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1.
Plastic deformation and fracture of binary TiAl-base alloys 总被引:4,自引:0,他引:4
The mechanical behavior of binary TiAl alloys containing 46 to 60 at. pct Al has been studied in bulk materials preparedvia rapid solidification processing. Bending and tensile tests were carried out at room temperature as a function of Al concentration.
A few alloys were also tested from liquid nitrogen temperature to ∼ 1000°C. Deformation substructures were studied by analytical
transmission electron microscopy and fracture modes by scanning electron microscopy (SEM). It was found that both microstructure
and composition strongly affect the mechanical behavior of TiAl-base alloys. A duplex structure, which contains both primary
y grains and transformedγ/α
2 lamellar grains, is more deformable than a single-phase or a fully transformed structure. The highest plasticities are observed
in duplex alloys containing 48–50 at. pct Al after heat treatment in the center of theγ + α phase field. The deformation of these duplex alloys is facilitated by 1/2[110] slip and {111} twinning, but very limited
superdislocation slip occurs. The twin deformation is suggested to result from a lowered stacking fault energy due to oxygen
depletion or an intrinsic change in chemical bonding. Other factors, such as grain size and grain boundary chemistry and structure,
are important from a fracture point of view. The results on the deformation and fracture modes as a function of test temperature
are also discussed. 相似文献
2.
S. C. Huang R. D. Field D. D. Krueger 《Metallurgical and Materials Transactions A》1990,21(3):959-970
Microscopy and room-temperature tensile tests were performed on as-spun and annealed ribbons of Ni-20 (at. pet) Al-Fe alloys
containing 20 to 40Fe. The ribbons had the duplex structures consisting of grains of ordered bec β-NiAl and grains of disordered
fee γ-Ni, which contains precipitates of γ′-Ni3Al. The 25 to 30Fe alloys exhibited high ductility (∼10 pet elongation) in both the as-spun and annealed conditions. These
results indicate that rapid solidification-induced effects, such as the suppression of ordering, do not enhance ductility
as previously reported. The ductile alloys were found to contain high dislocation densities in both they and β grains, with no evidence of stress-induced martensite formation in the β phase. Dislocation analysis revealed that
the vast majority of dislocations in theβ had ≤100≥Burgers vectors; however, ≤111≥ dislocations were also observed. Additionally, slip bands were frequently observed
meeting at γ-β grain boundaries. Since they tend to align across the interphase grain boundary, deformation transfer between γ and β is
inferred. The deformation transfer was found to be facilitated by a specific orientation relationship between the grains.
The unusual deformation of ββby ≤111≥ slip and by deformation transfer from neighboring grains may be responsible for the
high ductility. 相似文献
3.
《Acta Metallurgica Materialia》1991,39(6):1053-1060
Ternary TiAl-base alloys containing V have been studied, using rapid-solidification processed materials. The mechanical behavior has been characterized and related to other results on microstructure, lattice parameters, site occupancy, phase stability, and deformation mode. It was found that V additions enhance the plasticity of duplex γ + α2 alloys, altough not single-phase alloys. The suctibility increase which is attributable to the V addition is small (∼0.5%), compared with that attributable to the duplex structure effect (∼1%) as previously determined in a binary alloy study. The ductilization effect of V is partially due to its ability to occupy Al lattice sites and modify the TiAl bond. It is also partially due to its ability to promote twin formation, by modifying the Al partitioning and therefore the α2/γ volume ratio in transformed regions. 相似文献
4.
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
5.
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
6.
The hot deformation behavior, microstructure development, and fracture characteristics of a wrought two-phase γ-titanium aluminide
alloy Ti-45.5Al-2Nb-2Cr containing a fine, equiaxed microstructure were investigated with special reference to the influence
of temperature transients immediately pre-ceding plastic deformation. Specimens were soaked at 1321 °C or 1260 °C, cooled
directly to test temperatures of 1177 °C and 1093 °C, and upset under conditions of constant strain rate and tem-perature.
Plastic flow behavior and microstructure evolution occurring in tests involving prior tem-perature transients were compared
with those occurring in specimens which were directly heated to the test temperature and upset under identical deformation
conditions. Flow curves associated with prior exposure at 1321 °C exhibited very sharp peaks and strong flow softening trends
compared to those obtained under isothermal conditions,i.e., involving no temperature transients. During cooling from 1321 °C, the metastable α phase undergoes limited or complete decomposition
into α/α2 + γ lamellae, depending on the final temperature (1177 °C/1093 °C). Subsequent hot deformation leads to partial globularization
of the lamellae together with extensive kinking and reorientation of lamellae. In contrast, isothermal deformation at 1177
°C/1093 °C preserves the fine, equiaxed microstructure, through dynamic recrystallization of the γ grains. Cracking observed
in specimens deformed at 1093 °C and 1.0 s−1 after exposure at 1321 °C has been attributed to the low rate of globularization as well as the occurrence of shear localization.
Plastic flow behavior observed in this work is compared with that observed in several single-phase and two-phase gamma titanium
aluminide alloys in order to identify mechanism(s) responsible for flow softening. 相似文献
7.
Evolution of boride morphologies in TiAl-B alloys 总被引:2,自引:0,他引:2
M. E. Hyman C. McCullough C. G. Levi R. Mehrabian 《Metallurgical and Materials Transactions A》1991,22(7):1647-1662
The solidification of γ-TiAl alloys with relatively low (<2 at. pct) additions of boron is discussed. Binary Ti-Al alloys
containing 49 to 52 at. pct Al form primary α-(Ti) dendrites from the melt, which are subsequently surrounded by γ segregate
as the system goes through the peritectic reactionL + α →γ. Alloys between 45 and 49 at. pct Al go through a double peritectic cascade, forming primary β-(Ti) surrounded by α-(Ti) and
eventually by γ in the interdendritic spaces. Boron additions to these binary alloys do not change the basic solidifi-cation
sequence of the matrix but introduce the refractory compound TiB2 in a variety of mor-phologies. The boride develops as highly convoluted flakes in the leaner alloys, but needles, plates,
and equiaxed particles gradually appear as the B content increases above ∼1 at. pct. Increasing the solidification rate initially
promotes the formation of flakes over plates/needles and ultimately gives way to very fine equiaxed TiB2 particles in the interdendritic spaces of the metallic matrix. Furthermore, the primary phase selection in the 49 to 52 at.
pct Al range changes from α-(Ti) to β-(Ti) at supercoolings of the order of 200 K. The different boride morphologies are fully
characterized, and their evolution is rationalized in terms of differences in their nucleation and growth behavior and their
relationship to the solidification of the inter-metallic matrix.
Formerly Research Assistant, University of California-Santa Barbara (UCSB)
Formerly Professor of Materials and Dean of the College of Engineering at UCSB 相似文献
8.
The structure of transient scales formed on pure, Y-doped, and Zr-doped NiCrAI alloys was examined by transmission electron
microscopy. Oxidation for 0.1 hour in 1100 °C air produced many of the features observed in mature α-Al2O3 scales, but on a much finer degree: randomly oriented 0.1 to 0.2 μm grains, dispersed porosity decreasing in size and amount
toward the oxide-metal interface, and indications of strain and deformation. Other layers in the scale contained structures
which preceded the random α-Al2O3 layer: γ-Al2O3, α-(Al, Cr)2O3, or Ni(Al, Cr)2O4 oxides which were composed of 0.1 μm subgrains having nearly the same crystallographic orientation. These layers were densely
populated with internal precipitates and Moiré patterns. The underlying metal structures showed evidence of plastic flow (dislocations)
due to growth stresses in the oxide and recovery of these interface dislocations into low energy networks. The formation of
coherent layers of aluminum-depleted phases indicated the selective removal of aluminum, even for these very short times. 相似文献
9.
Microstructure evolution in tial alloys with b additions: Conventional solidification 总被引:10,自引:0,他引:10
M. E. Hyman C. McCullough J. J. Valencia C. G. Levi R. Mehrabian 《Metallurgical and Materials Transactions A》1989,20(9):1847-1859
Solidification microstructures of arc-melted, near-equiatomic TiAl alloys containing boron additions are analyzed and compared
with those of binary Ti-Al and Ti-B alloys processed in a similar fashion. With the exception of the boride phase, the matrix
of the ternary alloy consists of the same α2 (DO19) and γ (Ll0) intermetallic phases found in the binary Ti-50 at. pct Al alloy. On the other hand, the boride phase, which is TiB (B27)
in the binary Ti-B alloys, changes to TiB2 (C32) with the addition of Al. The solidification path of the ternary alloys starts with the formation of primary α (A3)
for an alloy lean in boron (∼1 at. pct) and with primary TiB2 for a higher boron concentration (∼5 at. pct). In both cases, the system follows the liquidus surface down to a monovariant
line, where both α and TiB2 are solidified concurrently. In the final stage, the α phase gives way to γ, presumably by a peritectic-type reaction similar
to the one in the binary Ti-Al system. Upon cooling, the α dendrites order to α2 and later decompose to a lath structure consisting of alternating layers of γ and α2. 相似文献
10.
11.
A two-phase alloy of composition Ti-47.5Al-2.5Cr has been studied under two heat-treated conditions in order to obtain different
microstructures. These consisted of lamellar and equiaxed distributions of y grains in which the α2 phase was distributed as long lamellae or smaller globules, respectively. The specific rotation relationships between γ/γ
and γ/α2 grains have been measured, and these have been used to understand their effect on the compatibility of deformation across
adjacent grains. For this, detailed analysis of active slip systems has been carried out by transmission electron microscopy
(TEM) observations of deformed samples. A theoretical calculation of a geometric compatibility factor characterizing the best
slip transfer across adjacent grains has been used in such a way that it has been possible to deduce the role played by the
type of orientation relationship between grains in producing active deformation systems that allow the maximum compatibility
of deformation. 相似文献
12.
S. C. Huang E. L. Hall K. M. Chang R. P. Laforce 《Metallurgical and Materials Transactions A》1986,17(10):1685-1692
A study of the effect of Cr additions (5, 10, and 12.5 at. pct) on the microstructure of melt-spun Ni3Al-base alloys has been carried out using analytical electron microscopy. The analyses showed that the formation of the β-NiAl
phase could not be totally avoided at all three levels of Cr additions studied. However, its volume fraction, morphology,
and distribution were affected by the Cr concentration as well as by changes in the rapid solidification conditions. The ordering
structure of the matrix γ contains anisotropic antiphase boundaries or cube-plane-oriented thin layers of disordered γ, depending
on the local chemistry. A series of microchemistry measurements along the radius of a grain was used to trace the route of
Al and Cr segregation, and an unexpected reverse segregation with a partition coefficient greater than 1 was observed for
Cr. The bend ductility and tensile properties of the melt-spun ribbons were also studied. It was shown that the Cr additions
substantially increased the ribbon strength and modified the mode of failure toward transgranular fracture. The improved tensile
properties were partially attributed to the fine matrix γdomains which had mixed interfaces of antiphase boundaries and y
thin layers. 相似文献
13.
T. K. G. Namboodhiri C. J. McMahon H. Herman 《Metallurgical and Materials Transactions B》1973,4(5):1323-1331
Decomposition of the α-phase in Ti-Al alloys in the range 6.8 to 19.2 at. pct Al was investi-gated using electrical resistivity
measurements and transmission electron microscopy. The solvus line between the α and α+ α2 regions was found to be somewhat higher than that re-ported by Blackburn. Short range ordering both above and below the solvus
in 6.8 and 10.0 pct Al alloys was indicated by the resistivity measurements. In alloys containing 14.2 to 19.2 pet Al the
precipitation process follows a C-curve behavior with a maximum rate at about 600°C.
T. K. G. Namboodhiri and H. Herman, formerly with the School of Metallurgy and Materials Science, University of Pennsylvania,
Philadelphia, Pa. 19174 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.
Interdiffusion in Ni-rich, Ni-Cr-Al diffusion couples was studied after annealing at 1100 and 1200 °C. Recession of γ′ (Ni3Al structure), β (NiAl structure), or α (bcc) phases was also measured. Aluminum and chromium concentration profiles were
measured in the γ (fcc) phase for most of the diffusion couples. The amount and location of Kirkendall porosity suggests that
Al diffuses more rapidly than Cr which diffuses more rapidly than Ni in the γ phase of Ni-Cr-Al alloys. The location of maxima
and minima in the concentration profiles of several of the diffusion couples indicates that both cross-term diffusion coefficients
for Cr and Al are positive and that DCrAl has a greater effect on the diffusion of Cr than does DA1Cr on the diffusion of Al. The γ/γ + β phase boundary has also been determined for 1200 °C through the use of numerous γ/γ+ β diffusion couples. 相似文献
16.
G. Babu Viswanathan Michael J. Mills Vijay K. Vasudevan 《Metallurgical and Materials Transactions A》2003,34(10):2113-2127
The effects of microstructure on the tensile properties and deformation behavior of a binary Ti-48Al gamma titanium aluminide
were studied. Tensile-mechanical properties of samples with microstructures ranging from near γ to duplex to fine grained, near- and fully-lamellar were determined at a range of temperatures, and the deformation structures
in these characterized by transmission electron microscopy (TEM). Microstructure was observed to exert a strong influence
on the tensile properties, with the grain size and lamellar volume fraction playing connected, but complex, roles. Acoustic
emission response monitored during the tensile test revealed spikes whose amplitude and frequency increased with an increase
in the volume fraction of lamellar grains in the microstructure. Analysis of failed samples suggested that microcracking was
the main factor responsible for the spikes, with twinning providing a minor contribution in the near-lamellar materials. The
most important factor that controls ductility of these alloys is grain size. The ductility, yield stress, and work-hardening
rate of the binary Ti-48Al alloy exhibit maximum values between 0.50 and 0.60 volume fraction of the lamellar constituent.
The high work-hardening rate, which is associated with the low mobility of dislocations, is the likely cause of low ductility
of these alloys. In the near-γ and duplex structures, slip by motion of 1/2<110] unit dislocations and twinning are the prevalent deformation modes at room
temperature (RT), whereas twinning is more common in the near- and fully-lamellar structures. The occurrence of twinning is
largely dictated by the Schmid factor. The 1/2<110] unit dislocations are prevalent even for grain orientations for which
the Schmid factor is higher for <101] superdislocations, though the latter are observed in favorably oriented grains. The
activity of both of these systems is responsible for the higher ductility at ambient temperatures compared with Al-rich single-phase
γ alloys. A higher twin density is observed in lamellar grains, but their propagation depends on the orientation and geometry
of the individual γ lamellae. The increase in ductility at high temperatures correlates with increased activity of 1/2<110] dislocations (including
their climb motion) and twin thickening. The role of microstructural variables on strength, ductility, and fracture are discussed.
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
17.
The effects of carbon content and ausaging on austenite γ ↔ martensite (α′) transformation behavior and reverse-transformed
structure were investigated in Fe-32Ni-12Co-4Al and Fe-(26,28)Ni-12Co-4Al-0.4C (wt pct) alloys. TheM
s
temperature, the hardness of γ phase, and the tetragonality of α′ increase with increasing ausaging time, and these values
are higher in the carbon-bearing alloys in most cases. The γ → α′ transformation behavior is similar to that of thermoelastic
martensite; that is, the width of α′ plate increases with decreasing temperature in all alloys. The αt’ → γ reverse transformation
temperature is lower in the carbon-bearing alloys, which means that the shape memory effect is improved by the addition of
carbon. The maximum shape recovery of 84 pct is obtained in Fe-28Ni-12Co-4Al-0.4C alloy when the ausaged specimen is deformed
at theM
s
temperature and heated to 1120 K. There are two types of reverse-transformed austenites in the carbon-bearing alloy. One
type is the reversed y containing many dislocations which were formed when the γ/α′ interface moved reversibly. The plane
on which dislocations lie is (01 l)γ if the twin plane is (112)α′. The other type of reverse-transformed austenite exhibits γ islands nucleated within the α′ plates. 相似文献
18.
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
19.
Sang H. Kim Nack J. Kim H. H. Chung Sung G. Pyo S. J. Hwang 《Metallurgical and Materials Transactions A》1998,29(9):2273-2283
The present study is concerned with γ-(Ti52Al48)100−x
B
x
(x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure
of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of α2 and γ with a mean grain size of 200 nm. Besides α2 and γ phases, binary and 0.5 pct B alloys contain Ti2AlN and Al2O3 phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing
treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the γ
grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility,
but very high yield strength >2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength
than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed
alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature
of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine
5 pct B alloy is dominated by the grain boundary sliding mechanism.
This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented
at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the
Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees. 相似文献
20.
G. -X. Wang B. Dogan F. -Y. Hsu H. -J. Klaar M. Dahms 《Metallurgical and Materials Transactions A》1995,26(3):691-701
Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct
Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist
of the intermetallic compounds y-TiAl, α2-Ti3Al, and ξ-Ti5(Si, Al)3. The microstructure can be described as a duplex structure(i.e., lamellar γ/α2 regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates
and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature
and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite
is true. Examinations of tensile deformed specimens reveal ξ-Ti5(Si, Al)3 particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along
lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the
alloys can tolerate ξ-Ti5(Si, Al)3 particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized. 相似文献