KINETICS OF CELLULAR REACTION IN ORIENTED BICRYSTALS OF PB-7AT PERCENT SN

The rate of growth, interlamellar spacing and proportion of the equilibrium amount of precipitate phase formed were measured as a function of temperature (-13° to 45°C) and misorientation about 〈001〉 (θ = 15° to 25°) in oriented bicrystals of Pb-7 at.% Sn. These data were analyzed on the basis of the Turnbull and the Cahn theories of the cellular reaction and of certain modifications of these theories. The cell boundary diffusivities (DB) calculated at θ = 25° from the modified Turnbull theory fell closest to the radiotracer grain boundary diffusivity data of Pb in Pb-1.55 at.% Sn reported by Stark and Upthegrove. The forms of the dependence of DB upon θ obtained from all theoretical approaches were compatible with the Stark-Upthegrove boundary diffusivity data in pure Pb. The activation energy of the cell boundary mobility, as determined from the Cahn theory, was 5-6 kcal/mole, in remarkably good agreement with published data on grain boundary migration in zone-refined Pb; this result is qualitatively consistent with Cahn's impurity drag theory of boundary migration. Both the Turnbull and the Cahn theories gave thoroughly unsatisfactory results for the interfacial energy of the broad faces of the cell lamellae. The cellular structure failed to develop when θ was reduced from 15° to 13°, in support of C.S. Smith's mechanism of this reaction. © 1968.