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1.
Sapphire-reinforced NiAl matrix composites with chromium or tungsten as alloying additions were synthesized using casting
and zone directional solidification (DS) techniques and characterized by a fiber pushout test as well as by microhardness
measurements. The sapphire-NiAl(Cr) specimens exhibited an interlayer of Cr rich eutectic at the fiber-matrix interface and
a higher interfacial shear strength compared to unalloyed sapphire-NiAl specimens processed under identical conditions. In
contrast, the sapphire-NiAl(W) specimens did not show interfacial excess of tungsten rich phases, although the interfacial
shear strength was high and comparable to that of sapphire-NiAl(Cr). The postdebond sliding stress was higher in sapphire-NiAl(Cr)
than in sapphire-NiAl(W) due to interface enrichment with chromium particles. The matrix microhardness progressively decreased
with increasing distance from the interface in both DS NiAl and NiAl(Cr) specimens. The study highlights the potential of
casting and DS techniques to improve the toughness and strength of NiAl by designing dual-phase microstructures in NiAl alloys
reinforced with sapphire fibers.
R. TIWARI, formerly Research Associate, Department of Chemical Engineering, Cleveland State University 相似文献
2.
A series of Al3Ni fiber reinforced composites with a matrix composition varying from pure aluminum to Al-3.3 wt pct Cu were prepared by directional
solidification of Al-Ni-Cu alloys. The solidification conditions were kept constant in all cases atG/R ≃ 104 °C · s/mm2 (G is the temperature gradient andR is the growth rate). The mechanical properties of the composites were studied in the as grown and in the heat treated conditions
and the results were discussed in terms of the structure and composition. With the techniques used, it was possible to preserve
the Al-Al3Ni eutectic composite structure while strengthening the matrix by copper addition. The addition of 1 wt copper to the matrix
caused a considerable increase in the mechanical strength, especially after heat treatment, without affecting the ductility.
Strength values of the order of 530 MN/m2 were reached in the heat treated composites which is higher than predicted by the rule of mixtures. This is attributed to
the high work hardening capacity of the matrix especially in the presence of θ’ phase. Massive Al3Ni rods and dendrites caused premature fracture and reduction in the strength of the composites containing 2 and 3 wt pct
copper. Eliminating these defects by using higherG/R values can produce composites with exceptionally high strength. 相似文献
3.
A series of Al3Ni fiber reinforced composites with a matrix composition varying from pure aluminum to Al-3.3 wt pct Cu were prepared by directional
solidification of Al-Ni-Cu alloys. The solidification conditions were kept constant in all cases atG/R ≃ 104 °C. s/mm2 (G is the temperature gradient andR is the growth rate). The mechanical properties of the composites were studied in the as grown and in the heat treated conditions
and the results were discussed in terms of the structure and composition. With the techniques used, it was possible to preserve
the Al-Al3Ni eutectic composite structure while strengthening the matrix by copper addition. The addition of 1 wt copper to the matrix
caused a considerable increase in the mechanical strength, especially after heat treatment, without affecting the ductility.
Strength values of the order of 530 MN/m2 were reached in the heat treated composites which is higher than predicted by the rule of mixtures. This is attributed to
the high work hardening capacity of the matrix especially in the presence of θ′ phase. Massive Al3Ni rods and dendrites caused premature fracture and reduction in the strength of the composites containing 2 and 3 wt pct
copper. Eliminating these defects by using higherG/R values can produce composites with exceptionally high strength. 相似文献
4.
5.
6.
The effect on structure of some of the possible changes in shape of directionally solidified Al-Al3Ni eutectic composites has been studied for two growth conditions. The shape changes investigated included both contraction
and divergence in cross-section of the grown part, as well as 90 deg bends in the center-line of the composites. The experimental
results showed that contractions in the solidifying cross section do not seriously affect the growth of composites, apart
from some coarsening of the structure near the surface. Fiber branching took place in the case of gradual divergence in the
solidifying cross section with the fibers deviating from the general growth direction by an angle determined by the shape
of equitemperature contours during solidification. Sharp changes in growth direction, 90 deg bend, gave rise to nucleation
of new grains of considerable misorientation and hypoeutectic alloys nucleated primary aluminum phase before the eutectic
structure was established. The relatively large under cooling needed for nucleation gave rise to high local growth rates in
the 90 deg bend area. As the Al3Ni fibers grow at right angles to equitemperature contours during solidification, it is concluded that control of the composite
structure can be achieved by controlling the growth conditions and mold design. Molten alloy-graphite reactions resulted in
the formation of aluminum carbides which were more extensive at higher temperatures and longer exposure times. 相似文献
7.
8.
S. A. David A. T. Santhanam H. D. Brody 《Metallurgical and Materials Transactions A》1976,7(7):1051-1055
The crystallographic relationship displayed by the niobium and niobium carbide <Nb2C> phases in an aligned eutectic sample with a lamellar carbide morphology is lamellar interface ∥ {110}NB ∥ (001)Nb
2C growth direction ∥<112>NB ∥ [010]Nb2C or [1-20]Nb
2C and for the rod-like carbide morphology rod interface (major axis) ∥{110}Nb ∥ (001)Nb
2C growth direction 11(H2)Nb II l010]Nb.,c or [210]NB2C.
The transition in morphology of the carbide phase is discussed in terms of the relative volume fraction of the phases, growth
rate, and orientation relationships. The carbide morphology is influenced by the growth rate and carbon content. For constant
growth rate increasing the volume fraction of the carbide phase favors the lamellar morphology. At low growth rates the lamellar
morphology is favored, and at high growth rates the rod-like morphology is favored. Growth crystallography has no direct influence
on the transition in carbide morphology. 相似文献
9.
Al-Si eutectic alloys have been directionally solidified in a horizontal resistance heated furnace. The temperature gradient,
G, ahead of the solid/liquid interface was kept fairly constant at 80°c/cm, while the growth rate,R, was varied between 0.28 and 131 μm/s. Microstructural studies show a definite alignment of the rod-like Si at low growth
rates. At growth rates higher than 14 /μm/s the microstructures appear irregular, although some preferential orientation of
the Si rods parallel to the growth direction can be observed. Tensile tests show higher values in both yield and ultimate
strengths than was found in previous investigations, most likely due to the careful sample preparation prior to testing in
the present work. The yield strength increases with the growth rate up to about 14 /μm/s, and only a slight increase is observed
at higher rates. The ultimate strength also increases with the growth rate, but shows less tendency toward saturation. Superposition
of hardness and yield data show excellent correlation, while comparison between hardness and ultimate strength shows higher
hardness than ultimate values with decreasing growth rates.
Formerly Metallurgist with Materials and Molecular Research Division, Lawrence Berkeley Laboratory. 相似文献
10.
J. Daniel Whittenberger S. V. Raj Ivan E. Locci Jonathan A. Salem 《Metallurgical and Materials Transactions A》2002,33(5):1385-1397
The eutectic composition Ni-33Al-33Cr-1Mo has been directionally solidified (DS) via a modified Bridgman technique at rates ranging from 7.6 to 508 mm/h to determine if the growth rate affects the mechanical
properties. Microstructural examination revealed that all DS rods had grain/cellular microstructures containing alternating
plates of NiAl and Cr alloyed with Mo. At slower growth rates (≤12.7 mm/h), the grains had sharp boundaries, while faster
growth rates (≥25.4 mm/h) led to cells bounded by intercellular regions. None of the growth conditions resulted in either
dendrites or third phases. Compressive testing between 1200 and 1400 K indicated that alloys DS at rates between 25.4 and
254 mm/h possessed the best strengths, while room-temperature toughness exhibited a plateau of about 16 MPa√m for growth rates
between 12.7 and 127 mm/h. Thus, a growth rate of 127 mm/h represents the best combination of fast processing and mechanical
properties for this system. 相似文献
11.
对Al-Fe-Mg-Si和Al-Fe-Mn-Si2个四元系进行热力学优化评估,并对这2个四元系富Al角的零变量平衡反应温度和液相成分进行了计算。采用光学显微镜,扫描电镜和电子探针技术,系统研究了多组元Al基合金Al356.1定向凝固的显微组织及显微偏析。计算模拟了3个多组元Al合金(Al356.1,Al356.2,Al518.2)的平衡凝固和非平衡凝固的显微组织及显微偏析。模型计算结果与实测数据很吻合,证实了所建立的多组元体系热力学及动力学数据库的可靠性。 相似文献
12.
《Acta Metallurgica》1987,35(2):427-438
From directionally solidified aluminium with a 〈100〉 fibre texture specimens of different orientations were cut and the textures after rolling (95% reduction) and recrystallization were determined. The results are discussed on the basis of the current concepts on deformation and recrystallization in single and polycrystalline materials. Due to the strong influence of the different starting textures, characteristic differences in the rolling textures are obtained. For the first time, a case is reported for which an experimental rolling texture is completely explicable in terms of the Taylor theory under full constraints condition. Further, the recrystallization textures, although appearing more uniform, exhibit clear differences which yield new evidence with respect to the mechanisms of formation of the cube texture. It shows that in the present case for obtaining a pronounced cube texture, both the conditions of oriented growth (in the sense of 40° 〈111〉 rotations) and oriented nucleation (in the sense of properly oriented transition bands) must occur. 相似文献
13.
Unlike many eutectic composites, the Ni-W eutectic exhibits extensive ductility by slip. Furthermore, its properties may be
greatly varied by proper heat treatments. Here results of studies of deformation in both monotonic and fatigue loading are
reported. During monotonie deformation the fiber /matrix interface acts as a source of dislocations at low strains and an
obstacle to matrix slip at higher strains. Deforming the quenched-plus-aged eutectic causes planar matrix slip, with the result
that matrix slip bands create stress concentrations in the fibers at low strains. The aged eutectic reaches generally higher
stress levels for comparable strains than does the as-quenched eutectic, and the failure strains decrease with increasing
aging times. For the composites tested in fatigue, the aged eutectic has better high-stress fatigue resistance than the as-quenched
material, but for low-stress, high-cycle fatigue their cycles to failure are nearly the same. However, both crack initiation
and crack propagation are different in the two conditions, so the coincidence in high-cycle fatigue is probably fortuitous.
The effect of matrix strength on composite performance is not simple, since changes in strength may be accompanied by alterations
in slip modes and failure processes. 相似文献
14.
15.
The eutectics NiAl-Cr with cylindrical chromium fibers and NiAl-Mo with faceted molybdenum fibers were heated at 1400°C to
determine the stability of the composite structure and to compare the stability of the nonfaceted fibers with that of the
faceted fibers in the NiAl matrix. Fiber size and size distribution and number of fibers per unit area were measured as a
function of time at temperature. The number of fibers in the NiAl-Cr eutectic decreased continuously reaching half the initial
value in about 30 h at temperature. Spheroidization of the fibers occurred and was complete in 160 h. In the NiAl-Mo eutectic,
the number of fibers per unit area remained constant to 150 h and the fiber size was constant to 331 h at 1400°C. In NiAl-Cr,
the cylindrical chromium fibers first formed pinchedoff segments at random diameter variations along the length of the fibers.
The segments gradually shortened and thickened and finally spheroidized. The faceted molybdenum fibers remained stable because
the Mo-NiAl interface is constrained to lie in the facet plane which inhibits the formation of faults leading to pinching
off of the fibers. 相似文献
16.
Unlike many eutectic composites, the Ni-W eutectic exhibits extensive ductility by slip. Furthermore, its properties may be greatly varied by proper heat treatments. Here results of studies of deformation in both monotonic and fatigue loading are reported. During monotonie deformation the fiber /matrix interface acts as a source of dislocations at low strains and an obstacle to matrix slip at higher strains. Deforming the quenched-plus-aged eutectic causes planar matrix slip, with the result that matrix slip bands create stress concentrations in the fibers at low strains. The aged eutectic reaches generally higher stress levels for comparable strains than does the as-quenched eutectic, and the failure strains decrease with increasing aging times. For the composites tested in fatigue, the aged eutectic has better high-stress fatigue resistance than the as-quenched material, but for low-stress, high-cycle fatigue their cycles to failure are nearly the same. However, both crack initiation and crack propagation are different in the two conditions, so the coincidence in high-cycle fatigue is probably fortuitous. The effect of matrix strength on composite performance is not simple, since changes in strength may be accompanied by alterations in slip modes and failure processes. 相似文献
17.
S. A. David A. T. Santhanam H. D. Brody 《Metallurgical and Materials Transactions A》1976,7(8):1051-1055
The crystallographic relationship displayed by the niobium and niobium carbide <Nb2C> phases in an aligned eutectic sample with a lamellar carbide morphology is lamellar interface ∥ {110}NB ∥ (001)Nb
2C growth direction ∥<112>NB ∥ [010]Nb2C or [1-20]Nb
2C and for the rod-like carbide morphology rod interface (major axis) ∥{110}Nb ∥ (001)Nb
2C growth direction 11(H2)Nb II l010]Nb.,c or [210]NB2C. 相似文献
18.
The crystallographic direction of growth in directionally solidified magnesium alloy AZ91 has been studied by TEM and EBSP
techniques in SEM. The main direction of growth is found to be
. The dendrites have sixfold symmetry around the main direction, with secondary arms lying along the traces of the (0001),
, and
-planes, respectively. The secondary arms lying in the basal plane are crystallographically of the same type as the main direction:
and
. 相似文献
19.
J. T. Guo C. Yuan H. C. Yang V. Lupinc M. Maldini 《Metallurgical and Materials Transactions A》2001,32(5):1103-1110
The creep-rupture behavior of the directionally solidified (DS) nickel-base superalloy DZ17G has been investigated over a
wide stress range of 60 to 950 MPa at high temperature (923 to 1323 K). In this article, the detailed creep deformation and
fracture mechanisms at constant load have been studied. The results show that all creep curves exhibit a short primary and
a dominant accelerated creep stage, which results in higher ductility of DS superalloy DZ17G compared to the conventionally
cast alloy. From the creep parameters and transmission electron microscopy (TEM) observations, it is suggested that the dominant
creep deformation mechanism has a change from gamma prime particles shearing by matrix dislocations in high stress region
to dislocation climb process in low stress region. It is found that the fracture mode of DS superalloy DZ17G is transgranular,
and it is controlled by the propagation rate of creep cracks initiated at both surface and inner microstructure discontinuities.
The creep rupture data follows the Monkman-Grant relationship under all the explored test conditions. 相似文献
20.
Simple theories developed in a series of recent papers, describing solid/liquid interface stability and solute redistribution
in three phase ternary alloys are summarized and extended to apply to ternary alloys that are two phase on equilibrium solidification.
Calculated results compare well with experiments on alloys from the Al-rich corner of the Al-Cu-Ni ternary phase diagram.
These alloys represent the type currently under development for gas turbine applications. At sufficiently high values ofG/R (thermal gradient divided by growth rate) the alloys grow with a fully plane front. At moderate values ofG/r they exhibit a unique structure of single phase cells and two phase intercellular root material which grows with a “plane
front”. Further decreases inG/R produce more typical ternary cellular structures with regions of one, two and three phases.
Formerly Graduate Student, Massachusetts Institute of Technology, Cambridge, Mass.
Formerly Research Associate, Massachusetts Institute of Technology, Cambridge, Mass. 相似文献