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1.
Several ingots (0.0254 m in diam × 0.10 m long) of nickel-30 wt pct copper, nickel-10 wt pct cobalt and iron-25 wt pct nickel
were solidified with various undercoolings up to about 200 K, prior to nucleation of the solid. The materials were mechanically
tested in the ascast condition. In nickel-30 wt pct copper and iron-25 wt pct nickel alloys the 0.2 pct offset yield strength,
ductility and fatigue strength increased with undercooling. A linear relationship was established between 0.2 pct offset yield
strength and the square root of secondary dendrite arm spacing in dendritic alloys (undercooled less than 170 K) or that of
grain diameter in nondendritic alloys (undercooled more than 170 K). In iron-25 wt pct nickel limited testing indicated improvements
in Charpy V-notch impact strength and in fracture toughness with undercooling. No improvement of tensile properties with undercooling
was observed in nickel-10 wt pct cobalt, an alloy which solidified normally with very low microsegregation. 相似文献
2.
Solidification of highly undercooled Sn- Pb alloy droplets 总被引:1,自引:0,他引:1
M. G. Chu Y. Shiohara M. C. Flemings 《Metallurgical and Materials Transactions A》1984,15(7):1303-1310
Experimental work is described on undercooling and structure of tin-lead droplets emulsified in oil. The droplets, predominantly
in the size range of 10 to 20 μm, were cooled at rates (just before nucleation) ranging from about 10-1 K per second to 106 K per second. The higher cooling rates were obtained by a newly developed technique of quenching the emulsified droplets
in a cold liquid. Measured undercoolings (at the lower cooling rates) ranged up to about 100 K. Structures obtained depend
strongly on undercooling, cooling rate before and after nucleation, and alloy composition. Droplets containing up to 5 wt
pct Pb were apparently single phase when undercooled and rapidly quenched. Droplets in the composition range of about 25 wt
pct to 90 wt pct Pb solidified dendritically, even at the most rapid quench rates employed, apparently because these alloys
undercooled only slightly before nucleation of the primary phase.
Formerly Graduate Research Assistant and Postdoctoral Associate in the Department of Materials Science and Engineering, Massachusetts
Institute of Technology. 相似文献
3.
4.
Toshihiko Koseki Merton C. Flemings 《Metallurgical and Materials Transactions A》1995,26(11):2991-2999
Solidification of undercooled Fe-Cr-Ni alloys was studied by high-speed pyrometry during and after recalescence of levitated,
gas-cooled droplets. Alloys were of 70 wt pct Fe, with Cr varying from 15 to 19.7 wt pct, balance was Ni. Undercoolings were
up to about 300 K. Alloys of Cr content less than that of the eutectic (18.1 wt pct) have face-centered cubic (fee) (austenite)
as their equilibrium primary phase, and alloys of higher Cr content have body-centered cubic (bcc) (ferrite) as their equilibrium
primary phase. However, except at low undercoolings in the hypoeutectic alloys, all samples solidified with bcc as the primary
phase; the bcc then transformed to fcc during initial recalescence for the lower Cr contents or during subsequent cooling
for the higher Cr contents. The bcc-to-fcc transformation, whether in the semisolid or solid state, was detected by a second
recalescence. In the hypoeutectic alloys, the growth of primary metastable bcc apparently results from preferred nucleation
of bcc. The subsequent nucleation of fcc may occur at bcc/bcc grain boundaries.
Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology 相似文献
5.
Y. Wu T. J. Piccone Y. Shiohara M. C. Flemings 《Metallurgical and Materials Transactions A》1987,18(6):925-932
High-speed optical temperature measurements were made of the solidification behavior of levitated metal samples within a transparent
glass medium. Two undercooled Ni-Sn alloys were examined, one a hypoeutectic alloy and the other of eutectic composition.
Recalescence times for the 9 mm diameter samples studied decreased with increasing undercooling from the order of 1.0 second
at 50 K under-cooling to less than 10−3 second for undercoolings greater than 200 K. Both alloys recalesced smoothly to a maximum recalescence temperature at which
the solid was at or near its equilibrium composition and equilibrium weight fraction. For the samples of hypoeutectic alloy
that recalesced above the eutectic temperature, a second nucleation event occurred on cooling to the eutectic temperature.
For samples which recalesced only to the eutectic temperature, no subsequent nucleation event was observed on cooling. It
is inferred in this latter case that both the α and β phases were present at the end of recalescence. The thermal data obtained
suggest a solidification model involving (1) dendrites of very fine structure growing into the melt at temperatures near the
bulk undercooling temperature, (2) thickening of dendrite arms with rapid recalescence, and (3) continued, much slower recalescence
accompanying dendrite ripening. 相似文献
6.
Y. Wu T. J. Piccone Y. Shiohara M. C. Flemings 《Metallurgical and Materials Transactions A》1987,18(6):915-924
Solidification of undercooled Ni-25 wt pct Sn alloy was observed by high-speed cinematography and results compared with optical
temperature measurements. Samples studied were rectangular in cross-section, and were encased in glass. Cinematographic measurements
were carried out on samples undercooled from 68 to 146 K. These undercoolings compare with a temperature range of 199 K from
the equilibrium liquidus to the extrapolated equilibrium solidus. At all undercoolings studied, the high-speed photography
revealed that solidification during the period of recalescence took place with a dendrite-like front moving across the sample
surface. Spacings of the apparent “dendrite” were on the order of millimeters. The growth front moved at measured velocities
ranging from 0.07 meters per second at 68 K undercooling to 0.74 meters per second at 146 K undercooling. These velocities
agree well with results of calculations according to the model for dendrite growth of Lipton, Kurz, and Trivedi. It is concluded
that the coarse structure observed comprises an array of very much finer, solute-controlled dendrites. 相似文献
7.
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
8.
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
9.
S. N. Tewari 《Metallurgical and Materials Transactions A》1987,18(5):525-542
Ni-35 at. pct Mo (eutectic) and Ni-38 at. pct Mo (hypereutectic) alloy specimens have been solidified from various levels
of undercooling in the differential thermal analysis (DTA) and the electromagnetic levitation (EML) units in a pyrex/vycor
bed. The evolution of the microstructure in the solidified specimens has been examined in terms of the degree of undercooling,
the nature of the first phase to nucleate from the melt, and the specimen cooling rate. The melt has been observed to undercool
more in the presence of intermetallic NiMo (β) phase as compared to that in the presence of nickel-rich solid solution (γ).
The “anomalous eutectic” type of microstructure has been shown to result from the initial formation of the dendritic skeleton
of either of the two phases, its segmentation due to convection and ripening, and the subsequent nucleation of the other phase
in the interdendritic liquid regions. The recalescence behavior has been examined as a function of undercooling and the nature
of the phase nucleating first in the melt. 相似文献
10.
Mingjun Li Ph.D. Candidate Gencang Yang Yaohe Zhou 《Metallurgical and Materials Transactions A》1999,30(11):2941-2949
Fe-Co alloy melts with Co contents of 10, 30, and 60 at. pct were undercooled to investigate the dependence of the primary
phase on grain coarsening. A pronounced characteristic is that the metastable fcc phase in the Fe-10 at. pct Co alloy and
the metastable bcc phase in the Fe-30 at. pct Co alloy will primarily nucleate when undercoolings of the melts are larger
than the critical undercoolings for the formation of metastable phases in both alloys. No metastable bcc phase can be observed
in the Fe-60 at. pct Co alloy, even when solidified at the maximum undercooling of ΔT = 312 K. Microstructural investigation shows that the grain size in Fe-10 and Fe-30 at. pct Co alloys increases with undercoolings
when the undercoolings of the melts exceed the critical undercoolings. The grain size of the Fe-60 at. pct Co alloy solidified
in the undercooling range of 30 to 312 K, in which no metastable phase can be produced, is much finer than those of the Fe-10
and Fe-30 at. pct Co alloys after the formation of metastable phases. The model for breakage of the primary metastable dendrite
at the solid-liquid interface during recalescence and remelting of dendrite cores is suggested on the basis of microstructures
observed in the Fe-10 and Fe-30 at. pct Co alloys. The grain coarsening after the formation of metastable phases is analyzed,
indicating that the different crystal structures present after the crystallization of the primary phase may play a significant
role in determining the final grain size in the undercooled Fe-Co melts. 相似文献
11.
Henry C. de Groh 《Metallurgical and Materials Transactions A》1994,25(11):2507-2516
The accepted primary mechanism for causing macrosegregation in directional solidification (DS) is thermal and solutal convection
in the liquid. This article demonstrates the effects of under-cooling and nucleation on macrosegregation and shows that undercooling,
in some cases, can be the cause of end-to-end macrosegregation. Alloy ingots of Pb-Sn were directionally solidified upward
and downward, with and without undercooling. A thermal gradient of about 5.1 K/cm and a cooling rate of 7.7 K/h were used.
Crucibles of borosilicate glass, stainless steel with Cu bottoms, and fused silica were used. High undercoolings were achieved
in the glass crucibles, and very low undercoolings were achieved in the steel/Cu crucible. During under-cooling, large, coarse
Pb dendrites were found to be present. Large amounts of macrosegregation developed in the undercooled eutectic and hypoeutectic
alloys. This segre-gation was found to be due to the nucleation and growth of primary Pb-rich dendrites, continued coarsening
of Pb dendrites during undercooling of the interdendritic liquid, Sn enrichment of the liquid, and dendritic fragmentation
and settling during and after recalescence. Eutectic ingots that solidified with no undercooling had no macrosegregation,
because both Pb and Sn phases were effectively nucleated at the start of solidification, thus initiating the growth of solid
of eutectic composition. It is thus shown that undercooling and single-phase nucleation can cause significant macrosegregation
by increasing the amount of solute rejected into the liquid and by the movement of unattached dendrites and dendrite fragments,
and that macrosegregation in excess of what would be expected due to diffusion transport is not necessarily caused by convection
in the liquid. 相似文献
12.
W. J. Boettinger L. Bendersky J. G. Early 《Metallurgical and Materials Transactions A》1986,17(5):781-790
Rapidly solidified powders of Al-8 wt pct Fe exhibit four distinct microstructures with increasing particle diameter in the
size range of 5 μm to 45 μm: microcellular α-Al; cellular α-Al; a-Al + Al6Fe eutectic; and Al3Fe primary intermetallic structure. Small powder particles (~10 μm or less) undercool significantly prior to solidification
and typically exhibit a two-zone microcellular-cellular structure in individual powder particles. In the two-zone microstructure,
there is a transition from solidification dominated by internal heat flow during recalescence with high growth rates (microcellular)
to solidification dominated by external heat flow and slower growth rates (cellular). The origin of the two-zone microstructure
from an initially cellular or dendritic structure is interpreted on the basis of growth controlled primarily by solute redistribution.
Larger particles experience little or no initial undercooling prior to solidification and do not exhibit the two-zone structure.
The larger particles contain cellular, eutectic, or primary intermetallic structures that are consistent with growth rates
controlled by heat extraction through the particle surface (external heat flow). 相似文献
13.
The droplet emulsion technique, which involves dispersal of a bulk liquid alloy into a collection of fine droplets (5 to 30μm), was applied to Sn-Sb alloys to yield high levels of controlled undercooling. The maximum undercooling levels achieved
varied from 179 °C for pure Sn to 113 °C for a Sn-16 at. pct Sb alloy. Analysis of hypoperitectic alloy samples (alloys with
an Sb content less than that of the liquid at the peritectic temperature) indicates that solute trapping occurs during solidification
at the levels of undercooling and cooling rate investigated, yielding nearly homogeneousβ-tin solid solutions with compositions approaching those of the bulk alloys. With increasing undercooling and/or cooling rate,
hyperperitectic alloys exhibit a transition from a highly segregated structure consisting of faceted primary intermetallic
phase and cellularβ to a structure consisting primarily of a supersaturated tin-rich solid solution. Lattice constant measurements confirm that
virtually complete supersaturation ofβ-tin was achieved in emulsion samples cooled at 200 °C ss−1 for compositions up to approximately 20 at. pct Sb. The development and characteristics of subsequent solid-state precipitation
were used to guide the interpretation of the often complex solidification reaction sequences in the hyperperitectic alloys.
The formation of supersaturatedβ-tin solid solutions in the undercooled samples is related to the appropriate metastable phase equilibria and the development
of solute trapping.
Formerly Graduate Student, Department of Materials Science and Engineering, University of Wisconsin-Madison 相似文献
14.
A model was developed to predict micro structural development in lead—61.9 wt pct Sn (eutectic) alloys which were undercooled
5 to 25 K below their equilibrium freezing temperature prior to being preferentially nucleated. While the initial solidification
velocity rapidly increases with increasing undercooling, the model predicts it to quickly decrease, prior to 10 pct solid
formation, after which growth continues near the equilibrium temperature. Experimentally, and in accordance with the prediction,
the eutectic emanated from the nucleation site with an initially fine spacing that increased with distance. However, in contrast
to the model, the eutectic grew outward in a spokelike manner with each arm surrounded by a globular structure, this being
attributed to the difficulty of lateral nucleation. Microstructural uniformity was further compromised by equiaxed eutectic
grains which grew ahead of the advancing interface in the now only slightly undercooled liquid. Consequently, while containerless
techniques may ensure sample purity and permit processing of high-temperature materials, development of a continuously fine
and uniformly aligned microstructure cannot be assumed.
This article is based on a presentation made in the symposium entitled “Microgravity Solidification, Theory and Experimental
Results” as a part of the 1993 TMS Fall meeting, October 17-21, 1993, Pittsburgh, PA, under the auspices of the TMS Solidification
Committee. 相似文献
15.
N. Liu F. Liu L. M. Lu X. Y. Gao X. J. Wang 《Metallurgical and Materials Transactions B》2014,45(3):815-820
Adopting a fluxing purification and cyclic superheating technique, Co-10 wt pct Si and Co-15 wt pct Si alloys had been undercooled to realize rapid solidification in this work. It was investigated that the solidification modes and microstructures of Co-Si alloys were deeply influenced by the undercooling of the melts. Both alloys solidified with a near-equilibrium mode in a low undercooling range; the peritectic reaction occurred between the primary phase and the remnant liquids, and it was followed by the eutectic reaction and eutectoid transformation. With the increase of undercooling, both alloys solidified with a nonequilibrium mode, and the peritectic reaction was restrained. As was analyzed, a metastable Co3Si phase was found in Co-10 wt pct Si alloy when a critical undercooling was achieved. 相似文献
16.
Directionally solidified samples of Mg-32.3 wt pct Al eutectic alloy were produced under an argon atmosphere in a vacuum Bridgman-type
furnace to study the eutectic growth with different growth velocities. Typical features such as steady-state lamellar eutectic
growth, lamellar branching at the quenching interface, and the formation of colony structures due to the impurity of the Mg-Al
binary alloy were observed using a JEOL 6301F scanning electron microscope (JEOL Ltd., Tokyo, Japan). The lamellar spacing
of the two eutectic phases was measured on the transverse sections of the samples. It was found that the relationship between
the measured lamellar spacing and growth velocity agreed well with the prediction of the Jackson-Hunt model. Subsequent studies
of Mg-Al eutectic growth were conducted using a numerical model based on the cellular automaton (CA) method. Taking account
of the solute diffusion, constitutional undercooling, and curvature undercooling, modeling of steady-state lamellar eutectic
growth was achieved. A systematic investigation of the eutectic growth morphology and lamellar spacing of the Mg-Al eutectic
was carried out under directional solidification with different undercoolings, initial lamellar spacings, temperature gradients,
and growth velocities. The results showed that under the interaction between solute diffusion and surface energy, the adjustment
of eutectic lamellar spacing was accomplished by nucleation, lamellar branching, lamellar termination, and overgrowth. The
simulated results were consistent with both the experimental results and the Jackson-Hunt eutectic theory. 相似文献
17.
Mahmoud M. Farag Roberto Matera Merton C. Flemings 《Metallurgical and Materials Transactions B》1979,10(3):381-388
γ/γ/’-δ eutectic alloy containing 21.5 wt pct Nb, 2.5 wt pct Al, balance Ni was directionally solidified under a thermal gradient
(G) of 500 K/cm and different schedules of growth rate (R). Under steady state growth rate conditions, the critical (G/R) for plane front solidification is 25 K h/cm2. At progressively lower (G/R) the structure becomes richer in γ-phase,i.e. hypoeutectic. Sudden increase in growth speed causes the structure to change from lamellar to cellular and gradual increase
in growth speed results in interlamellar spacings that are larger than the extremum values. Sudden decrease in growth speed
causes little disturbance in the structure but causes the eutectic grain size to increase. 相似文献
18.
S. N. Tewari 《Metallurgical and Materials Transactions A》1986,17(12):2279-2290
Pb-8 pct Au and Pb-3 pct Pd alloy specimens partially directionally solidified and then quenched have been examined in order
to characterize their dendritic microstructural details and solute composition profiles. Dendrite tip radii have been measured
by a controlled sectioning technique. Dendrite tip radius, solute content of quenched liquid at the dendrite tip, solute profile
within the interdendritic region and ahead of the dendrite tip, cell length, and the primary arm spacing values obtained experimentally
have been compared with the theoretical predictions. Two groups of models, one based on the minimum undercooled dendrite tip
criterion and the other based on the marginal stability at the dendrite tip, have been examined. The Burden and Hunt model,
based on the “minimum undercooling” approach, does not predict the observed behavior. However, a modification of the Burden
and Hunt's model recently proposed by Laxmanan shows a good fit to the experimentally observed parameters. The models based
on the marginal stability approach also predict most of the observed behavior well. It is concluded that quantitative comparison
of the primary arm spacing measurements can not form the basis of distinguishing among the various dendrite growth models
in a positive temperature gradient. There is a critical need to carry out carefully controlled directional solidification
experiments in a well characterized metallic alloy system to help distinguish between the minium dendrite tip undercooling
and the marginal stability approaches. New experiments based on simultaneous measurements of (a) dendrite tip radius, (b)
dendrite tip temperature, and (c) the solute profile ahead of the dendrite tip—all in a convection free atmosphere—are required. 相似文献
19.
R. P. Liu D. M. Herlach M. Vandyoussefi A. L. Greer 《Metallurgical and Materials Transactions A》2004,35(2):607-612
Electromagnetic levitation is applied to achieve containerless solidification of 10-mm-diameter droplets of Al-50 at. pct
Si. A maximum undercooling of 320 K is obtained. Phase morphologies on the droplet surfaces and on the deeply etched sections
of the samples solidified at different undercoolings are examined by scanning electron microscopy. The primary silicon shows
well-developed faceted dendrites at a small undercooling, but a fine granular form at a large undercooling. Stratified deposits
of aluminum are found within the primary silicon plates, arising from solute pileup during growth. The microstructural refinement
at a large undercooling has its origins in solute restriction of crystal growth and in fragmentation of the primary silicon
dendrites. The form of the Al-Si eutectic is also found to be changed into an anomalous form at a large undercooling. 相似文献
20.
Effects of Undercooling and Cooling Rate on Peritectic Phase Crystallization Within Ni-Zr Alloy Melt
The liquid Ni-16.75 at. pct Zr peritectic alloy was substantially undercooled and containerlessly solidified by an electromagnetic levitator and a drop tube. The dependence of the peritectic solidification mode on undercooling was established based on the results of the solidified microstructures, crystal growth velocity, as well as X-ray diffraction patterns. Below a critical undercooling of 124 K, the primary Ni7Zr2 phase preferentially nucleates and grows from the undercooled liquid, which is followed by a peritectic reaction of Ni7Zr2+L → Ni5Zr. The corresponding microstructure is composed of the Ni7Zr2 dendrites, peritectic Ni5Zr phase, and inter-dendritic eutectic. Nevertheless, once the liquid undercooling exceeds the critical undercooling, the peritectic Ni5Zr phase directly precipitates from this undercooled liquid. However, a negligible amount of residual Ni7Zr2 phase still appears in the microstructure, indicating that nucleation and growth of the Ni7Zr2 phase are not completely suppressed. The micromechanical property of the peritectic Ni5Zr phase in terms of the Vickers microhardness is enhanced, which is ascribed to the transition of the peritectic solidification mode. To suppress the formation of the primary phase completely, this alloy was also containerlessly solidified in free fall experiments. Typical peritectic solidified microstructure forms in large droplets, while only the peritectic Ni5Zr phase appears in smaller droplets, which gives an indication that the peritectic Ni5Zr phase directly precipitates from the undercooled liquid by completely suppressing the growth of the primary Ni7Zr2 phase and the peritectic reaction due to the combined effects of the large undercooling and high cooling rate. 相似文献