共查询到20条相似文献,搜索用时 31 毫秒
1.
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
2.
Toshihiko Koseki Merton C. Flemings 《Metallurgical and Materials Transactions A》1997,28(11):2385-2395
In Parts I and II of this series of articles, it was shown that a range of levitation-melted Fe-Cr-Ni alloys, both hypoeutectic
and hypereutectic, all solidified with the hypereutectic phase (bcc) as their primary phase, except for the hypoeutectic alloys
at low undercoolings. In this article, the effect of heat extraction on phase formation is studied by chill casting the undercooled
alloys before nucleation. Two of the previously studied alloys are examined; one hypoeutectic and the other hypereutectic.
Chill substrates employed were copper, stainless steel, alumina, zirconia, and a liquid gallium-indium bath. Contrary to the
case of levitation melting and solidification, it is found that the dominant primary phase to solidify in both alloys, independent
of chill substrate, is the hypoeutectic phase (fcc). It is concluded that chilling the undercooled melt results in nearly
concurrent nucleation of bcc and fcc. Two different mechanisms are considered as possible explanations of the subsequent fcc
phase selection during growth. These are termed “growth velocity” and “phase stability” mechanisms. Evidence favors a phase
stability mechanism, in which the bcc phase massively transforms to fcc early in solidification so that fcc then grows without
competition. It is suggested that this mechanism may also explain structures observed in welds and other rapid solidification
processes. 相似文献
3.
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
4.
Determination of the melting and solidification characteristics of solders using differential scanning calorimetry 总被引:1,自引:0,他引:1
Sinn-Wen Chen Chao-Ching Lin Chihming Chen 《Metallurgical and Materials Transactions A》1998,29(7):1965-1972
Differential scanning calorimetry (DSC) is used in the present study to determine the onset temperature of phase transformation
and the enthalpy of fusion of various solder alloys. The solders studied are Sn-Pb, Sn-Bi, Ag-Sn, In-Ag, and Sn-Pb-Bi alloys.
Very notable undercooling, such as 35 °C, is observed in the solidification process; however, a superheating effect is not
as significant in the heating process. Besides the direct measurements of reaction temperature and heat of fusion, the fraction
solid vs temperature has also been determined using a DSC coupled with a mathematical-model method. The heating and cooling curves
of the solders are first determined using DSC. By mathematically modeling the heat transfer of the DSC cells, the heat evolution
and absorption can be calculated, and then the melting and solidification curves of the solder alloys are determined. The
three ternary alloys, Sn-35 wt pct Pb-10 wt pct Bi, Sn-45 wt pct Pb-10 wt pct Bi, and Sn-55 wt pct Pb-10 wt pct Bi, displayed
similar DSC cooling curves, which had three reaction peaks. However, the solid fractions of the three alloys at the same temperature
in the semisolid state, which had been determined quantitatively using the DSC coupled with a mathematical method, were different,
and their primary solidification phases were also different. 相似文献
5.
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 Fe69Cr31-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
6.
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
7.
The effects of laser-processing parameters on the microstructure and microhardness of Fe-Cr-W-C quaternary alloy coatings were investigated experimentally. The coatings were developed by laser processing a powder mixture of Fe, Cr, W, and C at a weight ratio of 10:5:1:1 on a low-carbon steel substrate using a 10 kW continuous wave CO2 laser. Depending on the processing parameters, either hypoeutectic or hypereutectic microstructures were produced. The hypoeutectic microstructures comprised primary dendrites of nonequilibrium face-centered cubic (fcc) austenite γ phase and eutectic consisting of pseudohexagonal close-packed (hcp) M7C3 (M = Cr, Fe, W) carbides and fcc γ phase. The hypereutectic microstructures consisted of hcp M7C3 primary carbides and eutectic similar to that in the hypoeutectic microstructures. The formation of hypoeutectic or hypereutectic microstructures was influenced by the alloy composition, particularly the C concentration, which depends on the amount of powder delivered into the melt pool and the extent of substrate melting. The enhancement of the lattice parameter of the γ phase is associated with the significant dissolution of alloying elements and lattice strains resulting from rapid quenching. The higher hardness of the hypereutectic microstructures is principally attributed to the formation of hcp M7C3 primary carbides. The relatively lower hardness of the hypoeutectic microstructures is related to the presence of y phase in the primary dendrites, excessive dilution from the base material, and relatively low concentrations of Cr and C. The results provide insight into the significance of laser-processing conditions on the composition and hardness of Fe-Cr-W-C alloy coatings and associated solidification characteristics. 相似文献
8.
9.
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
10.
The purpose of the paper is to present a method of calculating the enthalpy of a dendritic alloy during solidification. The
enthalpies of the dendritic solid and interdendritic liquid of alloys of the Pb-Sn system are evaluated, but the method could
be applied to other binaries, as well. The enthalpies are consistent with a recent evaluation of the thermodynamics of Pb-Sn
alloys and with the redistribution of solute in the same during dendritic solidification. Because of the heat of mixing in
Pb-Sn alloys, the interdendritic liquid of hypoeutectic alloys (Pb-rich) of less than 50 wt pct Sn has enthalpies that increase
as temperature decreases during solidification. For some concentrations of Sn, the enthalpy of the dendritic solid at the
solid-liquid interface also increases with decreasing temperature during solidification. Of particular concern, in formulating
the energy equation, is the fact that the heat of fusion during solidification increases as much as 80 pct for hypoeutectic
alloys and decreases as much as 25 pct for hypereutectic alloys. Thus the often applied assumptions of a constant specific
heat and/or a constant heat of solidification could lead to errors in numerical modeling of temperature fields for dendritic
solidification processes. 相似文献
11.
Microstructural transitions during containerless processing of undercooled Fe-Ni alloys 总被引:4,自引:0,他引:4
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
12.
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
13.
The effect of fluid flow on eutectic microstructure is systematically examined in Al-Cu alloys of compositions varying from
19.5 to 45.0 wt pct Cu. It is shown that significantly different fluid-flow effects are present in hypo- and hypereutectic
alloys, since the modes of convection are different in these two cases. In hypoeutectic alloys, the rejected solute is copper,
which is heavier than aluminum, and fluid flow gives rise to radial solute segregation in cylindrical samples. In hypereutectic
alloys, a lighter aluminum is rejected that causes a double diffusive convection and gives rise to macrosegregation. These
composition variations are shown to produce nonuniform microstructures that vary either radially (in hypoeutectic alloys)
or axially (in hypereutectic alloys) and can give rise to a single phase-to-eutectic, lamellar-to-rod eutectic, or rod-to-lamellar
eutectic transition in a given sample. Composition measurements are carried out to characterize solute segregation due to
fluid flow. The fluid-flow effect on eutectic spacing in eutectic or near-eutectic alloys is found to be very small, whereas
it increases the eutectic spacing in hypoeutectic alloys for a given local composition and it can increase or decrease the
spacing in hypereutectic alloys, depending on the microstructure and solidification fraction. Theoretical models, based on
diffusive grwoth, are modified to predict the spatio-temporal variation in eutectic microstructure caused by fluid flow. 相似文献
14.
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
15.
The phase relationships between the liquid phase and the primary solid phases were investigated in the iron-rich corner of
the Fe-Cr-Ni system as part of a larger study of the Fe-Cr-Ni-C system. The investigation consisted of measurements and modeling
of tie-lines and the liquidus surfaces of the liquid-delta (bcc) and liquid-gamma (fcc) equilibria and the peritectic surface
involving all three phases in the iron-rich corner of the Gibbs triangle bounded by 0 to 25 wt pct Cr and 0 to 25 wt pct Ni
(bal Fe). The temperature ranged from the melting point of iron (1811 K) to about 1750 K. Compositions for tie-lines were
obtained from liquid-solid equilibrium couples and temperatures for the surfaces were obtained by differential thermal analysis.
Parameters for modeling the system were then selected in the subregular solution model to minimize the square of the difference
between experimental and calculated tie-lines. With one ternary parameter employed for each phase, calculations by the model
are in excellent agreement with the tie-line and liquidus measurements and in fair agreement with the temperatures for the
peritectic surfaceL + δ/L + δ + γ. The usefulness of the model is demonstrated by calculation of the solidification paths of selected alloys in the
composition field investigated for the limiting cases of (a) complete equilibrium followed by the alloy system, and (b) no
solid diffusion (i.e., segregation) with equilibrium maintained at the solidifying front and complete mixing in the liquid
phase. 相似文献
16.
A modified cellular automaton (MCA) model was developed and applied to simulate the evolution of solidification microstructures
of both eutectic and hypoeutectic Al-Si alloys. The present MCA model considers the equilibrium and metastable equilibrium
solidification processes in a multiphase system. It accounts for the aspects including the nucleation of a new phase, the
growth of primary α dendrites and two eutectic solid phases from a single liquid phase, as well as the coupling between the phase transformation
and solute redistribution in liquid. The effects of alloy composition and eutectic undercooling on eutectic morphology and
eutectic nucleation mode were investigated. The simulated results were compared with those obtained experimentally. 相似文献
17.
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
18.
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
19.
The role of solute in grain refinement of magnesium 总被引:53,自引:0,他引:53
Y. C. Lee A. K. Dahle D. H. StJohn 《Metallurgical and Materials Transactions A》2000,31(11):2895-2906
The effect of separate solute additions of Al, Zr, Sr, Si, and Ca on grain size of Mg has been investigated. Increasing the
Al content in hypoeutectic Mg-Al alloys resulted in a continuous reduction in grain size up to 5 wt pct Al, reaching a relatively
constant grain size for higher Al contents (above 5 wt pct). The effect of Sr additions was investigated in both low- and
high-Al content magnesium alloys, and it was found that Sr had a significant grain refining effect in low-Al containing alloys
but a negligible effect on grain size in Mg-9Al. Additions of Zr, Si, and Ca to pure magnesium resulted in efficient grain
refinement. The grain refinement is mainly caused by their growth restriction effects, i.e., constitutional undercooling, during solidification, but the effect of nucleant particles, either introduced with the alloying
additions or as secondary phases formed as a result of these additions, may enhance the grain refinement. A brief review of
grain refinement of magnesium alloys is included in this article to provide an update on research in this field. 相似文献
20.
Nucleation on ceramic particles in cast metal-matrix composites 总被引:6,自引:0,他引:6
In order to understand the nucleation on ceramic particles in the melts of metal-matrix composites (MMCs), the effect of segregation
of solute on the surface of reinforcement particles in the melt has been analyzed as a function of particle temperature and
the surface energy of the particle/liquid melt. The temperature of the particle in the melt, calculated analytically, was
found to become close to the melt temperature within a very short time of contact between the particle and the melt. The solute
concentration near the particle surface will, therefore, primarily be influenced by the surface energy of the particle and
the melt. Based on this, the undercooling due to solute segregation around the particle and the chemical free-energy change
due to the formation of the new solid phase on the particle were calculated in selected hypo- and hypereutectic Al-Si alloy
melts containing (1) SiC particles or (2) graphite particles. The chemical free-energy change (driving force for nucleation)
due to the formation of the new phase on the particle is lower for hypoeutectic compositions than for hypereutectic compositions
in the aluminum-silicon alloy systems; this is due to the higher undercooling in the hypereutectic alloys due to solute segregation
on the surface of the particle. This suggests that the formation of the primary phase on the surfaces of particles in the
melt should be more favorable in the hypereutectic compositions than for hypoeutectic compositions. This also indicates that
even when the particle temperature is not significantly lower than the liquidus temperature, nucleation on the particles can
take place due to the segregation of the solute on the particles. Experimental observations of the microstructure of several
cast metal-matrix composites, including Al-Si-SiC and Al-Si-graphite, show (1) the presence of silicon in contact with the
reinforcement particles in hypereutectic alloys, suggesting that nucleation and growth of primary silicon under certain conditions
occurs on silicon carbide and graphite particles, possibly due to solute segregation on the surface of the particles, and
(2) the presence of reinforcement particles in the last-freezing interdendritic regions of the primary phases in hypoeutectic
alloys, suggesting the absence of nucleation of primary phases on the reinforcement surface, as predicted by the analysis. 相似文献