Surface and grain-boundary amorphization: Thermodynamic melting of coesite below the glass transition temperature

Coesite, a high-pressure ${\mathrm{SiO}}_2$ polymorph, becomes amorphous during isothermal annealing below the glass transition temperature, ${\mathit{T}}_{\mathit{g}}$, at one-bar pressure. Transmission electron microscopy was used to examine this fusion process (vitrification) below ${\mathit{T}}_{\mathit{g}}$. The transformation is dominated by a heterogeneous nucleation-and-growth controlled process above the thermodynamic melting temperature, ${\mathit{T}}_{\mathit{m}}$ (875 K), but below ${\mathit{T}}_{\mathit{g}}$ (1480 K). Amorphous domains nucleate at free surfaces and grain boundaries, and the amorphous-crystalline interface propagates into the interior of the crystal. This ‘‘interface-mediated’’ amorphization (vitrification) is the same as ‘‘interface-mediated’’ melting, based on the thermodynamic, microstructural, and mechanistic aspects of the transformation. This amorphization process parallels electron-irradiation and pressure-induced amorphization in coesite. © 1996 The American Physical Society.