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 tha t incorporates differences in both bulk and interface energies for the phases concerned, shows that the microstructure of the system can lar gely influence the solid state amorphization behaviour. For a crystall ine Ni-crystalline Ti system the model predicts that amorphization can occur both along the Ni-Ti interface and along (high-angle) grain bou ndaries in the Ti sublayers. On the other hand for an amorphous Ni-cry stalline Ti system amorphization can occur primarily along (high-angle ) grain boundaries in the Ti sublayers. The model also implies the occ urrence of maximum thicknesses for the amorphous product layers. Exper imental data for Ni-Ti multilayers (498-548 K) support the model. Kine tic analysis suggests diffusion controlled growth of the amorphous pha se. The activation energies for diffusion in the amorphous phase are f ound to be 126 +/- 7 kJ/mol at the interface and 72 +/- 25 kJ/mol in t he Ti grain boundaries. (C) 1998 Acta Metallurgica Inc. Published by E lsevier Science Ltd. All rights reserved.