains during aging is similar to that of the massive γ grains. When the fine γ/γ lamellar sample, which is formed by preliminary aging at a lower temperature (1173 K), is aged at a higher temperature (1323
K), apparent changes in microstructure could not be recognized. This result indicates that the fine γ-grain formation is closely related to the α
2 → γ phase transformation in the fine α
2/γ lamellae.
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 Transformation
Committees. 相似文献
16.
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
17.
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
18.
Thermoelastic martensite and shape memory effect in B2 Base Ni-Al-Fe alloy with enhanced ductility 总被引:5,自引:0,他引:5
A ductile shape memory alloy of the Ni-AI-Fe system has been developed using principles through the control of microstructure.
Addition of Fe to the binary Ni-Al shape memory alloy allows the introduction of a ductile face-centered cubic (fcc) phase
in an otherwise extremely brittle β phase alloy, leading to an improvement in its ductility while retaining its ability to
exhibit shape memory arising from the martensitic transformation of theβ phase to Ll0 structure. It is shown that the transformation temperature in the ternary Ni-AI-Fe alloy can be easily controlled by the
preannealing in the β+ γ region. Experimental results on the effect of different annealing treatments on the microstructures and the shape memory
behavior in this alloy are presented and discussed. 相似文献
19.
Microstructural changes and phase transformations of eutectoid Zn-Al-based alloy ZnA120.2Cul.8 (wt pct) were studied under
tensile stress by using X-ray diffraction and scanning electron microscopy (SEM) techniques. It was found that the lamellar
microstructure of the heat-treated eutectoid Zn-Albased alloy changed partially into a spheroidized structure at the rupture
part of the specimen after tensile testing, while the lamellar structure at the bulk part of the specimen remained stable
in the original state. The X-ray diffraction identification results showed that two phase transformations,i.e., decomposition of metastable phase η′T and a four-phase transformation, α+ ε → T + η, occurred during tensile testing. It was concluded that the tensile stress affected not only microstructural change but
also phase transformation of the alloy. The SEM observation on the etched specimen showed clearly the morphology of the microstructural
change. 相似文献
20.
Williams Lefebvre Alain Menand Annick Loiseau 《Metallurgical and Materials Transactions A》2003,34(10):2067-2075
The composition and structure of Ti-48 at. pct Al alloys with various oxygen contents, quenched from a homogeneous α state, have been studied by means of one-dimensional atom-probe (1DAP) and transmission electron microscopy (TEM) analysis.
Two regimes are observed. The change from one regime to the other depends on the global oxygen content. If the oxygen content
is lower than 1.2 at. pct, the α→γ
m massive transformation is involved during the quench. The alloys, hence, exhibit massive γ
m-structure regions and regions having a two-phase (α
2+γ) ultrafine lamellar structure. Very thin α
2 plates, saturated with oxygen, are observed in γ
m regions. The precipitation of these α
2 plates is promoted by excess oxygen in the γ
m structure. Within ultra-fine lamellar-structure regions, oxygen is concentrated in α
2 lamellae (not saturated with oxygen) and is found to be responsible for the high volume fraction of α
2 phase. When the oxygen content is larger than 1.2 at. pct, the massive transformation is suppressed and the ultrafine lamellar
structure is only observed in quenched samples. Analysis of the α → α
2 chemical ordering in the classical lamellar structure, formed within the (α+γ) dual-phase field, shows that high oxygen contents favor the chemical-ordering reaction of α phase at high temperatures (e.g., 1423 and 1523 K). It has, hence, been inferred that, above 1.2 at. pct O, the α → γ
m massive transformation is suppressed and replaced by the α → α
2+y transformation paths.
This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented
at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint
Committee on Mechanical Behavior of Materials. 相似文献