DIFFUSIONAL REACTIONS IN COMPOSITE SYNTHESIS

The thermal stability of advanced composites is dominated by the behav ior of internal interfaces. In order to develop effective processing s trategies and stable composite designs, it is essential to consider th e relevant phase diagrams which are of ternary order or higher. In add ition to phase diagram information, kinetic data such as the interdiff usion pathway and reaction rates are required to understand and contro l 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 neglect ed, but recent results indicate new kinetic behavior can develop durin g intermediate phase nucleation in a large concentration gradient. Mul tilayer thin film samples are well suited for probing the intital kine tic path and structural evolution during interdiffusion reaction and p hase nucleation. In Al/Ni multilayer samples with compositional modula tion wavelengths between 10 and 400 nm, thermal signal onsets due to p hase nucleation have been examined to monitor the reaction kinetics an d to probe the interdiffusion that precedes phase nucleation. The anal ysis of both bulk diffusion couple and multilayer sample behavior offe rs the basis for phase compatibility control that can be applied in de veloping stable composite structures by in situ reaction processing.