共查询到20条相似文献,搜索用时 875 毫秒
1.
Ke Zhang Feng Liu Jun-Feng Xu Gen-Cang Yang 《Metallurgical and Materials Transactions A》2012,43(5):1578-1587
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
2.
Martin E. Valdez Pello Uranga Katsuhiro Fuchigami H. Shibata Alan W. Cramb 《Metallurgical and Materials Transactions B》2006,37(5):811-821
The objective of this study was to determine the conditions under which alumina can act as a heterogeneous nucleant to initiate
the solidification of undercooled liquid iron. The undercooling of a pure iron sessile droplet in contact with Al2O3 substrates was measured under controlled oxygen partial pressures by observing droplet recalescence. The experimental results
indicated that the undercooling of liquid iron, in contact with an Al2O3 substrate, did not have a unique value, varied from 0 °C to 290 °C, and was significantly affected by the oxygen content
of the gas phase and the degree of interaction between the oxide and the metal. Deep undercoolings are possible at low oxygen
potentials, provided the oxygen potential is such that substantial substrate decomposition does not occur. The measured undercooling
was a strong function of gas phase oxygen content and a maximum in undercooling of 290 °C was measured at PO2=10−19 atm. The variation in undercooling was related to the wetting of the substrate by the liquid metal, where the deepest undercoolings
occurred when the highest contact angle between the substrate and the liquid droplet was achieved. 相似文献
3.
Michael P. De Cicco Lih-Sheng Turng Xiaochun Li John H. Perepezko 《Metallurgical and Materials Transactions A》2011,42(8):2323-2330
Different types of nanoparticles in aluminum (Al) alloy A356 nanocomposites were shown to catalyze nucleation of the primary
Al phase. Nanoparticles of SiC β, TiC, Al2O3 α, and Al2O3
γ were added to and dispersed in the A356 matrix as nucleation catalysts using an ultrasonic mixing technique. Using the droplet
emulsion technique (DET), undercoolings in the nanocomposites were shown to be significantly reduced compared to the reference
A356. None of the nanocomposites had a population of highly undercooled droplets that were observed in the reference samples.
Also, with the exception of the A356/Al2O3 α nanocomposite, all nanocomposites showed a reduction in undercooling necessary for the onset of primary Al nucleation.
The observed nanocomposite undercoolings generally agreed with the undercooling necessary for free growth. The atomic structure
of the particles showed an influence on nucleation potency as A356/Al2O3 γ nanocomposites had smaller undercoolings than A356/Al2O3 α nanocomposites. The nucleation catalysis illustrates the feasibility of, and basis for, grain refinement in metal matrix
nanocomposites (MMNCs). 相似文献
4.
The cross-sectional and surface morphologies of highly undercooled bismuth samples are investigated by optical microscopy
and scanning electron microscopy. It is found that the grain morphology can be classified into three types. When the undercooling
is less than 49 K (49 °C), flaky grains with pronounced edges and faces are arranged parallel to each other, showing the feature
of lateral growth. When the undercooling is over 95 K (95 °C), refined equiaxial grains with several smooth bulges on the
surface of each grain are randomly arranged, showing the feature of continuous growth. In the undercooling region from 49 K
to 95 K (49 °C to 95 °C), the features of both lateral and continuous growth are observed. The microstructures within the
sample grains obtained at different undercooling regions are dissimilar, but they all show features of anisotropic growth.
Based on the critical growth-transition undercoolings, direct expressions that express the relationship between the solid-liquid
interface energy and temperature are determined. Homogenous nucleation undercooling is also predicted according to the solid-liquid
interface energy obtained from the critical growth-transition undercooling. The predicted results of homogenous nucleation
undercooling for bismuth are in good agreement with the experimental results. 相似文献
5.
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
6.
Ultrarapid cooling may result in material with extremely fine grain size and interdendritic spacing, or containing supersaturated
solutions or nonequilibrium phases. Unfortunately, ultrarapid cooling cannot be obtained in large masses of metal; thus the
interesting properties cannot be exploited commercially. The same structures can be obtained by denucleation, that is removal
or neutralization of the nucleants that act at low undercooling. A denucleated melt, regardless of its size, can be undercooled
to a temperature well below its freezing point, where it can be made to freeze rapidly on a chosen nucleant. Electro-lytically
refined aluminum crystallizes normally with a fairly coarse grain at 1 to 2°C undercooling. By removing all nucleants with
less than 5°C undercooling a grain size of less than 1 mm diam was achieved in a furnace cooled sample. In aluminum-manganese
alloys, denucleation produces refinement of the grain structure in continuously cast billets and retention in solution of
a higher percentage of manganese, in pure aluminum columnar grain and crystallization twins are found in plaster molds. By
denucleation of aluminum-iron alloys, the FeAl6 phase can be made to appear in samples cooled at commercial rates of a few °C/sec and eutectic structures can be obtained
in alloys with 4 pct Fe. In cast iron removal of nucleants with low undercooling shifts the crystallization from primary graphite
to primary austenite.
Formerly Metallurgist, Revere Copper and Brass Incorporated 相似文献
7.
Tomotsugu Aoyama Yuzuru Takamura Kazuhiko Kuribayashi 《Metallurgical and Materials Transactions A》1999,30(5):1333-1339
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
8.
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
9.
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
10.
Mingjun Li Kosuke Nagashio Ph.D. Kazuhiko Kuribayashi 《Metallurgical and Materials Transactions A》2002,33(8):2677-2683
A mullite (3Al2O3·2SiO2) sample has been levitated and undercooled in an aero-acoustic levitator, so as to investigate the solidification behavior
in a containerless condition. Crystal-growth velocities are measured as a function of melt undercoolings, which increase slowly
with melt undercoolings up to 380 K and then increase quickly when undercoolings exceed 400 K. In order to elucidate the crystal
growth and solidification behavior, the relationship of melt viscosities as a function of melt undercoolings is established
on the basis of the fact that molten mullite melts are fragile, from which the atomic diffusivity is calculated via the Einstein-Stokes equation. The interface kinetics is analyzed when considering atomic diffusivities. The crystal-growth
velocity vs melt undercooling is calculated based on the classical rate theory. Interestingly, two different microstructures are observed;
one exhibits a straight, faceted rod without any branching with melt undercoolings up to 400 K, and the other is a feathery
faceted dendrite when undercoolings exceed 400 K. The formation of these morphologies is discussed, taking into account the
contributions of constitutional and kinetic undercoolings at different bulk undercoolings. 相似文献
11.
K. L. Zeik D. A. Koss I. E. Anderson Howell P. R. 《Metallurgical and Materials Transactions A》1992,23(8):2159-2167
High-pressure inert gas atomization (HPGA) has been used to produce rapidly solidified Cu-21Nb-2Mo (weight percent) powders
with a range of particle sizes and microstructures. The associated microstructures depend on particle size. Specifically,
fine-scale particles (\s15 μm@#@) are characterized by a predominance of multiphase spheroids and a small population of Nb-based dendrites in an almost pure
matrix of Cu. In contrast, large particles (45 to 75 μm@#@) contain only Nb-based dendrites in a Cu matrix. The volume fraction of the dispersed constituent is much higher in the former
instance than in the latter. The change in microstructure with particle size is analyzed in terms of both the amount of undercooling
and cooling rate of the liquid droplets prior to and during solidification. In particular, the large undercoolings in the
fine particles are believed to induce a nonequilibrium liquid phase separation which results in a high volume fraction of
spheroidal, multiphase Nb-Cu particles within a Cu-rich matrix containing Nb-rich dendrites. High-temperature (900 °C) isothermal
annealing treatments have also been performed on consolidated material to determine the inherent thermal stability of these
microstructures. 相似文献
12.
A. W. Urquhart G. A. Colligan G. L. F. Powell 《Metallurgical and Materials Transactions A》1977,8(8):1291-1299
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
13.
B. Timothy Bassler William H. Hofmeister Gabriel Carro Robert J. Bayuzick 《Metallurgical and Materials Transactions A》1994,25(6):1301-1308
At large undercoolings (τ;10 pctT
M, 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 pctT
M (173 K) and men remains constant with increasing undercooling. This suggests that the velocity of the solid/liquid interface,
at undercoolings greater than 10 pctT
M, 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
14.
15.
Zengyun Jian Kosuke Nagashio Kazuhiko Kuribayashi 《Metallurgical and Materials Transactions A》2002,33(9):2947-2953
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
16.
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
17.
Microstructure of Y3Al5O12 garnet solidified from the melt undercooled beyond the hypercooling limit
The microstructural evolution of Y3Al5O12 garnet (YAG) in a wide undercooling range beyond the hypercooling limit (ΔT
hyp) was investigated by containerless solidification processing. The dendrite to cellular-dendrite transition at high-growth
velocity was observed at the undercooling beyond ΔT
hyp. This transition may be explained by the hypothesis that it is difficult to form the well-developed secondary-dendritic arms
from the hypercooled melt because of no remaining melt in the interdendritic regions. With a further increase in undercooling
beyond ΔT
hyp, a cellular microstructure disappeared, and copious amounts of small particles appeared at an undercooling of approximately
1000 K, which is near the glass-transition temperature where the viscosity is approximately 1012 Pas. It is suggested that multiple nucleation occurred in the highly viscous undercooled melt because of the high nucleation
rate. The grain size of YAG, which was analyzed as a function of undercooling, gradually decreased with increasing undercooling
even beyond ΔT
hyp, and no fragmentation of dendrites was observed. 相似文献
18.
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, ΔT 2, instead of ΔT 1, the undercooling for primary solidification. At a small ?T 2 [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 ?T 2 [≥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, ΔT 2 * [125 K < ΔT 2 * < 157 K (125 °C < ?T 2 * < 157 °C)], for the metastable/stable eutectic formation. 相似文献
19.
Colin McCullough Denise S. Miller Carlos G. Levi 《Metallurgical and Materials Transactions A》1993,24(7):1481-1496
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