共查询到20条相似文献,搜索用时 31 毫秒
1.
P. K. Rohatgi R. C. Sharma K. V. Prabhakar 《Metallurgical and Materials Transactions A》1975,6(3):569-575
Al-10.98 pct Si-4.9 pct Ni ternary eutectic alloy was unidirectionally solidified at growth rates from 1.39μm/sec to 6.95μm/sec. Binary Al-Ni and Al-Si eutectics prepared from the same purity metals were also solidified under similar conditions to characterize the growth conditions under the conditions of present study. NiAl3 phase appeared as fibers in the binary Al-Ni eutectic and silicon appeared as irregular plates in the binary Al-Si eutectic. However, in the ternary Al-Si-Ni eutectic alloy both NiAl3 and silicon phases appeared as irregular plates dispersed in α-Al phase, without any regular repctitive arrangement. The size and spacing of NiAl3 and Si platelets in cone shaped colonies decreased with an increase in the growth rate of the ternary eutectic. Examination of specimen quenched during unidirectional solidification indicated that the ternary eutectic grows with a non-planar interface with both Si and NiAl3 phases protruding into the liquid. It is concluded that it will be difficult to grow regular ternary eutectic structures even if only one phase has a high entropy of melting. The tensile strength and modulus of unidirectionally solidified Al-Si-Ni eutectic was lower than the chill cast alloys of the same composition, and decreased with a decrease in growth rate. Tensile modulus and strength of ternary Al-Si-Ni eutectic alloys was greater than binary Al-Si eutectic alloy under similar growth conditions, both in the chill cast and in unidirectionally solidified conditions. 相似文献
2.
Melt-spun Al-3 wt pct Si with and without ternary additions of Na and Sr has been heat-treated above the Al-Si eutectic temperature
in a differential scanning calorimeter to form a microstructure of Al-Si eutectic liquid droplets embedded in the α-Al matrix.
During subsequent cooling in the calorimeter, the heterogeneous nucleation temperature for solidification of Si in contact
with the surrounding Al matrix depends sensitively on the alloy purity, with a nucleation undercooling which increases with
increasing alloy purity from 9 to 63 K below the Al-Si eutectic temperature. These results are consistent with Southin’s hypothesis
that low levels of trace P impurities are effective in catalyzing Si nucleation in contact with the surrounding Al matrix.
With a low Al purity alloy, 0.1 wt pct Na addition increases the Si nucleation undercooling from 9 to 50 K, 0.15 wt pct Sr
addition does not affect the Si nucleation temperature, and 0.3 wt pct Sr addition decreases the Si nucleation undercooling
from 9 to 3 to 4 K. The solidified microstructure of the liquid Al-Si eutectic droplets embedded in the Al matrix depends
on the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form
one faceted Si particle; however, with high Si nucleation undercooling, each Al-Si eutectic droplet solidifies to form a large
number of nonfaceted Si particles embedded in Al.
Formerly with the Oxford Centre for Advanced Materials and Composites, Department of Materials, Oxford University 相似文献
3.
The effect of hot-rolling on the mechanical properties and microstructures of chill-cast Al-Al3Ni, chill-cast Al-Al2Cu, and unidirectionally solidified Al-Al3Ni eutectic alloys has been studied. The chill-cast eutectic alloys were produced by casting into preheated mild steel molds
placed on copper chills. This system promoted growth along the length of the ingot and not radially from the mold wall. Cellular
microstructures resulted with good alignment of Al3Ni fibers or Al2Cu lamellae within the cells and an interfiber/lamellar spacing of ~ 1 /urn. In contrast, the Al-Al3Ni eutectic alloy was also unidirectionally solidified at a growth rate of 3 x 10-1 m s-1 in a conventional horizontal crystal grower. This produced well-aligned Al3Ni fibers with an interfiber spacing of 1.2 ώm. Both the unidirectionally solidified and chill-cast Al-Al3Ni eutectic alloy can be hot-rolled at 773 K to reductions in area of greater than 95 pct. Deformation was achieved by Al3Ni fiber fracturing followed by separation of the broken fiber fragments in the rolling direction. Additionally, for the chill-cast
eutectic the cellular microstructure disappeared and the Al3Ni fibers were homogeneously distributed throughout the matrix, after area reductions of 60 to 70 pct. In both cases, the
eutectic microstructure was deformed with a constant volume fraction of Al3Ni/unit volume being maintained during rolling. The chill-cast Al-Al2Cu eutectic alloy can be hot-rolled at 773 K to an area reduction of ~50 pct, after the continuous brittle Al2Cu phase within the cells has been ‘broken up’ by coarsening at high temperature. The variations of room temperature tensile
properties for the chill-cast and unidirectionally solidified eutectic alloys were measured as a function of reduction of
thickness during hot-rolling and the results were compared with predicted strengths from discontinuous fiber reinforcement
theory.
Formerly with the Department of Engineering and Applied Sciences, Sussex University, Falmer, Brighton, BNl 9QT, Sussex, United
Kingdom, 相似文献
4.
S. Sengupta H. Soda A. McLean J. W. Rutter 《Metallurgical and Materials Transactions A》2000,31(1):239-248
A ternary eutectic alloy with a composition of 57.2 pct Bi, 24.8 pct In, and 18 pct Sn was continuously cast into wire of
2 mm diameter with casting speeds of 14 and 79 mm min−1 using the Ohno Continuous Casting (OCC) process. The microstructures obtained were compared with those of statically cast
specimens. Extensive segregation of massive Bi blocks, Bi complex structures, and tinrich dendrites was found in specimens
that were statically cast. Decomposition of γSn by a eutectoid reaction was confirmed based on microstructural evidence. Ternary eutectic alloy with a cooling rate of
approximately 1 °C min−1 formed a double binary eutectic. The double binary eutectic consisted of regions of BiIn and decomposed γSn in the form of a dendrite cell structure and regions of Bi and decomposed γSn in the form of a complex-regular cell. The Bi complex-regular cells, which are a ternary eutectic constituent, existed
either along the boundaries of the BiIn-decomposed γSn dendrite cells or at the front of elongated dendrite cell structures. In the continuously cast wires, primary Sn dendrites
coupled with a small Bi phase were uniformly distributed within the Bi-In alloy matrix. Neither massive Bi phase, Bi complex-regular
cells, nor BiIn eutectic dendrite cells were observed, resulting in a more uniform microstructure in contrast to the heavily
segregated structures of the statically cast specimens. 相似文献
5.
J. R. Cahoon M. C. Chaturvedi K. N. Tandon 《Metallurgical and Materials Transactions A》1998,29(3):1101-1111
Three Al-4 wt pct Cu alloy ingots, 10 mm in diameter and 25-mm long, were unidirectionally solidified in microgravity during
the flight of a sounding rocket, with solidification rates of about 1.6 × 10−4 m/s and temperature gradients of about 2600 K/m. The apparatus was comprised of three muffle furnaces, which melted the ingots
prior to the launch of the rocket. Unidirectional solidification of the ingots was accomplished by chill plates attached to
the furnaces, which were withdrawn from the ingots during the microgravity portion of the flight, bringing the chill plates
into contact with the bases of the capsules containing the ingots. Solidification was complete in less than 4 minutes. For
comparison, several ground-based ingots were solidified in unit gravity under similar conditions. Metallographic analysis
of the solidified ingots showed that the macrostructures of the unit-gravity and microgravity ingots were similar, all exhibiting
columnar grains. However, the microstructures were significantly different, with the microgravity ingots exhibiting primary
dendrite spacings about 40 pct larger than the unit-gravity ingots and secondary dendrite arm spacings about 85 pct larger.
The larger dendrite spacings for the ingots solidified in microgravity are explained by lower dendrite growth velocities.
The absence of convective mixing in the microgravity ingots slightly increased temperature gradients in the liquid portion
of the alloy during solidification, which resulted in decreased growth velocities. 相似文献
6.
The microstructural evolution of a spray-formed and extruded hypereutectic aluminum-30 pct silicon-5 pct copper-2 pct magnesium
alloy heated into the semisolid state has been investigated. Liquid is formed initially by a quaternary eutectic reaction
and then by a ternary melt reaction. These reactions occur relatively quickly. However, the binary Al-Si eutectic melt reaction
takes a significant time—around several hours depending on the temperature. The coarsening rate constants (K) for the growth of the silicon particles are approximately three to four orders of magnitude lower than those for the majority
of metal spray-formed alloys. This may be associated with difficulties in addition or removal of atoms from the low index
silicon facets. Where growth does occur, agglomeration of silicon particles may play a large role, especially at higher liquid
contents. Electron backscatter diffraction (EBSD) gives evidence of agglomeration, and furthermore shows that high-angle silicon-silicon
boundaries are not wetted with liquid. 相似文献
7.
P. K. Rohatgi A. K. Bhutani K. P. Abraham 《Metallurgical and Materials Transactions A》1979,10(3):333-342
Attempts were made to produce directionally solidified, specifically grain aligned Al-6 wt pct Ni eutectic alloy using a laboratory
scale ESR unit. For this purpose sand cast alloy electrodes were electroslag remelted under different mold conditions. The
grain structure of the ingots obtained from these meltings showed that insulated silica molds gave the best vertical alignment
of grains along the length of the ingot. The NiAl3 fibers within the grains tended to fan out and there was only a preferred alignment of fibers along the growth direction
under the conditions of our experiments. The ESR parameters most suitable for vertical alignment of eutectic grains have been
identified. In some electroslag remelting trials ingots were grown on a seed ingot. This resulted in a fewer vertical grains
compared to the case when no seed ingot was used. The sand cast specimen of the eutectic exhibited a maximum tensile strength
of around 88.2 MN/m2 (9.0 kg/mm2) whereas conventional ESR using water cooled mold gave strength value of 98.0 MN/m2 (10 kg/mm2). The directionally solidified ESR material showed longitudinal tensile strength as high as 213.7 MN/m2 (21.8 kg/mm2) which could be further increased to 220.6 MN/m2 (22.5 kg/mm2) by using the seed ingot. The average growth rate was varied between 5 to 25 mm/min during electroslag remelting in this
study. The flow stresses, tangent modulus and ultimate tensile strength of directionally solidified eutectic increased with
increasing growth rates.
Formerly Research Fellow, Indian Institute of Science, Bangalore 560012 is now 相似文献
8.
J. A. Taylor G. B. Schaffer D. H. StJohn 《Metallurgical and Materials Transactions A》1999,30(6):1651-1655
The mechanism by which iron causes casting defects in the AA309 (Al-5 pct Si-1.2 pct Cu-0.5 pct Mg) may be related to the
solidification sequence of the alloy. Superimposing calculated segregation lines on the liquidus projection of the ternary
Al-Si-Fe phase diagram suggests that porosity is minimized at a critical iron content when solidification proceeds directly
from the primary field to the ternary Al-Si-βAl5FeSi eutectic point. Solidification via the binary Al-βAl5FeSi eutectic is detrimental to casting integrity. This hypothesis was tested by comparing the critical iron content observed
in the standard AA309 alloy to that of a high-silicon (10 pct Si) variant of this alloy. 相似文献
9.
J. R. Cahoon M. C. Chaturvedi K. N. Tandon 《Metallurgical and Materials Transactions A》1998,29(13):1101-1111
Three Al-4 wt pct Cu alloy ingots, 10 mm in diameter and 25-mm long, were unidirectionally solidified in microgravity during
the flight of a sounding rocket, with solidification rates of about 1.6×10−4 m/s and temperature gradients of about 2600 K/m. The apparatus was comprised of three muffle furnaces, which melted the ingots
prior to the launch of the rocket. Unidirectional solidification of the ingots was accomplished by chill plates attached to
the furnaces, which were withdrawn from the ingots during the microgravity portion of the flight, bringing the chill plates
into contact with the bases of the capsules containing the ingots. Solidification was complete in less than 4 minutes. For
comparison, several ground-based ingots were solidified in unit gravity under similar conditions. Metallographic analysis
of the solidified ingots showed that the macrostructures of the unit-gravity and microgravity ingots were similar, all exhibiting
columnar grains. However, the microstructures were significantly different, with the microgravity ingots exhibiting primary
dendrite spacings about 40 pct larger than the unit-gravity ingots and secondary dendrite arm spacings about 85 pct larger.
The larger dendrite spacings for the ingots solidified in microgravity are explained by lower dendrite growth velocities.
The absence of convective mixing in the microgravity ingots slightly increased temperature gradients in the liquid portion
of the alloy during solidification, which resulted in decreased growth velocities.
K.N. TANDON, formerly Associate Professor, Materials Engineering Laboratory, Department of Mechanical and Industrial Engineering,
University of Manitoba 相似文献
10.
Chromium white cast irons are widely used in environments where severe abrasion resistance is a dominant requirement. To improve
the wear resistance of these commercially important irons, the United States Bureau of Mines and CSIRO Australia are studying
their solidification and solid-state transformation kinetics. A ternary Fe-Cr-C iron with 17.8 wt pct (pct) Cr and 3.0 pct
C was compared with commercially available irons of similar Cr and C contents with Si contents between 1.6 and 2.2 pct. The
irons were solidified and cooled at rates of 0.03 and 0.17 K · s-1 to 873 K. Differential thermal analysis (DTA) showed that Si depresses the eutectic reaction temperature and suggests that
is has no effect upon the volume of eutectic carbides formed during solidification. Microprobe analysis revealed that austenite
dendrites within the Si alloyed irons cooled at 0.03 and 0.17 K·s-1 had C and Cr contents that were lower than those of dendrites within the ternary alloy cooled at the same cooling rate and
a Si alloyed iron that was water quenched from the eutectic temperature. These lower values were shown by image analysis to
be the result of both solid-state growth (coarsening) of the eutectic carbides and some secondary carbide formation. Hardness
measurements in the as-cast condition and after soaking in liquid nitrogen suggest an increase in the martensite start temperature
as the Si content was increased. It is concluded that Si’s effect on increasing the size and volume fraction of eutectic carbides
and increasing the matrix hardness should lead to improved wear resistance over regular high-chromium white cast irons. 相似文献
11.
Small laboratory samples of binary lead-tin alloys, about 20 mm diameter, 50 mm high, and weighing about 180 grams, were solidified
unidirectionally upward, with both cooling rate and thermal gradient being closely controlled. A total of nine ingots were
produced; six of these ingots had a nominal composition of Pb-15 wt pct Sn, and the remaining three were Sn-15 wt pct Pb.
Detailed thermal measurements, chemical composition measurements (dendrite composition, fraction of interdendritic eutectic
formed), and microstructural characterization (primary and secondary dendrite arm spacing measurements) were carried out.
Normal macrosegregation was observed in the Pb-rich alloys, with Sn content being highest at the top of the ingot. In the
Pb-15 wt pct Sn alloys, macrosegregation was found to increase with increasing thermal gradients, and also with decreasing
cooling rates. In some of our experiments with Pb-rich alloys it was possible to induce severe segregation, in the form of
deep freckles. The freckles were in the form of a column of tin-rich material which extended over the upper half of the ingot.
No significant macrosegregation was evident in the Sn-rich alloy for the growth conditions employed here. Segregation behavior
of all ingots was correlated with the measured thermal history, compositional data, and the fraction eutectic phase in the
finally solidified samples.
This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research”
presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD
Thermodynamic Data Committee and the Material Processing Committee. 相似文献
12.
In the ternary system Al-Ge-Si, the binary eutectic reactions, L (liquid) ⇔ (Al) + (Si) and L ⇔ (Al) + (Ge) are connected
by a monovariant valley, L ⇔ (Al) + (SiGe), falling from 578 °C at 12.7 wt pct Si to 424 °C at 53 wt pct Ge. The binary, solid
eutectic surface extends from the Al corner of the ternary phase diagram (1.65 wt pct Si to 5.2 wt pct Ge) across to the continuous
(SiGe) solid solution which contains very little Al: several tie triangles, L-(A1)-(SiGe), have been determined using thermal
analysis and electron microprobe analysis (EMPA). Optical and scanning electron microscopy (SEM) examination reveals that
coring in the normal (SiGe) phase is discontinuous, showing composition banding, which indicates that stationary 111 facets
of the solid solution were exposed to the liquid for extended periods up to 500 seconds; details of this interrupted coring
were quantified by EMPA. Similar, smaller, and more gradual variations could also be detected in the (Al) matrix, and these
compositional fluctuations are considered to reflect discontinuities in the local eutectic growth rates. Modification and
twin-ning, induced by Na, are observed in both Al-Si and Al-Ge but decrease progressively with Ge content; coring in the modified
ternary alloys is more continuous. 相似文献
13.
Compositions on the eutectic liquidus line between Co-25 wt pct Cr-3.5 wt pct C and Co-45 wt pct Cr-2.2 wt pct C wherein the simultaneous freezing of a metal matrix and a carbide occur have been solidified unidirectionally. The composite structures formed consist of a cobalt matrix containing substantial amounts of soluble chromium with the carbide phase, (Cr, Co)7C3, as an aligned fibrous dispersion. The structure of one monovariant eutectic composition has been examined as a function of solidification velocity. Cellular growth was observed at rates above approximately 4 cm per hr. However, the cell cusps only gradually deepen with increasing growth rate and the carbide phase remains essentially aligned even at 50 cm per hr. The concentration of carbide fibers in the transverse section was measured at various growth rates and was found to be approximately a linear function of rate. The structure of the alloy was also examined by transmission electron microscopy. These results indicate that the preferred growth direction of the carbide is more important than the solid-solid surface energy in controlling the growth. 相似文献
14.
M. Basaran T. Z. Kattamis R. Mehrabian M. C. Flemings 《Metallurgical and Materials Transactions B》1973,4(10):2429-2434
The effect of high temperature homogenization treatment and hot isostatic pressing on microporosity in unidirectionally solidified
AISI 4330 low alloy steel ingots was investigated. The volume fraction and size distribution of micropores in the as-cast,
homogenized, and hot isostatically pressed conditions were determined using X-ray microradiography and optical metallography.
The vol pct of spherical, nonspherical and total microporosity increased with distance from the chill in a linear fashion
in both the as-cast and the homogenized conditions. The vol pct of microporosity decreased with increasing homogenization
time. Nonspherical micropores gradually spheroidized during the treatment. Elimination of microporosity was substantially
accelerated by carrying out homogenization under high isostatic pressure. Measured vol pct microporosity at 3 cm from the
chill before and after vacuum homogenization at 1315°C for 30 h were 2 × 10−2 and 1 × 10−2, respectively. All observable microporosity was eliminated by hot isostatic pressing for 1 h at 1260 and 1O38°C with corresponding
pressures of 29,000 and 27,000 psi, respectively. It is proposed that vacancy diffusion to grain boundaries is the rate controlling
mechanism. 相似文献
15.
A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum
alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy
and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied
in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain
refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance
from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis
and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture
surface analysis (phase identification), respectively. The results showed that the local solidification time(viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced
effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain
boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching.
The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al5FeSi, α- Al15(Fe,Mn)3Si2, and Al2Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties
to DAS allows for the use of the latter as an indicator of the expected properties of the alloy. 相似文献
16.
A study has been made of the microstructure and some high temperature mechanical properties of a unidirectionally solidified,
pseudobinary Fe-Cr-Nb eutectic alloy. The alloy consists of 22 volume pct of an intermetallic phase in an iron-chromium solid
solution matrix. At all solidification rates investigated the eutectic solidified with a cellular solid/liquid interface.
The minor phase adopted a fibrous growth morphology except at cell boundaries where it had a plate-like morphology. The system
was found to be resistant to microstructural changes at temperatures of up to 1100°C. Fiber reinforcement however was not
observed; the strength of the alloy at elevated temperatures was mainly governed by the properties of the matrix phase.
Formerly Research Officer, BHP Melbourne Research Laboratories, Melbourne, Victoria, Australia 相似文献
17.
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
18.
Hypereutectic Al-18 wt pct Si alloy is widely used in automotive industry as a wear-resistant alloy for engine components. However, in the last few years, this traditional composition is being considered for processing by different rapid solidification methods. Positive points include its low thermal expansion and uniform distribution of surface oxides. Nevertheless, the microstructural aspects of Al-Si powders of 18 wt pct Si are still need to be addressed, such as, the eutectic Si morphology, size, and distribution generated by different process conditions during rapid solidification. Based on a detailed quantitative analysis of the microstructures of rapid solidified Al-18 wt pct Si in this work, solidification conditions that yield specific Si morphologies, Si spacing, and thermal cooling conditions are outlined. The focus is determining the solidification conditions that will yield a specified shape of eutectic Si. It is shown that Si morphology is dependent on a combination of growth velocity (based on modified JH model) and temperature gradient. Furthermore, the highest hardness is achieved with globular morphologies of Si. The processing conditions required to achieve these properties are outlined. 相似文献
19.
Microstructural characteristics of Ni-Sb eutectic alloys under substantial undercooling and containerless solidification conditions 总被引:1,自引:0,他引:1
Both Ni-36 wt pct Sb and Ni-52.8 wt pct Sb eutectic alloys were highly undercooled and rapidly solidified with the glass-fluxing
method and drop-tube technique. Bulk samples of Ni-36 pct Sb and Ni-52.8 pct Sb eutectic alloys were undercooled by up to
225 K (0.16 T
E
) and 218 K (0.16 T
E
), respectively, with the glass-fluxing method. A transition from lamellar eutectic to anomalous eutectic was revealed beyond
a critical undercooling ΔT
1*, which was complete at an undercooling of ΔT
2*. For Ni-36 pct Sb, ΔT
1*≈60 K and ΔT
2*≈218 K; for Ni-52.8 pct Sb, ΔT
1*≈40 K and ΔT
2*≈139 K. Under a drop-tube containerless solidification condition, the eutectic microstructures of these two eutectic alloys
also exhibit such a “lamellar eutectic-anomalous eutectic” morphology transition. Meanwhile, a kind of spherical anomalous
eutectic grain was found in a Ni-36 pct Sb eutectic alloy processed by the drop-tube technique, which was ascribed to the
good spatial symmetry of the temperature field and concentration field caused by a reduced gravity condition during free fall.
During the rapid solidification of a Ni-52.8 pct Sb eutectic alloy, surface nucleation dominates the nucleation event, even
when the undercooling is relatively large. Theoretical calculations on the basis of the current eutectic growth and dendritic
growth models reveal that γ-Ni5Sb2 dendritic growth displaces eutectic growth at large undercoolings in these two eutectic alloys. The tendency of independent
nucleation of the two eutectic phases and their cooperative dendrite growth are responsible for the lamellar eutectic-anomalous
eutectic microstructural transition. 相似文献
20.
T. S. Prasanna Kumar K. Narayan Prabhu 《Metallurgical and Materials Transactions B》1991,22(5):717-727
Heat flow at the metal/chill interface of bar-type castings of aluminum base alloys was modeled as a function of thermophysical
properties of the chill material and its thickness. Experimental setup for casting square bars of Al-13.2 pct Si eutectic
and Al-3 pet Cu-4.5 pct Si long freezing range alloys with chill at one end exposed to ambient conditions was fabricated.
Experiments were carried out for different metal/chill combinations with and without coatings. The thermal history at nodal
locations in the chill obtained during the experiments was used to estimate the interface heat flux by solving a one-dimensional
Fourier heat conduction equation inversely. Using the data on transient heat flux q, the heat flow at the casting/chill interface
was modeled in two steps: (1) The peak in the heat flux curve qmax was modeled as a power function of the ratio of the chill thickness d to its thermal diffusivity a, and (2) the factor (q/qmax) X α0.05 was also modeled as a power function of the time after the solidification set in. The model was validated for Cu-10 pct Sn
-2 pct Zn alloy chill and Al-13.2 pct Si and Al-3 pct Cu-4.5 pct Si as the casting alloys. The heat flux values estimated
using the model were used as one of the boundary conditions for solidification simulation of the test casting. The experimental
and simulated temperature distributions inside the casting were found to be in good agreement.
Formerly Assistant Professor with Karnataka Regional Engineering College 相似文献