SURFACE AND GRAIN-BOUNDARY AMORPHIZATION - THERMODYNAMIC MELTING OF COESITE BELOW THE GLASS-TRANSITION TEMPERATURE

Coesite, a high-pressure SiO2 polymorph, becomes amorphous during isot hermal annealing below the glass transition temperature, T-g, at one-b ar pressure. Transmission electron microscopy was used to examine this fusion process (vitrification) below T-g. The transformation is domin ated by a heterogeneous nucleation-and-growth controlled process above the thermodynamic melting temperature, T-m (875 K), but below T-g (14 80 K). Amorphous domains nucleate at free surfaces and grain boundarie s, and the amorphous-crystalline interface propagates into the interio r of the crystal: This ''interface-mediated'' amorphization (vitrifica tion) is the same as ''interface-mediated'' melting, based on the ther modynamic, microstructural,and mechanistic aspects of the transformati on. This amorphization process parallels electron-irradiation and pres sure-induced amorphization in coesite.