A Study of Solid/Liquid Interfacial Phenomena in Hypereutectic Al-Si with High Resolution in situ Transmission Electron Microscopy

Solid-liquid interfaces are extensively encountered in materials processing and applications. The structure and properties of these interfaces affect solidification, wetting behavior, and crystalline growth mechanisms, including epitaxial growth. The exact structural, dynamic, and chemical nature of these interfaces have been historically difficult to study at high resolution due to the fact that one phase is a liquid. Recently, advances in in situ high-resolution transmission electron microscopy experimental methods have progressed to enable the study of solid- liquid interfaces in metals with resolution on the atomic scale. Reported results thus far include direct observation of step/ledge growth, the presentation of various crystallographic facets during growth/dissolution, ordering of liquid layers adjacent to crystalline solids, and chemical segregation at interfaces.

This work reports the first observations of a number of new interfacial phenomena associated with the dynamic behavior of the solid/liquid phase boundary at equilibrium. The structural dynamics of silicon {111}-type planes were tracked near the vapor/liquid/solid triple-point as the solid/liquid interface fluctuated between roughened, smoothly faceted, and smoothly rounded morphologies. Both the roughened and smoothly faceted configurations were unstable from frame-to-frame while the faceted configuration was observed to persist for as many as three consecutive time-steps. Adjacent to the triple-point, the facets {100}, {112}, {117}, and {113} were observed along the interface between with the solid and liquid phases.

In contrast to previous experimental and theoretical work at disordered solid/solid interfaces, no characteristic length between interfacial fluctuations was observed. The magnitude of the planar motion of individual {111} planes at the interface between a stable {113} silicon facet and liquid-phase aluminum alloy showed a bias to quantize changes in lengths corresponding to multiples of the {113} inter-planar spacing. This bias towards quantized fluctuation magnitudes has not been shown in previous work modelling the fluctuations present at solid/liquid interfaces in metallic systems; this quantization is likely a result of the strong directional nature of the covalent bonds present in the silicon lattice. In contrast to previous experimental and theoretical work at disordered solid/solid interfaces, no characteristic length between interfacial fluctuations was observed.

Finally, a metastable copper precursor phase was observed to heterogeneously nucleate at the boundary between the solid and liquid phases. These copper islands ranged widely in length along the interface, from 4–132 ̊A; the range of distances that they extended off of the interface into the liquid phase ranged from 2–13 ̊A. Each copper island observed shared the same (1 ̄13)Si∥(076)Cu;[110]Si∥(100)Cu orientation relationship with the silicon substrate below.