Color metallography of diffusion-induced grain boundary migration in Cu-Zn and Cu-As alloys

Abstract

Diffusion-induced grain boundary migration is a recently recognized phenomenon that leads to unexpected motion of grain boundaries. Vastly enhanced mass transport characterizes the low-temperature aspect of the phenomenon, since the grain boundaries provide easy paths for diffusional redistribution of atoms in the regions traversed by the boundaries. Diffusion of a solute into or out of polycrystalline materials when only grain boundary diffusion is significant has been observed to induce grain boundaries to migrate in a number of alloys. To investigate this phenomenon further, boundary migration in binary Cu-Zn and Cu-As was studied by use of color metallography. Alloyed surfaces were revealed in the regions traversed by the boundaries after zincification of pure copper in the temperature range 600–900 K and after diffusion of arsenic from the vapor into pure copper in the temperature range 573–643 K. The extent of migration, degree of alloying, and morphology of the grain boundary migration were studied using a variety of metallographic preparative techniques. Because the migrating boundaries sweep across grains, mixing in solute, and because no compositional changes occur except in regions through which migrating boundaries pass, changes in the color of surface regions were used to detect and to extract the extent of the phenomenon.