The transition from Harrison type-B to type-A kinetics in grain-boundary tracer diffusion

This paper is concerned with the lattice and grain boundary diffusivities t hat can be extracted from the tracer concentration depth profile resulting from diffusion of tracers from a thin film source in the presence of equall y spaced parallel boundary slabs. We address this problem by mapping a grid over the phenomenologically conceived system and explore this grid with in dependent particles using Monte Carlo methods. It is shown that the transit ion from Harrison type-A kinetics (where the Hart equation provides the eff ective diffusivity) to Harrison type-B kinetics (where the lattice and grai n boundary sections of the depth profile are delineated) occurs at a much s maller diffusion length than previously thought. In addition to the usual m odel where the mobility of tracers at the (surface) tracer source is matche d to the immediate substrate, we also investigated a model which this mobil ity is made equal to the grain-boundary mobility. Similar behaviour was fou nd.