3. From segregation to intergranular precipitation

Strong intergranular segregation leads to saturation of the joint with solute atoms, often resulting in the formation of precipitates. There are two other reasons for the appearance and development of a second phase at grain boundaries:

(i) the variation in free enthalpy ΔG, which accompanies the appearance of a second-phase seed at a grain boundary, is much lower than that required for a seed in matrix,

(ii) the intergranular diffusion coefficient, much higher than the volume diffusion coefficient, enables rapid growth of this germ.

The fact that intergranular precipitation of a compound of the element S is closely dependent on the level of segregation of this element at grain boundaries is well revealed in an yttrium-doped alumina polycrystal. Grain growth – and, consequently, a reduction in the volume occupied...