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1.
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
2.
Directionally solidified (DS) alloys with the nominal composition Ni-30 at. pct Fe-20 at. pct Al having eutectic microstructures
were used to study slip transfer across interphase boundaries and dislocation nucleation at the interfacial steps. The slip
transfer from the ductile second phase, γ(fcc) containing ordered γ′(L12) precipitates, to the ordered β(B2) phase and the generation of dislocations at the interface steps were interpreted using the mechanisms proposed for similar
processes involving grain boundaries in polycrystalline single-phase materials. The criteria for predicting the slip systems
activated as a result of slip transfer across grain boundaries were found to be applicable for interphase boundaries in the
multiphase ordered Ni-Fe-Al alloys. The potential of tailoring the microstructures and interfaces to promote slip transfer
and thereby enhance the intrinsic ductility of dislocation-density-limited intermetallic alloys is discussed. 相似文献
3.
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
4.
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
5.
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
6.
The effects of Cr additions to y-base alloys have been investigated, using bulk materials consolidated from rapid solidification-processed
ribbons. The composition ranges studied were 0 to 4 at. pet Cr and 44 to 54 at. pet Al. It was found that Cr additions do
not affect the deformation behavior of single-phase γ alloys. However, they significantly enhance the plasticity of Al-lean
duplex alloys which contain grains of single-phase γ and grains of lamellar γ/α2. Other Cr effects on microstructure, phase stability, site occupancy, and deformation sub-structures were characterized and
correlated to the observed mechanical behavior. It was concluded that the ductilization effect of Cr in duplex alloys is partially
due to the tendency of Cr to occupy Al lattice sites. Ductilization is also partially due to the ability of Cr to modify the
Al partitioning and, therefore, the thermal stability of transformed α2 laths. 相似文献
7.
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
8.
T. Furuhara H. J. Lee E. S. K. Menon H. I. Aaronson 《Metallurgical and Materials Transactions A》1990,21(6):1627-1643
Interphase boundary structures generated during diffusional transformations in Ti-base alloys, especially the proeutectoid
α and eutectoid reactions in a β-phase matrix, are reviewed. Partially coherent boundaries are shown to be present whether
the orientation relationship between precipitate and matrix phases is rational or irrational. Usually, these structures include
both misfit dislocations and growth ledges. However, grain boundary α allotriomorphs (GBA’s) do not appear to develop misfit
dislocations at partially coherent boundaries. Evidently, these dislocations can be replaced by ledges which provide a strain
vector in the plane of the interphase boundary. The bainite reaction in Ti-X alloys produces a mixture of eutectoid α and
eutectoid intermetallic compound. Both eutectoid phases are partially coherent with theβ matrix, and both grow by means of the ledge mechanism, though unlike pearlite the ledge systems of the two phases are structurally
independent. Even after deformation and recrystallization, the boundaries between the eutectoid phases and theβ matrix, as well as between these phases, are partially coherent. Titanium and zirconium hydrides have partially coherent
interphase boundaries with respect to theirβ matrix. The recent observation of ledgewise growth of γ TiH within situ high-resolution transmission electron microscopy (HRTEM) suggests that, repeated suggestions to the contrary, these hydrides
do not grow by means of shear transport of Ti atoms at rates paced by hydrogen diffusion.
This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the
1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee. 相似文献
9.
10.
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
11.
Jörg M.K. Wiezorek Andreas Kulovits Xiao-Dong Zhang Hamish. L. Fraser 《Metallurgical and Materials Transactions A》2011,42(3):605-612
The role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically
twinned (PST) crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant
polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces
parallel and perpendicular to the loading directions to produce hard mode slip activity in both the γ and the α
2 phases. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases
and to study details of the hard mode of the slip transfer across heterophase interfaces. The results are discussed with respect
to the mechanical behavior of PST-TiAl. 相似文献
12.
Low-cycle fatigue behavior of INCONEL 718 superalloy with different concentrations of boron at room temperature 总被引:1,自引:0,他引:1
L. Xiao M. C. Chaturvedi D. L. Chen 《Metallurgical and Materials Transactions A》2005,36(10):2671-2684
Symmetrical push-pull low-cycle fatigue (LCF) tests were performed on INCONEL 718 superalloy containing 12, 29, 60, and 100
ppm boron (B) at room temperature (RT). The results showed that all four of these alloys experienced a relatively short period
of initial cyclic hardening, followed by a regime of softening to fracture at higher cyclic strain amplitudes (Δɛ
t
/2≥0.8 pct). As the cyclic strain amplitude decreased to Δɛ
t
/2≤0.6 pct, a continuous cyclic softening occurred without the initial cyclic hardening, and a nearly stable cyclic stress
amplitude was observed at Δɛ
t
/2=0.4 pct. At the same total cyclic strain amplitude, the cyclic saturation stress amplitude among the four alloys was highest
in the alloy with 60 ppm B and lowest in the alloy with 29 ppm B. The fatigue lifetime of the alloy at RT was found to be
enhanced by an increase in B concentration from 12 to 29 ppm. However, the improvement in fatigue lifetime was moderate when
the B concentration exceeded 29 ppm B. A linear relationship between the fatigue life and cyclic total strain amplitude was
observed, while a “two-slope” relationship between the fatigue life and cyclic plastic strain amplitude was observed with
an inflection point at about Δɛ
p
/2=0.40 pct. The fractographic analyses suggested that fatigue cracks initiated from specimen surfaces, and transgranular
fracture, with well-developed fatigue striations, was the predominant fracture mode. The number of secondary cracks was higher
in the alloys with 12 and 100 ppm B than in the alloys with 29 and 60 ppm B. Transmission electron microscopy (TEM) examination
revealed that typical deformation microstructures consisted of a regularly spaced array of planar deformation bands on {111}
slip planes in all four alloys. Plastic deformation was observed to be concentrated in localized regions in the fatigued alloy
with 12 ppm B. In all of the alloys, γ″ precipitate particles were observed to be sheared, and continued cyclic deformation reduced their size. The observed cyclic
deformation softening was associated with the reduction in the size of γ″ precipitate particles. The effect of B concentration on the cyclic deformation mechanism and fatigue lifetime of IN 718 was
discussed. 相似文献
13.
The mechanical properties of two-phase Zircaloy-4-oxygen alloys at thermal equilibrium have been determined. The strength
of these alloys depends to a large extent on their microstructure. The strengthening behavior for alloys having isolateda grains in the softer β matrix is similar to the dispersed particle strengthening. The yield strength of these alloys is found
to obey the Petch relationship,
(MPa), whereλ
β is the mean free path of β phase inμm. As the volume fraction ofa phase increases, its aspect ratio also increases. This allows more effective load transfer from the matrix to the hardera phase. It has been shown that the strength of these alloys obeys a modified rule of mixtures. The alloys having equiaxeda grains in the β matrix show large strain rate sensitivities at low strain rate. The deformation behavior is interpreted in
terms of dislocation slip in the β matrix and diffusion assisted climb near the interphase boundary. 相似文献
14.
M. Raghavan 《Metallurgical and Materials Transactions A》1977,8(7):1071-1078
Decomposition of a Cu-30 pct Ni-1 pct Nb alloy on aging in the range of 866 K (600°C) to 1073 K (800°C) was investigated.
The initial decomposition, concomitant with age hardening, occurred through the precipitation of body centered tetragonal
metastable Ni3Nb-γ” precipitates on the 100 matrix planes. Equilibrium orthorhombicβ phase formed either through a grain boundary cellular reaction at low temperature (≤973 K (700°C)) or as Widmanstaettenplatelets
on the 1ll planes at higher temperatures (≥1073 K (800°C)) with the following crystallographic relationship: (0l0)β//111γ [100]β//[1•11]γ. Based on the observations, a schematic transformation sequence is presented. 相似文献
15.
Ti-Fe binary alloys were rapidly solidified by the melt-spinning technique, and four compositions were examined: Ti-5 wt pct
Fe, which is the critical composition for theβ to ω athermal transformation; Ti-10 wt pct Fe, which represents a hypoeutectoid composition; the eutectoid composition Ti-15
wt pct Fe; and Ti-20 wt pct Fe, as an example of a hypereutectoid alloy. The Ti-5 wt pct Fe rapidly solidified ribbons are
composed of two different structures. The first consists of α′-martensite plates inβ matrix and the second, athermal ω particles inβ matrix. The Ti-10, 15, and 20 wt pct Fe alloys are also composed of two structures. These areβ grains and isothermal-like ω particles inβ matrix. A solidification model is suggested which explains the existence of two different microstructures at the same composition
and the for-mation of two kinds of ω particles. 相似文献
16.
T. Khaled G. H. Narayanan S. M. Copley 《Metallurgical and Materials Transactions A》1978,9(12):1883-1890
The stability of theβ phase in the Ti-V, Ti-Mo, and Ti-V-Mo alloy systems was investi-gated, and theβ/α + β phase boundaries in these systems were determined in the range 300 to 600° C. The results indicate that Mo is more potent
than V in stabilizing theβ phase with respect to α phase formation and in retarding the β → α reaction kinetics. It is shown that increasing the oxygen
concentration in the alloys tends to enhance α phase formation in Mo-lean alloys (Mo contents < 15 wt pct), whereas it leads
to the formation of an oxide phase in Mo-rich alloys (Mo contents ≥15 wt pct).
Formerly Research Assistant, Department of Materials Science, University of Southern California 相似文献
17.
The DO22 lattice of the NbAl3 intermetallic compound shows very limited ductility at room temperature. In this study the slip and twinning systems that
are active during the deformation process were investigated. Evaluation of the possible deformation modes was performed and
contrast analysis in the transmission electron microscope revealed both expected and unexpected deformation modes. Two types
of dislocations were found in the deformed structure, namely thea 〈110〉 superdislocation on the {112} plane and loops of unidentified dislocations on the {010} plane. No evidence of 〈201〉
superdislocations was found, probably due to the fact that this type of dislocation is expected to move in groups of four.
Twins of the {112} type were found to play an important role in the deformation process since they supply a component of shear
perpendicular to the (001) plane. 相似文献
18.
W. S. Walston I. M. Bernstein A. W. Thompson 《Metallurgical and Materials Transactions A》1991,22(6):1443-1451
The microstructure of a single-crystal nickel-base superalloy, PWA 1480, has been varied by heat treatment and hot isostatic
pressing in order to study the role of the γ/yγ′ eutectic and porosity on subsequent tensile behavior. The level of porosity
was found not to affect any of the tensile properties, while the γ/γ′ eutectic strongly influenced ductility. Eliminating
the γ/γ′ eutectic increased ductility which was attributed to the cleavage fracture of this constituent. It is proposed that
such cleavage of the γ/γ′ eutectic is initiated by the stress created from impinging slip bands, promoting shear localization,
and final fracture along {111} slip planes. The precise nature of this fracture process is discussed with emphasis on the
role of the γ/′ micro-structure. The deformation structure of PWA 1480 was also studied, and while different in some respects
from many other single-crystal superalloys, its fracture process appears to be similar.
Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University. 相似文献
19.
The fracture behavior of single crystals of the PWA 1480E nickel-base superalloy was studied using both scanning electron
microscopy (SEM) and transmission electron microscopy (TEM) techniques. Notched single crystals with seven different crystal
growth orientations near [100], [110], [111], [013], [112], [123], and [223] were tensile tested at 22 °C in a helium atmosphere
at 34 MPa. Gamma prime particles were orderly and closely aligned with the cube edges along the [100], [010], and [001] directions
of theγ matrix. The cuboid morphology of theγ’ precipitate was not influenced by the crystal growth orientation. The specimen with the [110] orientation was the strongest,
while the crystal with the [100] orientation was the weakest. A stereoscopic technique, combined with the use of planary’ morphologies, was applied to identify the cleavage plane orientation. All specimens failed predominately by {lll}-type cleavage
which originated from combined slip on various {111} planes. In most cases, deformation was found to occur inhomogeneously
in intense slip bands lying on {111} planes and aligned parallel to the different slip directions. Both SEM and TEM studies
indicated that {lll}-type slip was the controlling factor during cleavage fracture of single crystals of the PWA 1480E nickel-base
superalloy.
Formerly Graduate Student, Auburn University 相似文献
20.
Role of Solute in the Texture Modification During Hot Deformation of Mg-Rare Earth Alloys 总被引:1,自引:0,他引:1
Jason P. Hadorn Kerstin Hantzsche Sangbong Yi Jan Bohlen Dietmar Letzig James A. Wollmershauser Sean R. Agnew 《Metallurgical and Materials Transactions A》2012,43(4):1347-1362
Although conventional Mg alloys develop strong crystallographic textures during deformation that persist during annealing,
the addition of rare earth (RE) elements can induce comparably weaker textures. The texture weakening effect is explored using
hot-rolled Mg-Y alloys of a single phase to focus on the possibility of solute effects. Of the studied compositions, the richer
alloys (≥0.17 at. pct) show the weakening effect, whereas the most dilute alloy (≤0.03 at. pct) does not. Electron backscattered
diffraction (EBSD) analysis of intragranular misorientation axes (IGMA) indicate that the geometrically necessary dislocation
(GND) content in dilute, hot-rolled alloys contain primarily basal 〈a〉 dislocations. At higher concentrations, the dislocations
are predominantly prismatic 〈a〉 type. This change in the GND content suggests a change in dynamic recrystallization (DRX)
mode. For example, nonbasal cross slip has been associated with continuous DRX. Furthermore, nonbasal slip might also promote
more homogenous shear banding/twinning. Both of these mechanisms have been shown previously to give rise to more randomly
oriented nuclei during DRX. Energy dispersive X-ray spectroscopy performed through transmission electron microscopy shows
that Mg-Y exhibits significant grain boundary solute segregation, consistent with recent observations of solute clustering.
Slow grain growth may be explained by solute drag. It is hypothesized that limited grain boundary mobility suppresses conventional
discontinuous DRX, which has been shown to retain the deformation texture. The promotion of nonbasal slip and suppression
of grain boundary mobility are proposed as solid solution-based mechanisms responsible for the observed texture weakening
phenomenon in Mg rare earth alloys. 相似文献