A THEORETICAL-ANALYSIS FOR THE FORMATION OF PERIODIC LAYERED STRUCTURE IN TERNARY DIFFUSION COUPLES INVOLVING A DISPLACEMENT TYPE OF REACTIONS

Periodic layered structures were found to form in numerous ternary diffusion couples of the type B(delta)'' C/A involving a displacement type of reactions. In all of these couples, A is a dominant diffusing species within the diffusion zone and C is essentially immobile. Occurrence of this type of layered structure was rationalized in terms of thermodynamic arguments. Moreover, a diffusion model was formulated to describe the growth of this structure. An approximate solution to the diffusion equations was obtained when simplified assumptions were made. The relative stability of a periodic layered structure versus that of a simple layered structure is governed by the relative mobilities of A and B within the diffusion zone. The simple layered structure is formed when B has a high mobility relative to that of A. On the other hand, with increasing mobility of A, the periodic layered structure tends to be stable.