Intergranular pressure solution in internally wetted polycrystals: Effect of additives

Wetting of grain boundaries in polycrystalline materials leads to considerable changes in their physicochemical and mechanical properties. Under a constant compressive load, internally wetted materials display an enhanced deformability; creep rate increases sometimes by several orders of magnitude. The dominant creep mechanism is known as dissolution–precipitation or pressure solution; a stress-induced excessive chemical potential provides a driving force for dissolution of material within grain contacts, diffusion through the grain boundary solution film and re-precipitation elsewhere. Sensitivity of pressure solution rate to the chemical composition of the intergranular liquid was reported earlier, but the underlying mechanisms were poorly understood. In the present work, the creep experiments were carried out on poly- or monocrystalline sodium chloride in the presence of NaCl aqueous solution (pure or containing additives such as copper, magnesium and lead chlorides, K4Fe(CN)6 and urea). In all cases, pressure solution has been shown to be the main deformation mechanism. Creep rate decreases in the presence of additives which are known to affect the dissolution and growth processes of sodium chloride or its concentration in the brine. Rate-limiting stage (dissolution or diffusion) in various environments has been identified.