The Grain Boundary Grooving of Iron in Liouid Sodium

Abstract

When a polycrystalline metal is annealed at a sufficiently high temperature, grain boundary grooves are produced at the intersection of the grain boundaries with the metal surface. The grooves are formed by surface-tension-driven mass transport and have a profile as indicated schematically in Fig. 1. With the development by Mullins(1,2) of a theory of grain boundary grooving, it has been possible to distinguish between the mass transport mechanisms of surface and volume diffusion and to determine the diffusion coefficient. In recent years many studies of surface self-diffusion of metals have been performed by measuring grooving kinetics in vacuum, hydrogen, or inert gas environments.(3) Relatively few such studies have been made in a liquid metal environment and none, to the authors’ knowledge, in liquid sodium. Because of the use of sodium as a reactor coolant and the interest in its behavior as a corrodent, a study of grain boundary groove development on α-iron (b.c.c.) surfaces exposed to liquid sodium is currently underway in this laboratory. The purpose is to determine the mass transport mechanism and diffusion coefficient for a-iron in a sodium environment and to compare these results with those of surface self-diffusion of iron in vacuum.(4) Interference microscopy is being used to determine surface topography. The experimental program is nearly complete, and preliminary results of the study are presented here. Open image in new window Fig. 1 Schematic cross section of a grain boundary groove.