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1.
The microstructure and relative amounts of fcc and bcc phases have been studied for rapidly solidified Type 304 stainless
steel powders produced by vacuum gas atomization (VGA) and centrifugal atomization (CA). The VGA powder solidifies with a
cellular microstructure while the CA powder has a dendritic microstructure. The volume fraction of fcc phase in the CA powder
is found to increase from 40 Pct to 97 Pct with increasing particle size from 30 to 125 μm. In the VGA powder, the volume
fraction of fcc phase is found to decrease from about 90 Pct to 77 Pct over the same range of particle sizes. The origins
of the fcc and bcc phases in each powder are considered. It is concluded that bcc is present as both a primary crystallization
phase in the smaller CA particles (<75 μm) and as compositionally stabilized eutectic ferrite at the cell walls of particles
of both CA and VGA powders in which fcc was the primary crystallization phase. 相似文献
2.
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
3.
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
4.
The microstructure of martensitic stainless steel powders produced by inert gas atomization was investigated. Depending upon
the powder particle size, the microstructure was found to exhibit a cellular, dendritic, or martensitic morphology. Relationships
between the microstructure scale and the particle diameter were identified. It was found that at a critical particle diameter
of 25 to 30 μm, the structure changed from cellular/dendritic (96.5 vol pct bcc and 3.5 vol pct fcc) to martensite. The solidification
path of the powder particles below and above 25 to 30 μm in size was considered. High-temperature X-ray diffraction (HTXRD) measurements revealed that there is a delay in the appearance
of the fcc phase for the small particle size. The delay in the appearance of the fcc phase is a result of different nucleation
sites for the fcc phase between the large and the small particle size. 相似文献
5.
This paper investigates amorphous phase formation and rapid solidification characteristics of a CoCr alloy. High cooling rate and high undercooling-induced rapid solidification of the alloy was achieved by impulse atomization in helium atmosphere. Two atomization experiments were carried out to generate powders of a wide size range from liquid CoCr at two different temperatures. Amorphous fraction and kinetic crystallization properties of impulse atomized powders were systematically quantified by means of differential scanning calorimetry. In addition, different but complementary characterization tools were used to analyze the powders microstructures. The fraction of amorphous phase within the investigated powders is found to be promoted by high cooling rate or smaller powder size. The critical cooling rate for amorphous phase formation, which is influenced by the oxygen content in the melt, is found to be ~3 × 10 4 K s ?1 and corresponds to a 160- µm-diameter powder atomized in helium. Hardness of the powders is found to follow a trend that is described by the Hall–Petch relation when a relatively high fraction of crystalline structures is present and decreases with the fraction of amorphous phase. 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.
This report focuses on the phase relations and transformations in a (Fe 0.05Co 0.95) 89Zr 7B 4 melt-spun alloy with an emphasis on crystallization and its effects on thermomagnetic properties. When as-spun ribbons are
annealed at relatively low temperatures (near primary crystallization), the nucleation and growth of nonequilibrium body-centered-cubic
(bcc) crystallites occurs in a residual amorphous matrix, as determined by transmission electron microscopy (TEM) and X-ray
diffraction (XRD). At intermediate temperatures, bcc crystallites continue to grow with the addition of a small volume fraction
of the equilibrium face-centered-cubic (fcc) phase. It is expected that after the bcc nuclei are formed, the grains coarsen
as bcc phase and do not transform to the more stable fcc phase at intermediate temperatures. At temperatures where the amorphous
matrix phase dissociates into Zr intermetallics, the bcc phase is transformed into fcc and the grains coarsen significantly.
Thermodynamic modeling has been used to support the nucleation of the nonequilibrium bcc phase during the early stages of
crystallization. Thermomagnetic results show little reduction in the saturation magnetization as a function of annealing temperature
up to the primary crystallization temperature (~420 °C).
This article is based on a presentation made in the symposium “Phase Transformations in Magnetic Materials” which occurred
during the TMS Spring meeting, March 12–16, 2006, in San Antonio, TX under the auspices of the Joint TMS-MPMD and ASMI-MSCTS
Phase Transformations Committee.
相似文献
8.
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
10.
采用CaO-SiO2-Na2O-CaF2-Al2O3-MgO渣系,通过测定熔渣的熔化温度、凝固温度和结晶温度,研究连铸保护渣的熔化温度凝固温度和结晶温度与化学成分之间的关系。熔化温度高于凝固温度和结晶温度。凝固温度和结晶温度之间的关系与连铸保护的玻璃性能有关。 相似文献
12.
Crystal nucleation during rapid solidification generally occurs rather slowly except at active heterogeneities or unless very large undercoolings are achieved. Many typical microstructures are then essentially of columnar morphology, for example from the bottom surface of a melt-spun ribbon or from a heterigeneity on the surface of a powder particle. In such cases the microstructure may be considerably refined by the presence of many active nucleants for heterogeneous nucleation distributed throughout the melt. Such microstructural refinement is analysed here during rapid solidification of a laser-melted surface containing fine TiB 2 particles. Simulation of the cooling and solidification conditions confirms these particles to be highly effective nucleating substrates, capable of greatly increasing nucleating rates. As a result it is possible to obtain materials possessing greatly refined grain sizes. 相似文献
13.
In this study, the effects of solidification conditions on the grain refinement capacity of heterogeneous nuclei TiC in directionally solidified Ti6Al4V alloy were investigated using experimental and numerical approaches. Ti6Al4V powder with and without TiC particles in a Ti6Al4V sheath was melted and directionally solidified at various solidification rates via the floating zone melting method. In addition, by using the phase field method, the microstructural evolution of directionally solidified Ti6Al4V was simulated by varying the temperature gradient G and solidification rate V. As the solidification rate increased, the increment of the prior β grain number by TiC addition also increased. There are two reasons for this: first, the amount of residual potent heterogeneous nuclei TiC is larger. Second, the amount of TiC particles that can nucleate becomes larger. This is because increasing the constitutional undercooling ΔTc leads to the activation of a smaller radius of heterogeneous nuclei and a higher nucleation probability from each radius. At a cooling rate R higher than that in the floating zone melting experiment (R = 3 to 1000 K/s), the maximum degree of constitutional undercooling ΔTc,Max has a peak value, which suggests that constitutional undercooling ΔTc has a smaller contribution at higher cooling rates, such as those that occur during electron beam melting (EBM), including laser powder bed fusion (LPBF). 相似文献
14.
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
15.
Analytical and numerical methods have been developed to analyze the solidification kinetics of a mass of liquid droplets dispersed
in a fluid or solid matrix using classical nucleation theory. The resulting analytical expressions and numerical calculations
can be compared directly with calorimetric measurements of the droplet solidification exotherms to obtain information about
the nucleation mechanism. With increasing contact angle at the solid-liquid-matrix triple point, the solidification onset,
peak, and end temperatures and exothermic peak height all decrease sharply and the droplet solidification exotherms become
broader. Decreasing either the droplet radius or the number of potential catalytic nucleation sites produces a similar but
smaller effect. Distributions in droplet radius, contact angle, and nucleation sites have no effect on the solidification
peak temperature, but the droplet solidification exotherms become broader and more symmetric. The solidification onset temperature
is independent of cooling rate in the calorimeter, but the solidification peak and end temperatures decrease and the exothermic
peak height increases with increasing cooling rate. Predicted droplet solidification exotherms are in excellent agreement
with detailed experimental measurements on 10-nm-radius Cd droplets embedded in a solid Al matrix. Analytical predictions
give best-fit values of 43 deg and 430 for the contact angle and the number of potential catalytic nucleation sites per droplet,
respectively; numerical predictions give best-fit values of 43 deg and 750 for the contact angle and the number of potential
catalytic nucleation sites per droplet, respectively. 相似文献
16.
The rapid solidification of a peritectic alloy is studied. Various 2D and 3D characterization techniques were effectively utilized to investigate the effect of cooling rate on both the phase fractions and the shrinkage porosity. Particles of Al-36 wt pct Ni were produced using a drop tube impulse system. Neutron diffraction and Rietveld analysis were used to quantify the phases formed during solidification. The microstructure of the produced particles was analyzed using SEM and X-ray microtomography. It was found that increasing cooling rate resulted in decreasing the Al 3Ni 2 to Al 3Ni ratio. Also, quantitative analysis of the microtomography images revealed that the volume percent of porosity increased with increasing particle size. The distribution of porosity was found to be significantly different in small and large particles. It was concluded that the extensive growth of Al 3Ni 2 at lower cooling rates followed by the peritectic reaction made the feeding of the shrinkages more difficult, and as a result, the volume percent of porosity increased. Other findings showed that high cooling rate during solidification would result in the formation of a quasicrystalline phase, known as D-phase, and suppression of the primary Al 3Ni 2. Also, investigation of the 3D structure of the solidified particles revealed that large particles of Al-36 wt pct Ni contain multiple nucleation sites, while smaller particles contain only one single nucleation site. 相似文献
17.
对钢液凝固温度下钛的化合物、Al2O3、MnS等基底与形核相铜元素的二维点阵错配度进行了计算,并对其成为铜元素非均质形核核心的有效性进行了分析.结果表明:基底与形核相的错配度δ越小,越有利于非均质形核.Ti2O3、MnS和Al2O3与铜元素的错配度较小,具有良好的匹配关系,可以作为铜元素形核的质点并促进其异质形核.Ti2O3和MnS是钢中残余铜元素非均质形核的最有效核心;Al2O3为中等有效核心;TiC、TiN为无效核心. 相似文献
18.
In order to investigate the mechanism by which electromagnetic vibrations affect the solidification structure of metallic
alloys, an experimental apparatus which enables the simultaneous application of electric and magnetic fields under different
cooling conditions (ranging from rapid to furnace cooling) is developed. The objective is followed by inducing vibrations
in a hypereutectic Al-Si alloy melt containing suspended silicon particles and interrupting the process at different temperatures
before and after the start of solidification by water quenching. Interrupting the process at temperatures higher than the
liquidus has revealed the effects of vibrations independent of the influences of the nucleation and growth phenomena. Establishing
the conditions for obtaining identical cooling rates in experiments with different experimental conditions has lead to the
exclusion of effects resulting from the differences in cooling rates and recognition of the effects caused only by electromagnetic
vibrations. It is found that the application of either of the two fields alone has no significant effect on the solidified
structure, while profound effects are observed when the two fields are applied simultaneously. An increase in the number of
suspended silicon particles and a reduction in their average size are the effects noticed before the start of solidification.
In this research, it has for the first time been clearly verified, through microscopic observation of the quenched samples,
that these effects are brought about by the cavitation phenomenon. After the start of solidification, particles are locally
agglomerated and expelled toward the surrounding walls under a combined influence of electromagnetic vibrations and pinch
force squeezing the liquid. The final structure obtained is composed of an almost completely eutectic matrix surrounded by
agglomerates of silicon particles along the outer surface. 相似文献
19.
The microstructure of the Co 3V alloy formed by heat treatment at various temperatures is studied by transmission electron microscopy. Two ordering–separation phase transitions are revealed at temperatures of 400–450 and 800°C. At the high-temperature phase separation, the microstructure consists of bcc vanadium particles and an fcc solid solution; at the low-temperature phase separation, the microstructure is cellular. In the ordering range, the microstructure consists of chemical compound Co 3V particles chaotically arranged in the solid solution. The structure of the Co 3V alloy is shown not to correspond to the structures indicated in the Co–V phase diagram at any temperatures. 相似文献
20.
An important parameter affecting microstructure development during solidification is the amount of undercooling prior to nucleation.
The undercooling potential of aluminum has been assessed by thermal analysis measurements on powder dispersions of the liquid
metal. A number of variables have been identified which influence the undercooling of powder Al samples including powder coating,
powder size, melt cooling rate, and melt superheat. Surface analysis by Auger electron spectroscopy indicates that changing
the medium in which the powders are produced is an effective method of altering the coating chemistry. Factorial design analysis
has been employed to quantify the potential of processing variables to increase the undercooling level obtainable in aluminum.
The factorial analysis indicates that control of the powder coating through changing the medium in which the powders are produced
is most effective in decreasing the nucleation temperature. Additionally, the finest powders produced in the medium which
induces the least catalytic coating, when cooled at high rates, T = 500 °C/s, from low superheats, T
s = 710 °C, are found to achieve the deepest undercooling, ΔT = 175 °C. These studies provide the basis for further increases
in undercooling and for future investigations into the solidification reactions which produce both stable and metastable structures
in aluminum alloys.
Formerly Research Assistant in the Department of Metallurgical and Mineral Engineering, University of Wisconsin-Madison 相似文献
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