Solid state amorphization in Ni-Ti systems: The effect of structure on the kinetics of interface and grain-boundary amorphization

Application of a thermodynamic model for solid state amorphization that incorporates differences in both bulk and interface energies for the phases concerned, shows that the microstructure of the system can largely influence the solid state amorphization behaviour. For a crystalline Ni-crystalline Ti system the model predicts that amorphization can occur both along the Ni-Ti interface and along (high-angle) grain boundaries in the Ti sublayers. On the other hand for an amorphous Ni-crystalline Ti system amorphization can occur primarily along (high-angle) grain boundaries in the Ti sublayers. The model also implies the occurrence of maximum thicknesses for the amorphous product layers. Experimental data for Ni-Ti multilayers (498-548 K) support the model. Kinetic analysis suggests diffusion controlled growth of the amorphous phase. The activation energies for diffusion in the amorphous phase are found to be 126 +/- 7 kJ/mol at the interface and 72 +/- 25 kJ/mol in the Ti grain boundaries. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.