Solidification Of Undercooled Melts Research Articles

Direct observation of dendrite fragmentation in the solidification of undercooled melts

The fragmentation of dendrites immediately following the recalescence phase of growth during the solidification of undercooled melts has been invoked to explain various rapid solidification microstructures. Despite this, little direct evidence of such a fragmentation process usually survives in the as-solidified material. We report on the rapid solidification of the single phase, congruently melting intermetallic β-Ni3Ge. During equilibrium solidification this material solidifies to the chemically ordered L12 crystal structure. Conversely, during rapid solidification, disorder trapping results in solidification to a random fcc solid solution, thereby providing a means to distinguish the rapidly solidified structures. We present results which show a range of microstructures in which the dendrite fragmentation process has been captured in progress. Results from EBSD Euler mapping reveal that dendrite fragmentation is a potential, but not particularly efficient, route to grain refinement.

Crystallization kinetics in Si-1 at%Sn during rapid solidification in undercooled melt

In order to elucidate the cause of the morphological transition of crystals growing in an undercooled melt of semiconducting materials, we carried out the containerless solidification of undoped Si and Si-1at%Sn using a CO2 laser-equipped electromagnetic levitator (EML). The crystallization of these materials was successfully achieved under controlled undercooling. The relation between the shape of growing crystals and the degree of undercooling in Si-1at%Sn was similar to that in undoped Si; that is, plate-like needle crystals were observed at low undercooling, whereas at medium and high undercooling the shape of growing crystals changed to massive dendrites. The grain-size of as-solidified samples of Si-1at%Sn was remarkably small compared with that of undoped Si. The surface morphologies of samples solidified by dropping the melt onto a chill plate of mirror-polished silicon consisted of typical twin-related <110> dendrites. On the other hand, samples that were dropped from the undercooled state consisted of twin-free <100> dendrites. The nucleation rate of two-dimensional nuclei calculated on the basis of two mechanisms, which are the twin-plane re-entrant edge mechanism and the twin-free mechanism, suggested that the morphological transition to twin-free <100> dendrites from twin-related <110> dendrites occurs when the degree of undercooling becomes larger than the critical value. These results indicate that the cause of the morphological transition of Si growing in the undercooled melt is not the roughening transition of the crystal–melt interface but the transition of the nucleation kinetics to the twin-free mechanism from the twin-related mechanism.

Decoupled growth mechanism of Fe40Ni40P14B6 eutectic alloy

Abstract

Baseline Method for Determining the Solid Fraction

A baseline method to determine directly the solid fraction using the cooling curve was proposed for solidification of undercooled melts. To construct the baselines, a sudden function ξα(x) is introduced. In terms of present method, the solid fractions during solidification of Zr metal and Al-7wt.%Si alloy were predicted. The predictions coincide with the results of the available methods.

Determination of Solid Fraction from Cooling Curve

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.

Directional Solidification of Undercooled Hypoeutectic Silver-Copper Alloy Melt

Directional solidification of the undercooled melt which combines melt undercooling with conventional directional solidification is a new method of obtaining directional structure. This method is used to realize the directional solidification of the undercooled Ag70.3Cu29.7 melt. The experiment is performed in two steps: The undercooled melt is obtained by glass fluxing, and the above specimen is remelted and excited with Ga-In-Sn coolant. By this method, directional solidification dendrites of Ag70.3Cu29.7 alloy are achieved successfully. The columnar dendrites are straight and fine. The primary arm spacing is 21µm in average for a 65K undercooling, as compared to 38µm for a 32K undercooling. This confirms that higher undercooling promotes finer dendritic microstructures.

The solidification of undercooled melts via twinned dendritic growth

The solidification of undercooled Cu-x wt pct Sn (x=1, 2, 3, or 4) alloys has been studied by a melt-encasement (fluxing) technique. It was found that below undercoolings of ΔT≈90 K, the preferred dendrite growth orientation in each of these alloys was along the 〈111〉 direction: moreover, the 2 and 3 wt pct Sn alloys also displayed evidence of twinned growth. Above ΔT≈90 K, the preferred growth direction returned to the more usual 〈100〉 orientation.

The solidification of undercooled melts via twinned dendritic growth

The solidification of undercooled Cu–x wt.% Sn (x=1, 2, 3, 4) alloys has been studied by a melt encasement (fluxing) technique. It was found that below undercoolings of ΔT≈90 K the preferred dendrite growth orientation in each of these alloys was along the 〈1 1 1〉 direction. Moreover the 2 and 3 wt.% Sn alloys also displayed evidence of twinned growth. Above ΔT≈90 K the preferred growth direction returned to the more usual 〈1 0 0〉 orientation.

Preparation and characterization of SiO 2 non-catalytic nucleation coating for the solidification of undercooled melt*

Abstract SiO2 non-catalytic nucleation films were coated onto a ceramic substrate and the inner surface of a ceramic crucible by the Sol-Gel method. The surface morphology of the coatings was characterized with an atomic force microscope and a scanning electronic microscope. The non-catalytic effect of the coating on the heterogeneous nucleation of metal melts was tested through measuring the nucleation undercooling. Investigation results indicated that the prepared coating could act as a baffle-wall to prevent the metal melt from nucleating heterogeneously on the inner surface of the crystalline crucible. The tiny holes with a diameter below 2 μm and heaves on the coating surface have no evident effect on the undercooling of metal melts. For a poly-layer coating the super-layercan cover the cracks in the sublayers which prevents the cracks in the coating from spreading in the direction vertical to the coating layers and thus eliminates the effect of cracks on the coating.

Preparation and characterization of SiO2 non-catalytic nucleation coating for the solidification of undercooled melt

Preparation and characterization of SiO2 non-catalytic nucleation coating for the solidification of undercooled melt

Metastable phase formation during solidification of undercooled melt

Undercooling behaviour of micron-sized droplets entrained in the matrix of the primary phase of Ag–Ge, Al–Ge and Cu–Ag alloys was investigated. Thermal cycle consisted of equilibration of hypoeutectic alloys above the eutectic temperature in two phase liquid–solid region followed by their slow cooling or quenching from the mushy state. Repeated thermal cycling generated a wide size range of droplets. During slow cooling droplets exhibited a large undercooling below the eutectic temperature of alloys. Solidification structure of the undercooled droplets revealed presence of metastable phases in Ag–Ge and Al–Ge alloys and a non-equilibrium solid solution phase in Cu–Ag alloy. Solidification behaviour of undercooled melt and formation of metastable microstructure are discussed.

Grain refinement through fragmentation of dendrites in undercooled melts

The widely observed microstructural transitions from coarse dendritic to fine equiaxed grains in solidification of undercooled melts have been puzzling for many years. Here we describe recent progress made in solving this puzzle. We review the basic elements of a simple model that assumes that these transitions result from the fragmentation of dendrites by remelting during the period following recalescence when the interdendritic melt solidifies. Furthermore, we review the results of detailed experiments on Ni–Cu alloys specifically designed to test the predictions of this model. A good overall quantitative agreement is obtained between the model predictions and the experimental observations. In particular, the model predicts the existence of four grain refinement transitions, three of which are observed experimentally. It also describes the dependence of the microstructural transition undercoolings on the cooling rate that follows recalescence and on the alloy composition, in agreement with the experimental findings.

Rapid directional solidification from highly undercooled Cu-5wt.%Ni alloy melts

For decades solidification of highly undercooled alloy melt has been widely studied [1–6]. The study of that is meaningful for two reasons. First, high undercooling (1T ) before solidification can lead to rapid solidification once the melt nucleates, which results in refined microstructures and improved properties. Second, the relative slowly cooled but highly undercooled melt opens up the possibilities of direct observations of the rapid solidification processes under nonequilibrium conditions, which is important both to the consummation of rapid solidification theory and to the optimum of production techniques. If the undercooling of alloy melts and directional solidification technology are combined organically, we will obtain a simple solidification model of rapid solidification of highly undercooled alloy melt. That is valuable both in the quantitative research of solidification of highly undercooled melts and in the production of materials which require aligned crystal orientation with much less time and expenses. Tarshis et al. [7] have studied the microstructures of Cu-Ni alloy solidified at different initial undercoolings. They found that coarse and dendritic microstructures could exist in an undercooling range, from 85 K to 150 K. Beyond this undercooling range equiaxed structures were found. Lux et al. [8] demonstrated the feasibility of producing superalloy castings having directional microstructures from undercooled melt. Ludwig et al. [9] developed a so-called autonomous directional solidification technology and obtained single crystal superalloy turbine blades from undercooled superalloy CMSX-6 melt. Schwarz et al. [10] proposed a mechanism which described the observed transitions in solidification of undercooled melts from a coarse grained dendritic to a grain refined equiaxed microstructure. It was thought that the refinement was caused by remelting and coarsening of primary formed dendrites. Research team of DLR [11, 12] had done much work on the triggered rapid solidification of highly undercooled electromagnetically levitated alloy melt. They observed the microstructures which grew radially from the trigger point. Kiminami and Sahm [13] had studied the undercooling and subsequent solidification of Pd77.5Cu6Si16.5 alloy melt. Also they obtained the radial microstructures which grew from the nucleation point. However, detailed studies about the directional solidification from highly undercooled melts are still scarce. In this paper, directional solidification and rapid solidification from highly undercooled alloy melt are combined using a Cu-5wt.%Ni alloy in a set of selfmade experimental apparatus, therefore, rapid directional solidification from highly undercooled alloy melts are realized, and expected directional solidified structures are obtained. The experiments were divided into two stages. First, undercooling of Cu-5wt.%Ni alloy melt; second, the rapid directional solidification of highly undercooled Cu-5wt%Ni alloy. The 810 mm Cu-5wt %Ni master alloy ingots were prepared from 99.972% Cu and 99.99% Ni in an induction furnace using a graphite crucible under vacuum, then the ingots were cut into pieces with cylindric shape weighing about 5 g (the surface layers of the ingots were cut off in advance). Fig. 1a shows the apparatus used during the first experimental stage. A procedure to get high undercooling, which has been employed effectively in a number of investigations [4–6], was applied by superheating and cooling the molten metal several times whether immersed in a molten high purity B2O3 glass or not (both in a high purity fused silica tube). High frequency induction melting technology was employed here. When the power was off, the sample began to cool and reached its maximum undercooling. The temperature was measured with an infrared thermometer with precision of ±1.0%. Before measurement it was calibrated by standand WRe3-WRe25 thermocouples under the similar experimental conditions as the subsequent undercooling and directional solidification experiments. Fig. 1b shows a sketch of the self-made apparatus used during the directional solidification experiments of undercooled Cu-5wt.%Ni alloy melts. The sample obtained during the first experimental stage was remelted in a funnel-like quartz crucible with 8 4 mm in diameter and formed a liquid cylinder. Because of the electromagnetic levitation force and the surface tension the melt could be held in the crucible. Then when the melt was undercooled to a predetermined undercooling, liquid Ga-In-Sn alloy (it was at room temperature) was used to nucleate the undercooled melt. Then rapid directional solidification would occur in the undercooled melt. The actual undercooling was determined according to the recorded cooling curve. Solidified samples

Nonequilibrium solidification of hypercooled Co–Pd melts

Co–Pd alloy melts are characterized by a low heat of fusion and, as a consequence, by a comparably small critical undercooling for the hypercooling limit ΔThyp of the order of 300 K. It is shown that containerless processing of bulk melts by electromagnetic levitation offers undercooling levels of ΔT≈350 K thus exceeding the hypercooling limit considerably. Solidification of undercooled melts from the hypercooling regime leads to rapid crystallization of the entire sample under nonequilibrium conditions. The electromagnetic levitation technique in combination with time-resolved recalescence detection was used to measure the growth velocity of Co–Pd alloy melts as a function of undercooling prior to solidification. The growth velocities of undercooled metallic melts were measured in an undercooling range exceeding the hypercooling limit. The experimental results are discussed within current theory of dendritic growth.

Kinetic Competition in Undercooled Liquid Alloys

ABSTRACTMost alloy systems can develop more than one solid phase for a given composition during solidification. For metastable undercooled liquids a thermodynamic analysis can provide insight into the range of structural and compositional options that are possible. The numerous examples of metastable phase formation during solidification of undercooled melts demonstrates that the actual phase selection is dominated by kinetics. Indeed, the observation of undercooling is linked to kinetic control. Different forms of kinetic control can be identified and include suppression of heterogeneous nucleation during slow cooling and constrained growth during rapid quenching. In both cases the kinetic competition involved in phase selection is mediated by the thermal history. Nucleation controlled kinetics can be examined in fine powders of undercooled liquids where crystallization rates and metastable phase diagrams provide a basis for analysis. Similarly, constrained growth can be examined during controlled cooling where microstructures and kinetic models can be used for analysis. The existing kinetics models appear to be adequate but the processing conditions are often dynamic and in nonsteady state conditions so that critical tests are difficult unless specific kinetic transitions are available.

Physical Mechanism of Grain Refinement in Solidification of Undercooled Melts

Physical Mechanism of Grain Refinement in Solidification of Undercooled Melts

Physical Mechanism of Grain Refinement in Solidification of Undercooled Melts

It is proposed that the widely observed transitions in solidification of undercooled melts from a coarse grained dendritic to a grain refined equiaxed microstructure result from the fragmentation of dendrites by remelting during the period following recalescence where the interdendritic melt solidifies. This mechanism is supported by the experimental demonstration in Cu-Ni alloys that the transition undercoolings vary with cooling rate in a way which is relatively well described quantitatively by a simple fragmentation model. The latter also predicts the occurrence to two transitions, both of which are observed.

Structures and magnetic properties of a Nd15Fe77B8 alloy produced by twin-roller quenching

The relationship between the magnetic properties and microstructures of flakes of a Nd15Fe77B8 alloy produced by twin-roller quenching was studied. Flakes produced at low to intermediate roller speeds (1.4–8 m/s) showed a dendritic structure of Nd2Fe14B oriented normal to the chilled surface, while flakes produced at higher roller speeds showed fine, equiaxed Nd2Fe14B grains. The flakes with a dendritic structure were found to be magnetically anisotropic and showed a high-energy product (BH)max (15.2 MGOe) in the direction perpendicular to the chilled surface. The flakes that had fine equiaxed grains were magnetically isotropic. The maximum coercivity of the flakes exceeded 20 kOe. The microstructural transition from columnar dendrites to equiaxed grains is explained in terms of morphological changes in the solidification of undercooled melts. Optimization of twin-roller quenching parameters for the production of desirable microstructures in Nd-Fe-B magnets is discussed.

The solidification behaviour of undercooled AlCu alloys in contact with the primary phases

The near eutectic AlCu liquid is undercooled in contact with both α-Al and θ-Al 2 Cu phase by entrained droplet technique to study and understand the evolution of solidification microstructures at different undercoolings and for different substrates. Four different types of microstructures could be observed in droplets in contact with α-Al and two distinct morphologies could be observed in the droplets in contact with θ-Al 2 Cu. The results can be qualitatively explained with the help of an extrapolated metastable phase diagram and present understanding of solidification in undercooled melt. Finally the knowledge gained is extended to analyse the solidification behaviour of AlCu eutectic melt pool obtained by pulsed laser processing.

On the operative mechanisms in the solidification of undercooled melts

The role of undercooling in controlling the operative mechanisms of solidification and in promoting the formation of extended solid solutions in the AgCu and CdZn systems is discussed. Use is made of a novel expression for the free energy of an undercooled liquid in arriving at several characteristic temperatures controlling the mechanism of solidification. The temperature T 0, of equal free energy of solid and liquid phases, the temperature T f at which fluctuations in the liquid state can become potent nucleants and the temperature T a at which the driving forces for the processes of solute redistribution and of diffuionless solidification become equal are determined for the two systems. The prominent role of the entropy of fusion in determining the mixing parameters from phase diagram data is highlighted.