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 dif fusion couples of the type B(delta)'' C/A involving a displacement typ e of reactions. In all of these couples, A is a dominant diffusing spe cies within the diffusion zone and C is essentially immobile. Occurren ce of this type of layered structure was rationalized in terms of ther modynamic arguments. Moreover, a diffusion model was formulated to des cribe the growth of this structure. An approximate solution to the dif fusion equations was obtained when simplified assumptions were made. T he 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 forme d 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 t o be stable.