Faceted primary silicon structures in directionally solidified Al-Si Directional solidification of eutectic Al-Si at a velocity of 1 micron per second in a thermal gradient of 7 K/mm results in a microstructure consisting of faceted star-shaped silicon rods in an aluminum solid solution (upper). These structures are highly textured along a <100> direction. Under these conditions, the Si particles grow in a dendritic or cellular fashion, with four secondary "plates" emanating radially from the central core in <110> directions. Twinning in the central core permits a second set of four side-plates, resulting in eight-pointed star-shapes (lower). We are currently investigating the dynamics of the side-branching process, the growth mechanisms at the central core, and the dendritic or cellular coupling responsible for the interface morphology and spacing selection.
	Measurements reveal anisotropy of crystal-melt interfacial free energy We have reported for the first time the experimental determination of the 3D Wulff-shape (upper left) for a metallic crystal-melt system (Al-Sn). The anisotropy was reported using the leading terms of the relevant cubic harmonics, determined to be epsilon_1 =(1.81±0.36)x10 -2 and epsilon_2 =(-1.12±0.13)x10 -2 . Corresponding normal stiffnesses and a generalized stiffness (lower left) were quantified and compared with predictions from atomistic simulations.
	Solidification texture and morphology suggest dual eutectic/cellular character in Al-Si We have reported on the mechanistic/diffusive selection of a twinned bicrystalline faceted primary growth structure in near eutectic Al-Si alloys. Through serial milling, 3D reconstruction, and orientation imaging with electron backscatter diffraction patterns, we have uncovered the branching processes leading to the selection <100> oriented twinned bicrystal dendrites (shown in cross-section at left).
	Velocity limits in Free-jet melt spinning High-speed imaging during melt-spinning of an Fe-Si-B alloy was used to quantify the melt-pool behavior. Through analysis of shape oscillations and melt pool length variation, velocity limits for stable and reproducible melt-spinning were identified. Comparison of the relevant time scales revealed that surface-controlled melt-pool oscillation may govern the onset of unsteady thermal conditions at the lower limit. Analysis of momentum and volume constraints yielded an upper limit. Results are supported by experiment.
	Phase equilibria in Fe-Si binary system We have investigated the solidus and liquid boundaries in the iron-rich portion of the Fe-Si phase diagram. Quantifying the heating-rate dependence, we employed differential thermal analysis (DTA) and reported phase boundaries that differ considerably from previous reports, with implications of the importance of A2-B2 ordering and the associated thermodynamic models.
	High-rate morphological transitions in Al-Si We are investigating irregular eutectic growth in an aluminum-silicon alloy using directional solidification. The high-rate transition from flake to fiber morphology is being characterized in terms of the three dimensional microstructure. Results have shown that the transition occurs over the velocity range from 100 to 1000 mm/s, and two regimes of behavior are observed with respect to a characteristic length scale that defined by the spatial variation in local volume fraction.
	"MST" as local order parameter for 2D array We have employed x-ray topography (example - lower left) and a minimum spanning tree (MST) analysis (upper left) to quantify the degradation of crystal perfection in a Ni-based single-crystal casting. The MST parameters were found to be rather insensitive to differences in <em>local </em> order due to statistical sampling size effects, particularly for 200 points or fewer. These effects were quantified. </p>