Precipitation in Microscopic Cracks

It is shown that precipitation occurs in Griffith-type surface cracks in solids from solutions or from vapors under conditions in which a plane surface would be dissolved or vaporized. A reduction in the equilibrium vapor pressure at the crack tip from that of a plane surface is produced by the driving force to reduce the surface energy of the crack. An equation is derived relating vapor pressure at the tip of the crack to vapor pressure over an infinite plane. This relation is used in deriving an expression for the fill-up of cracks with time. Experimental data on the measurement of the retreat of the crack edge with time appear to be in good agreement with the derived expression. A discussion of the effect of such crack healing on the mechanical properties of the solids is included.