In recent years it has become well established that fast diffusion along grain boundaries plays a key role in many important metallurgical processes including cases where net mass is transported along boundaries which act as sources and/or sinks for the fluxes of atoms. In addition, considerable advances have been made in understanding grain boundary structure, and new techniques have become available for studying kinetic phenomena in grain boundaries. This lecture will attempt to review our current knowledge of the atomistic mechanisms responsible for these grain boundary diffusion phenomena. Relevant aspects of the structure of grain boundaries and the point and line defects which may exist in grain boundaries are described first. The important experimental observations are then discussed. Diffusion models are then taken up, and it is concluded that the atomic migration occurs by a point defect exchange mechanism which, in at least the vast majority of boundaries in simple metals, most likely involves grain boundary vacancies. The grain boundary sources and/or sinks required to support divergences in the atomic (vacancy) fluxes are grain boundary dislocations. Phenomena therefore occur which resemblethe Kirkendall Effect in the bulk lattice in certain respects. Additional topics are discussed which include effects of boundary structure on boundary diffusion and the question of whether or not boundary diffusion is faster along migrating than stationary boundaries.

Grain boundary diffusion mechanisms in metals

In recent years it has become well established that fast diffusion along grain boundaries plays a key role in many important metallurgical processes including cases where net mass is transported along boundaries which act as sources and/or sinks for the fluxes of atoms In addition, considerable advances have been made in understanding grain boundary structure, and new techniques have become available for studying kinetic phenomena in grain boundaries This lecture will attempt to review our current knowledge of the atomistic mechanisms responsible for these grain boundary diffusion phenomena Relevant aspects of the structure of grain boundaries and the point and line defects which may exist in grain boundaries are described first The important experimental observations are then discussed Diffusion models are then taken up, and it is concluded that the atomic migration occurs by a point defect exchange mechanism which, in at least the vast majority of boundaries in simple metals, most likely involves grain boundary vacancies The grain boundary sources and/or sinks required to support divergences in the atomic (vacancy) fluxes are grain boundary dislocations Phenomena therefore occur which resemblethe Kirkendall Effect in the bulk lattice in certain respects Additional topics are discussed which include effects of boundary structure on boundary diffusion and the question of whether or not boundary diffusion is faster along migrating than stationary boundaries

Thermodynamic properties of coincidence boundaries in aluminum

Transformation of the grain boundary structure in iron by phosphorus segregation

Identification of Grain Boundary Atomic Structure in Gold by Matching Lattice Imaging Micrographs with Simulated Images

The structure of the Sigma =5 tilt grain boundary in aluminium was simulated by molecular dynamics using the Morse potential. Two types of structure were obtained. The local density of states (LDS) of phonons was calculated by the recursion method developed by Haydock and colleagues. The Debye parameters ( theta D(n)) were found to be very small near the boundary plane and there were large mean atomic displacements in the direction perpendicular to the boundary plane.

Y Ishida

Papers

Computer simulation of the structure and atomic vibration of the Σ=5 tilt boundary in aluminium