On the roles of temperature and interfaces in irradiation and thermally induced solid state amorphization

The roles of irradiation temperature and interfaces (free surfaces and grain boundaries) in irradiation- and thermally-induced amorphization of ceramics (coesite, apatie, olivines and spinels) have been studied by transmission electron microscopy (TEM). The irradiations were performed with 1.5 MeV Kr+, 200 keV and 1 MeV electrons over a wide temperature range (20--700 K). The critical amorphization dose at which amorphization is complete, Dc, increased with increasing irradiation temperature for most material except coesite (a high pressure polymorph of SiO2) which showed a decreasing Dc with increasing temperature under 1 MeV electron irradiation. Although amorphization may occur directly within a displacement cascade or by cascade overlap, this study shows that free surfaces and grain boundaries are favorable sites for nucleation of amorphous volumes. Once the amorphous volume is formed at interfaces, it may grow rapidly under continued irradiation. Coesite which has a glass transition temperature higher than its melting temperature underwent spontaneous amorphization during thermal annealing at 1,200 K. This thermally-induced amorphization also started at free surface and grain boundaries and propagated into the interior of the crystal. The interface-mediated amorphization is analogous to the process of thermodynamic melting