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 TE) and 218 K (0.16 TE), respectively, with the glass-fluxing method. A transition from lamellar eutectic to anomalous eutectic was revealed beyond
a critical undercooling ΔT1*, which was complete at an undercooling of ΔT2*. For Ni-36 pct Sb, ΔT1*≈60 K and ΔT2*≈218 K; for Ni-52.8 pct Sb, ΔT1*≈40 K and ΔT2*≈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
Phase selection and microstructure evolution in nonequilibrium solidification of ternary eutectic Fe40Ni40B20 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
Studies were made of structure and solute distribution in undercooled droplets of nickel-25 wt pct tin alloy and the eutectic
nickel-32.5 wt pct tin alloy. Structures of levitation melted droplets of the Ni-25 wt pct Sn alloy showed a gradual and continuous
transition from dendritic to fine-grained spherical with increasing initial undercooling up to about 180 K. Results suggest
that all samples solidified dendritically and that the final structures obtained were largely the result of ripening. Experimental
data on minimum solute composition in the samples produced are bounded by two calculated curves, both of which assume equilibrium
at all liquid-solid interfaces during recalescence and subsequent cooling. One assumes complete diffusion in the solid during
recalescence; the other assumes limited diffusion, but partial remelting to avoid superheating of the solid. Several observations
support the view that the eutectic alloy solidifies dendritically, much as the hypoeutectic alloy does. Surface dendrites
were seen in regions of surface shrinkage cavities and a coarse “dendritic” structure can be discerned on polished sections,
which seems to correspond to the large surface “dendrites” seen by high-speed photographs of the hypoeutectic alloy. The structure
of highly undercooled eutectic samples is composed fully of an anomalous eutectic. Samples solidified with intermediate amounts
of undercooling possess some lamellar eutectic which, it is believed, solidified after recalescence was complete. 相似文献
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
A new method to determine directly the solid fraction using the cooling curve was proposed for solidification of undercooled
melts. Then, to construct three different baselines, a sudden function ξα(x) is introduced. In terms of the ξα(x) function, accordingly, the solid fractions during solidification of Ni-3.3 wt pct B, Al-7 wt pct Si, Al-14 wt pct Cu, and
Fe-4.56 wt pct Ni alloys were predicted. The predictions of the primary, the regular lamellar eutectic, the anomalous eutectic,
and the peritectic phases from cooling curves of the solidified samples coincide with the results of measurement or the available
methods. 相似文献
The solidification of undercooled Ti3Sn melts was investigated using electromagnetic levitation and electrohydrodynamic atomization experiments followed by extensive
microstructural char- acterization. The study was motivated by several reports on the kinetic preference for the body- centered
cubic (bcc) phase over more closely packed disordered and ordered structures during competitive crystallization from undercooled
melts. At low undercoolings, Ti3Sn melts yield the equilibrium ordered hexagonal DO19 structure, which is retained without change upon cool- ing. Undercoolings between ~100 and ~300 K yield primary dendrites
with hexagonal sym- metry but a final microstructure which is clearly martensitic in origin. Two previously unknown metastable
forms of Ti3Sn were identified: an ordered base-centered orthorhombic derived from the α martensite and an ordered monoclinic phase related
to the face-centered orthorhombic martensite observed in the Ti-V system. Both phases are believed to evolve from the solid
state transformation of a high temperature β phase, but the dendritic structure clearly indicates the formation of a hexagonal
phase different from DO19,i.e., α. The latter forms in preference to β, which has a larger driving force in at least part of the undercooling regime studied.
It is proposed that the primary α transforms to β as a consequence of recalescence, which subse- quently transforms martensitically
and orders to yield the observed metastable forms of Ti3Sn. 相似文献
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
The change of eutectic solidification mode in undercooled Ni-3.3 wt pct B melt was studied by fluxing and cyclic superheating. The eutectic structure is mainly controlled by the undercooling for eutectic solidification, ΔT2, instead of ΔT1, the undercooling for primary solidification. At a small ?T2 [e.g., 56 K (56 °C)], the stable eutectic reaction (L → Ni3B + Ni) occurs and the eutectic morphology consists of lamellar and anomalous eutectic; whereas at a larger ?T2 [≥140 K (140 °C)], the metastable eutectic reaction (L → Ni23B6 + Ni) occurs and the eutectic morphology consists of matrix, network boundary, and two kinds of dot phases. Further analysis declares that the regularly distributed dot phases with larger size come from the metastable eutectic transformation and are identified as α-Ni structure, whereas the irregularly distributed ones with smaller size are a product of the metastable decomposition and tend to have a similar structure to α-Ni as it grows. Calculation of the classical nucleation theory shows that the competitive nucleation between Ni23B6 and Ni3B leads to a critical undercooling, ΔT2* [125 K < ΔT2* < 157 K (125 °C < ?T2* < 157 °C)], for the metastable/stable eutectic formation. 相似文献
Ni75B25 alloy was solidified at various undercooling. The formation and subsequent transformation of metastable Ni23B6 phase were clearly identified. If undercooling prior to nucleation is less than a critical value of 240 K (240 °C), the alloy solidifies completely into Ni3B phase. At larger undercooling, metastable Ni23B6 phase primarily forms in the melt but then is decomposed into α-Ni and Ni3B through a eutectoid reaction. The decomposition simultaneously triggers the rapid solidification of residual liquid, due to which a second temperature recalescence occurs. The α-Ni/Ni3B eutectoid is partially remelted if temperature exceeds the eutectic temperature during the second recalescence. Then, residual Ni3B grows into coarse round grains while the remaining liquid re-solidifies into α-Ni/Ni3B eutectic structure in the remelted region. In the case that the eutectic temperature is not reached, the eutectoid product with dot α-Ni distributing in Ni3B matrix is retained in the solidification structure. A longer delay time between the two temperature recalescence events means less residual liquid, lower recalescence temperature and thus depressed remelting. The formation competition between Ni3B and Ni23B6 phases in the alloy melt is nucleation controlled. The heterogeneous site in Ni75B25 alloy melt is a better nucleation substrate for Ni23B6 phase than for Ni3B phase.
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
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
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.
Using an electromagnetic levitation facility with a laser heating unit, silicon droplets were highly undercooled in the containerless
state. The crystal morphologies on the surface of the undercooled droplets during the solidification process and after solidification
were recorded live by using a high-speed camera and were observed by scanning electron microscopy. The growth behavior of
silicon was found to vary not only with the nucleation undercooling, but also with the time after nucleation. In the earlier
stage of solidification, the silicon grew in lateral, intermediary, and continuous modes at low, medium, and high undercoolings,
respectively. In the later stage of solidification, the growth of highly undercooled silicon can transform to the lateral
mode from the nonlateral one. The transition time of the sample with 320 K of undercooling was about 535 ms after recalescence,
which was much later than the time where recalescence was completed. 相似文献
The crystal growth behavior of a semiconductor from a very highly undercooled melt is expected to be different from that of
a metal. In the present experiment, highly pure undoped Si and Ge were undercooled by an electromagnetic levitation method,
and their crystal growth velocities (V) were measured as a function of undercooling (ΔT). The value of V increased with ΔT, and V=26 m/s was observed at ΔT=260 K for Si. This result corresponds well with the predicted value based on the dendrite growth theory. The growth behaviors
of Si and Ge were found to be thermally controlled in the measured range of undercooling. The microstructures of samples solidified
from undercooled liquid were investigated, and the amount of dendrites immediately after recalescence increased with undercooling.
The dendrite growth was also observed by a high-speed camera. 相似文献
Directional solidification of two eutectic alloys in the Ni-In system results in aligned microstructures. The eutectic at
26.5 at. pct In solidifies as ribbons and lamellae of nickel-base solid solution in a matrix of ε-Ni2In. Peritectoid formation of Ni3In on cooling drastically alters the morphology, but this reaction can be suppressed by quenching. The peritectoid reaction
can then be studied by isothermal heat treatments. The eutectic at 46.6 at. pct In solidifies partly as rods of ε-Ni2In in a matrix of β-NiIn, partly in a lamellar morphology. Both phases transform on cooling, but because little composition
change is involved, the effect on eutectic morphology is slight. 相似文献
The Jackson and Hunt (JH) theory has been modified to relax the assumption of isothermal solid/liquid interface used in their
treatment. Based on the predictions of this modified theory, the traditional definitions of regular and irregular eutectics
are revised. For regular eutectics, the new model identifies a range of spacing within the limits defined by the minimum undercooling
of the α and β phases. For the irregular Al-Si eutectic system, two different spacing selection mechanisms were identified: (1) for a particular
growth rate, a nearly isothermal interface can be achieved at a unique minimum spacing λI; (2) the average spacing (λav>λI) is essentially dictated by the undercooling of the faceted phase. Based on the modified theoretical model, a semiempirical
expression has been developed to account for the influence of the temperature gradient, which is dominant in the irregular
Al-Si system. The behavior of the Fe-Fe3C eutectic is also discussed. The theoretical calculations have been found to be in good agreement with the published experimental
measurements. 相似文献
Containerless processing and rapid solidification techniques were used to process Nb-Si alloys in the Nb-rich eutectic range.
Electromagnetic ally levitated drops were melted and subsequently splat quenched from different temperatures. A variety of
eutectic morphologies was obtained as a function of the degree of superheating or undercooling of the drops prior to splatting.
Metallic glass was observed only in drops quenched from above the melting temperature. Micro-structures of splats deeply undercooled
prior to quenching were very fine and uniform. These results are discussed in terms of classic nucleation theory concepts
and the expected heat evolution at different regions of the splat during the rapid quenching process. The locations of the
coupled-zone boundaries for the α-Nb + Nb3Si eutectic are also suggested.
Formerly Graduate Student, Vanderbilt University. 相似文献
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
At large undercoolings (τ;10 pctTM, present theories relating solidification velocity to degree of undercooling do not agree well with reported experimental
data for the solidification velocity of nickel as a function of undercooling. The present work shows that this discrepancy
is due to two factors. First, the majority of previously reported results overestimate the solidification velocity of nickel
at large undercoolings. Second, the scatter in experimental data is so large that a functional relationship between undercooling
and velocity is not evident. In this study, the solidification velocity of undercooled nickel was measured using a linear
array of 38 photodiodes. The results indicate that the velocity of the thermal field generated by the solid/liquid interface
approaches a maximum velocity of 20 m s−1 atΔT} ≈ 10 pctTM (173 K) and men remains constant with increasing undercooling. This suggests that the velocity of the solid/liquid interface,
at undercoolings greater than 10 pctTM, could be limited by attachment kinetics at the interface.
GABRIEL CARRO, formerly Research Associate, Department of Applied and Engineering Sciences, Vanderbilt University 相似文献