Diffusional reactions in composite synthesis

The thermal stability of advanced composites is dominated by the behavior of internal interfaces. In order to develop effective processing strategies and stable composite designs, it is essential to consider the relevant phase diagrams which are of ternary order or higher. In addition to phase diagram information, kinetic data such as the interdiffusion pathway and reaction rates are required to understand and control the possible interfacial chemical reactions. With this information, it may be possible to bias the reactions and to alter pathways. Often the initial nucleation stage of interfacial reactions has been neglected, but recent results indicate new kinetic behavior can develop during intermediate phase nucleation in a large concentration gradient. Multilayer thin film samples are well suited for probing the initial kinetic path and structural evolution during interdiffusion reaction and phase nucleation. In Al/Ni multilayer samples with compositional modulation wavelengths between 10 and 400 nm, thermal signal onsets due to phase nucleation have been examined to monitor the reaction kinetics and to probe the interdiffusion that precedes phase nucleation. The analysis of both bulk diffusion couple and multilayer sample behavior offers the basis for phase compatibility control that can be applied in developing stable composite structures by in situ reaction processing.