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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.
The solidification behavior of undercooled Fe-Cr-Ni melts is analyzed with respect to the competitive formation of body-centered
cubic (bcc) phase (ferrite) and face-centered cubic (fcc) phase (austenite). The activation energies of homogeneous nucleation
and growth velocities for both phases as functions of undercooling of the melt are calculated on the basis of current theories
of nucleation and dendrite growth using data of thermodynamic properties available in the literature. As model systems for
numerical calculations, the alloys Fe-18.5Cr-11Ni forming primary ferrite and Fe-18.5Cr-12.5Ni forming primary austenite under
near-equilibrium solid-ification conditions are considered. Nucleation of the bcc phase is always promoted in the under-cooled
primary ferrite alloy, whereas the barrier for bcc nucleation falls below that for fcc nucleation for large undercooling in
primary austenite alloys. With rising undercooling, tran-sitions of the fastest growth mode were found from bcc to fcc and
subsequently from fcc to bcc for the primary ferrite forming alloy and from fcc to bcc for the primary austenite forming alloy.
The results of the calculations provide a basis for understanding contradictory experi-mental findings reported in the literature
concerning phase selection in rapidly solidified stainless steel melts for different process conditions.
Formerly Visiting Scientist at the Institut fur Raumsimulation 相似文献
3.
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 Co 3Si phase was found in Co-10 wt pct Si alloy when a critical undercooling was achieved. 相似文献
4.
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.
Results are reported on microstructures of Fe-Cr-Ni alloys, solidified over a range of undercoolings and quenched during or
after recalescence. Alloys studied contained 70 wt pct Fe and with Cr varying from approximately 15 to 20 wt pct. The three
lower Cr alloys were hypoeutectic (with fee as primary phase in equilibrium solidification); the two higher Cr alloys were
hypereutectic (with bcc as primary phase in equilibrium solidification). Results obtained are in agreement with predictions
based on thermal analyses previously presented; they confirm and extend the understanding gained in that work. The primary
phase to solidify in the hypoeutectic alloys is bec when undercooling is greater than an amount which decreases with increasing
Cr content. At the lower Cr contents, the stable fcc phase then forms by solid-state transformation of the metastable phase
and its subsequent engulfment by additional fcc. At the higher Cr content, transformation is by a peritectic-like reaction
in the semisolid state, except near the surface at higher undercoolings where the transformation is massive. In the hypereutectic
alloys, primary solidification at all undercoolings is the stable bcc phase. Partial transformation to fcc occurs in the semisolid
or solid state, depending on composition and undercooling.
Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology 相似文献
6.
The selection of the primary solidifying phase in undercooled stainless steel melts is theoretically analyzed in terms of
nucleation theory. Nucleation phenomena are considered using different models for the solid-liquid interface energy. The classical
nucleation theory for sharp interfaces and an improved modification, the diffuse interface theory, are applied. The influence
of deviations of the nucleus composition from the overall alloy composition is also revealed. A preferred nucleation of the
metastable bcc phase in fcc equilibrium solidification-type alloys is predicted. The critical undercooling of metastable crystallization
as a function of alloy composition is calculated for an isoplethal section at 69 at. pct Fe of Fe 69Cr 31-x
Ni
x
alloys. The results are summarized in a phase selection diagram predicting the primary solidification mode as a function
of undercooling and melt composition. 相似文献
7.
The solidification behavior of undercooled Fe-Cr-Ni melts of different compositions is investigated with respect to the competitive
formation of δ-bcc (ferrite) and γ-fcc phase (austenite). Containerless solidification experiments, electromagnetic levitation melting and drop tube experiments
of atomized particles, show that δ (bcc) solidification is preferred in the highly undercooled melt even at compositions where δ is metastable. Time-resolved detection of the recalescence events during crystallization at different undercooling levels
enable the determination of a critical undercooling for the transition to metastable bcc phase solidifcation in equilibrium
fcc-type alloys. Measurements of the growth velocities of stable and metastable phases, as functions of melt undercooling
prior to solidification, reveal that phase selection is controlled by nucleation. Phase selection diagrams for solidification
processes as functions of alloy composition and melt undercooling are derived from two types of experiments: X-ray phase analysis
of quenched samples and in situ observations of the recalescence events of undercooled melts. The experimental results fit well with the theoretical predictions
of the metastable phase diagram and the improved nucleation theory presented in an earlier article. In particular, the tendency
of metastable δ phase formation in a wide composition range is confirmed. 相似文献
8.
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 10 6 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
9.
Grain refinement phenomena during the microstructural evolution upon nonequilibrium solidification of deeply undercooled Ni-20 at. pct Cu melts were systematically investigated. The dendrite growth in the bulk undercooled melts was captured by a high-speed camera. The first kind of grain refinement occurring in the low undercooling regimes was explained by a current grain refinement model. Besides, for the dendrite melting mechanism, the stress originating from the solidification contraction and thermal strain in the FMZ during rapid solidification could be a main mechanism causing the second kind of grain refinement above the critical undercooling. This internal stress led to the distortion and breakup of the primary dendrites and was semiquantitatively described by a corrected stress accumulation model. It was found that the stress-induced recrystallization could make the primary microstructures refine substantially after recalescence. A new method, i.e., rapidly quenching the deeply undercooled alloy melts before recalescence, was developed in the present work to produce crystalline alloys, which were still in the cold-worked state and, thus, had the driven force for recrystallization. 相似文献
10.
The microstructural development associated with solidification in undercooled Fe-Ni alloys has been reported in different
studies to follow various pathways, with apparent dissimilarities existing as a function of sample size and processing conditions.
In order to identify the possible hierarchy of microstructural pathways and transitions, a systematic evaluation of the microstructural
evolution in undercooled Fe-Ni alloys was performed on uniformly processed samples covering seven orders of magnitude in volume.
At appropriate undercooling levels, alternate solidification pathways become thermodynamically possible and metastable product
structures can result from the operation of competitive solidification kinetics. For thermal history evaluation, a heat flow
analysis was applied and tested with large Fe-Ni alloy particles (1 to 3 mm) to assess undercooling potential. Alloy powders
(10 to 150 μm), with large liquid undercoolings, were studied under the same composition and processing conditions to evaluate
the solidification kinetics and microstructural evolution, including face-centered cubic (fcc)/body centered cubic (bcc) phase
selection and the thermal stability of a retained metastable bcc phase. The identification of microstructural transitions
with controlled variations in sample size and composition during containerless solidification processing was used to develop
a microstructure map which delineates regimes of structural evolutions and provides a unified analysis of experimental observations
in the Fe-Ni system. 相似文献
11.
Co-20.5 at. pct Sn and Ni-21.4 at. pct Si eutectic alloys have been levitated and undercooled in an electromagnetic levitator
(EML) and then solidified spontaneously at different undercoolings. The original surface and cross-sectional morphologies
of these solidified samples consist of separate eutectic colonies regardless of melt undercooling, indicating that microstructures
in the free solidification of the eutectic systems are nucleation controlled. Regular lamellae always grow from the periphery
of an independent anomalous eutectic grain in each eutectic colony. This typical morphology shows that the basic unit should
be a single eutectic colony, when discussing the solidification behavior. Special emphasis is focused on the anomalous eutectic
formation after a significant difference in linear kinetic coefficients is recognized for terminal eutectic phases, in particular
when a eutectic reaction contains a nonfaceted disordered solid solution and a faceted ordered intermetallic compound as the
terminal eutectic phases. It is this remarkable difference in the linear kinetic coefficients that leads to a pronounced difference
in kinetic undercoolings. The sluggish kinetics in the interface atomic attachment of the intermetallic compound originates
the occurrence of the decoupled growth of two eutectic phases. Hence, the current eutectic models are modified to incorporate
kinetic undercooling, in order to account for the competitive growth behavior of eutectic phases in a single eutectic colony.
The critical condition for generating the decoupled growth of eutectic phases is proposed. Further analysis reveals that a
dimensionless critical undercooling may be appropriate to show the tendency for the anomalous eutectic-forming ability when
considering the difference in linear kinetic coefficients of terminal eutectic phases. This qualitative criterion, albeit
crude with several approximations and assumptions, can elucidate most of the published experimental results with the correct
order of magnitude. Solidification modes in some eutectic alloys are predicted on the basis of the present criterion. Future
work that may result in some probable errors is briefly directed to improve the model. 相似文献
12.
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
13.
The theory of dendritic growth into undercooled alloy melts is extended to the case of large undercoolings, i.e. to high growth rates. This is done by applying the results of the complete stability analysis of a plane interface to the tip of an Ivantsov dendrite. For small Péclet numbers this model corresponds to a model published previously. For large Péclet numbers i.e. large undercoolings, however, the stability parameters become functions of Péclet numbers and cause drastic changes in the growth behaviour of the dendrite. Furthermore the limit of absolute stability is predicted when the undercooling is equal to the sum of the thermal unit undercooling and the equilibrium freezing range of the alloy. 相似文献
14.
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 Δ T
1*, which was complete at an undercooling of Δ T
2*. For Ni-36 pct Sb, Δ T
1*≈60 K and Δ T
2*≈218 K; for Ni-52.8 pct Sb, Δ T
1*≈40 K and Δ T
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-Ni 5Sb 2 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
15.
Phase selection and microstructure evolution in nonequilibrium solidification of ternary eutectic Fe 40Ni 40B 20 alloy have been studied. It is shown that γ-(Fe, Ni) and (Fe, Ni) 3B prevail in all the as-solidified samples. No metastable phase has been found in the deeply undercooled samples. This is
explained as resulting from the size effect of undercooled solidification. At small and medium undercoolings, the dendrite
γ-(Fe, Ni) appears as the leading phase. This is ascribed to the existence of the skewed coupled growth zone in FeNiB alloy.
With increasing undercooling, the amount of dendrites first increases and then decreases, accompanied by a transition from
regular eutectic to anomalous eutectic. The formation mechanisms of the anomalous eutectics are discussed. Two kinds of microstructure
refinement are found with increasing undercooling in a natural or water cooling condition. However, for melts with the same
undercooling, the as-solidified microstructure refines first, and then coarsens with an increasing cooling rate. The experimental
results show that the nanostructure eutectic cell has been obtained in the case of Ga-In alloy bath cooling with an initial
melt undercooling of approximately 50 K (50 °C). 相似文献
16.
Small liquid Ge droplets (0.3–0.5 mm diameter) have been undercooled 150–415 ± 20° C below Tm in B 2O 3 flux before solidifying to the diamond cubic phase. A correlation was found between initial undercooling and final grain size. Droplets undercooled <300°C exhibited a coarse grain structure. At greater undercoolings, the grain size became progressively finer. This correlation may be subsidiary to the dependence of grain size on interfacial undercooling. Ge droplets lightly doped with Sn solidified dendritically for undercoolings greater than 250°C. Twinned dendrites have been observed at small undercoolings (~ 10°C) in other experiments. It appears that larger interfacial undercoolings are necessary to grow the twin-free dendrites which we have observed. The correlation between grain size and the presence of dendrites suggests that the grain refinement observed in Ge samples undercooled > 300°C stems from dendritic break-up during solidification. 相似文献
17.
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
18.
A systematic experimental investigation on microsegregation and second phase fraction of Mg-Al binary alloys (3, 6, and 9 wt pct Al) has been carried out over a wide range of cooling rates (0.05 to 700 K/s) by employing various casting techniques. In order to explain the experimental results, a solidification model that takes into account dendrite tip undercooling, eutectic undercooling, solute back diffusion, and secondary dendrite arm coarsening was also developed in dynamic linkage with an accurate thermodynamic database. From the experimental data and solidification model, it was found that the second phase fraction in the solidified microstructure is not determined only by cooling rate but varied independently with thermal gradient and solidification velocity. Lastly, the second phase fraction maps for Mg-Al alloys were calculated from the solidification model. 相似文献
19.
The rate of solidification of dilute tin-lead alloys has been measured as a function of the initial undercooling (up to 45°C)
and the solute content (up to 2 wt pct lead). Solidified specimens were examined by metallography and X-ray diffraction to
obtain information on the solidification process and the resulting grain structure. Over an intermediate range of undercoolings,
it was found that dendrites grow in the tin-lead alloys as much as four times faster than in pure tin at the same undercooling.
This result is inconsistent with any present theories for dendrite growth kinetics in binary alloys. At both lower and higher
undercoolings there is no evidence for growth by simple extension of dendrites along the specimen, and solidification rate
measurements made under these conditions are probably not indicative of normal dendrite growth kinetics.
A. W. Urquhart and G. L. F. Powell were formerly at the Thayer School of Engineering. 相似文献
20.
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
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