KINETICS OF 1ST-ORDER PHASE-TRANSITIONS INITIATED BY DIFFUSION OF PARTICLES FROM THE SURFACE INTO THE BULK

The kinetic equations are formulated and solved to describe a first-order phase transition initiated by diffusion of particles from the surface into the bulk. The scenario of phase transformation is assumed to involve the following three stages. At first, the particles diffuse in the bulk almost freely because the grain concentration is low. Then, with increasing grain concentration, the particles are trapped predominantly near the interface and the space distribution of particles shifts to the interface. Finally, the dense phase formed near the interface blocks diffusion of particles from the interface into the bulk and prevents further uptake. General results are used to interpret the experimental data on the hydrogen uptake by Pd-coated Mg at low temperatures. In this system, the asymptotic hydrogen uptake as a function of time decreases with increasing hydrogen pressure. This unusual feature of the hydrogen uptake is shown to be qualitatively explained by hydride nucleation and growth near the Pd/Mg interface.