The role of melt-pool behavior in free-jet melt-spinning
R.E. Napolitano

Melt-pool behavior has a significant influence on the competition between nucleation of crystalline solidification products and glass formation during melt-spinning process. This work is focused on understanding the melt pool behavior and its role in governing the conditions tht ultimately give rise to microstructural transition during melt spinning.

The figure to the left shows crystalline nodules on the wheelside surface of a melt-spun Fe-Si-B ribbon. Such crystalline phases are observed at both high and low velocities. We have investigated the upper and lower limits to the observed “window” for amorphous ribbons.

Measurement of ribbon geometry and wheel-side surface roughness, direct imaging of the melt-pool shape, and the observation of multiphase crystalline nodules suggest that there exists three distinct velocity regimes of melt-pool behavior.
The entrapment of gas and the formation of gas pockets at the wheel-side surface is an important factor in the transition to the high rate regime, characterized by increasing surface roughness and the presence of crystalline phases.

A clear periodicity was observed in the ribbon width, exhibiting a characteristic frequency (314 s-1) rather than a characteristic spatial wavelength (see figure to left). While the melt-pool geometry is clearly different from a freely oscillating drop, an oscillating sphere analysis employing a mass corrected for the melt-pool shape indicates that the lowest order modes are indeed operative. Comparison between the oscillation time scale and the melt residence time reveals that, while the oscillations are present even at very high spinning rates, they have a significant effect on melt-pool dynamics only in the low velocity regime and may be a key contributor to the unsteady behavior observed below 10 m/s.

Measurement of anisotropy of crystal-melt interfacial energy using three-dimensional reconstructions liquid droplet shapes
R.E. Napolitano

We have used serial sectioning and three-dimensional reconstruction to measure the shape of liquid droplets within a surrounding single-crystal solid in Al-Sn binary alloys. The phase compositions are constrained by the temperature of coexistence so we report the anisotropy as a function of temperature only. The reported coefficients reflect the relative dominance of the first two terms in the Kubic harmonics. The figure shown here illustrates the equilibrium shape of the liquid droplets, which deviates only slightly from that of a sphere. The color scale has been superimposed onto the droplet shape to better illustrate the peaks which lie outside the unit sphere (red) and the valleys which lie inside the unit sphere (blue).

The Al-Sn system was chosen here for experimentals due to its nearly vertical liquidus between 250ºC and 400ºC. Because of this feature we can vary the temperature with little change in the composition of the two phases and, thus, isolate the temperature dependence.

Experimental measurement of anisotropy of crystal-melt interfacial free energy

The anisotropy in crystal-melt interfacial energy has been determined experimentally using an equilibrium shape technique for measuring the shape of liquid droplets entrained in single-phase solid. Such measurements are ongoing and results have been reported for for the Al-Cu and Al-Si binary systems. The figure here is an arbitrary cross-section in an Al-4wt%Cu alloy, showing the typical distribution of fine liquid (quenched) droplets. Using such droplets, the associated anisotropy parameter has been determined as e4=0.0097±0.0008. From similar experiments with Al-2wt%Si, anisotropy was measured as e4=0.0169. These values represent the first quantitative measurements of interface energy anisotropy in metallic systems.

S. Liu, R.E. Napolitano, and R.Trivedi, Acta Mater., 49 (2001) 4271.
R.E. Napolitano, S.Liu, and R. Trivedi, Interface Science, 10 (2002) 217.

Equilibrium morphology of coupled crystal-melt interfacial grain boundary grooves
Contact: R.E. Napolitano

A general analytical solution is presented for the fully coupled grain boundary groove morphology at a crystal-melt interface in a thermal gradient. The analysis employs a variational solution for the solid-liquid grain boundary groove and incorporates a general Fourier-series description of anisotropic interfacial energy in two dimensions. Supporting numerical calculations of groove morphologies illustrate the dramatic effect of anisotropy on the coupled groove depth and overall shape. In the case where the included grains are oriented symmetrically about the grain boundary, the junction condition was rigorously shown to be equivalent to that given by Herring’s equation. For asymmetric groove configurations, however, Herring’s equation does not adequately address the constraints of local equilibrium. Because of the mismatch in the normal interface energies, asymmetric grain orientations give rise to an effective remote migration force, not accounted for by local force-balance at the junction. Moreover, calculations of asymmetric groove energy reveal that the equilibrium structure must exhibit a tilted grain boundary, where the angle is sufficient to balance the migration force.

 

We have performed large-scale molecular dynamics simulations of
coexisting solid and liquid phases using 4/r^n interactions, for n=9
and n=12, and for Lennard-Jones systems, in order to calculate the
equilibrium melting curve. The coexisting systems evolve rapidly
toward the melting temperature. The P-T melting curves agree well with
previous calculations, as do the other bulk phase properties. The
agreement with other calculations are shown in the figure.

J. R. Morris and X. Song, J. Chem. Phys, 116 (2002) 9352.

We have calculated the complete anisotropic crystal-melt interfacial free energy of aluminum, using molecular dynamics simulations of the interfaces in equilibrium. This utilizes a recently developed approach that examines the fluctuations of the rough interface. A snapshot of the rough, dynamically fluctuating interface is shown in the figure. The results are in good agreement with experiment, including local measurements of the anisotropy and may help shed light on the selection of preferred growth directions in aluminum and aluminum alloys.

J. R. Morris, Phys. Rev. B, 66 (2002) 144104.