SOLID STATE AMORPHIZATION IN Ni±Ti SYSTEMS: THE EFFECT OF STRUCTURE ON THE KINETICS OF INTERFACE AND GRAIN-BOUNDARY AMORPHIZATION R. BENEDICTUS{, K. HAN{, C. TRáHOLT, A. BO È TTGER} and E. J. MITTEMEIJER} Delft University of Technology, Laboratory of Materials Science, Rotterdamseweg 137, 2628 AL Delft, The Netherlands (Received 10 December 1997; accepted 15 May 1998) AbstractÐApplication of a thermodynamic model for solid state amorphization that incorporates dier- ences in both bulk and interface energies for the phases concerned, shows that the microstructure of the system can largely in¯uence the solid state amorphization behaviour. For a crystalline Ni±crystalline Ti sys- tem 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 diusion controlled growth of the amorphous phase. The activation energies for diusion in the amorphous phase are found to be 126 27 kJ/mol at the interface and 72 225 kJ/mol in the Ti grain boundaries. # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. 1. INTRODUCTION Starting from crystalline parent phases, annealing of certain metal±metal couples may lead to amor- phous alloy product phase development: solid state amorphization (SSA) [1]. In view of the known phase diagrams, which usually indicate which stable phases can be expected to occur for certain alloy systems, such an observation is unexpected as the amorphous counterpart of a crystalline solid sol- ution has a larger (less negative) Gibbs free energy of formation. Thus kinetic reasons have been invoked to understand the observed formation of amorphous phases by reaction diusion in crystal- line metal±metal couples [2]. Yet, it has been shown recently that for a large number of metal±(non)me- tal systems occurrence of SSA can be understood on the basis of thermodynamics alone [3]. The prin- ciple of this model for the thermodynamics of SSA is based on a comparison of the interface energies and the bulk energies, occurring before and after a possible solid state reaction, where the product phase can be amorphous or crystalline. In general, an interface between an amorphous phase and a crystalline phase has a smaller energy than an inter- face between two crystalline phases [3]. Hence, the relatively small crystalline±amorphous interface energy favours the formation of the amorphous product phase. By contrast, the relatively large bulk energy of the amorphous phase counteracts its for- mation. As the gain in energy by the drop in inter- face energy in the initial stage of SSA can be as large as the mixing energy, development of an amorphous product phase, instead of a crystalline product phase, can be preferred thermodynamically. However, for late stages of SSA the pro®table role of the interface energy becomes less pronounced and the bulk energy becomes decisive; this implies that the thermodynamic model also indicates a maximum thickness for the amorphous product layer. It has been observed that SSA can occur not only at the interface between the two metals, but also along grain boundaries connected with this inter- face. The thermodynamic model discussed above can also predict whether or not SSA can occur along such grain boundaries. In the light of the above discussion it appears interesting to test the qualitative (i.e. whether SSA occurs or does not occur at interfaces and/or grain boundaries) and quantitative (i.e. composition and thickness of the amorphous product layer) predic- tions of the thermodynamic model. To this end special metal±metal couples can be devised. If one Acta mater. Vol. 46, No. 15, pp. 5491±5508, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 1359-6454/98 $19.00 + 0.00 PII: S1359-6454(98)00191-8 {Present address: Hoogovens Research Laboratory, P.B. 10000, 1974 CA IJmuiden, The Netherlands. {Present address: Center for Materials Science, Los Alamos National Laboratory, Los Alamos, New Mexico, U.S.A. }To whom all correspondence should be addressed. }Also at: Max Planck Institute for Metals Research, Seestrasse 92, 70174 Stuttgart, Germany. 5491