S c r i p t a METALLURGICA Vol. 22, pp. 715-719, 1988 Pergamon Press plc Printed in the U.S.A. All rights reserved WETTING AND PENETRATION OF KC1 AND NaC1 GRAIN BOUNDARIES BY WATER AND METHANOL T. Baykara and G. M. Pharr Department of Materials Science Rice University Houston, Texas 77251 (Received February 22, 1988) Introduction As a result of recent studies of creep enhanced by an intergranular liquid phase in alkali halide salts, it has been suggested that the process by which liquid residing in grain boundary triple junctions penetrates into adjacent two grain interfaces can play an important role in the deformation process (1,2). This is par- ticularly important when the stresses across the two grain interfaces are compressive, since penetration pro- rides a mechanism by which some liquid can be retained there, even though the action of the applied stress is to squeeze it out. This concept has been used to meehanisticaUy model the experimentally observed creep behavior of porous, polycrystalline potassium chloride salt containing various liquids in its porosity (1). The basis of the model is that when the equilibrium dihedral angle of the liquid on the solid grain boundaries, ~, is small (assumed to be 0"), the boundaries are penetrated by the liquid in order to maintain local surface tension equilibrium. The penetration occurs by dissolution and diffusional transport of the solid through the liquid, in a manner similar to that which occurs during grain boundary grooving in the presence of a liquid (3,4). Since penetration leads to undercutting of the boundary, the cross sectional area of the load bearing neck between the grains is reduced, and the stresses in the neck are correspondingly increased. When critical values are exceeded, localized plasticity and/or crushing can occur, producing an increment of strain and resulting in neck growth (1,5). A steady state can be achieved when the rate of increase of neck area resulting from plasticity and/or crushing is balanced by the rate of decrease in neck area produced by undercutting. In one important limit, deformation is rate controlled by transport of dissolved solid away from the neck by diffusion through the liquid. An important experimental observation which led to the development of this model is that the magni- tude of the creep enhancement in porous KC1 is different for different liquids (1). For example, experiments showed that the degree to which methanol enhances the creep rate is not nearly as great as that of water. It was hypothesized that this is due to a solubility effect, since the solubility of KC1 in methanol is small in comparison to its solubility in water (1.37 weight% in methanol; 35.7 weight % in water (6,7)). This was further supported by experiments in which mixtures of water and methanol were used as the liquid phase. The solubility of KC1 in these mixtures is dependent on the water-to-methanol ratio, and it was found that the rate of creep increases in direct proportion to this solubility. The fact that the creep rate correlates with the solubility of the solid in the liquid has been used as evi- dence that deformation is rate controlled by diffusion of the solid through the liquid (1). However, an as- sumption implicit in these arguments, as well as in the development of the undercutting model for deformation, is that water, methanol, and mixtures thereof exhibit perfect wetting on KCI grain boundaries. Under these circumstances, the driving force for penetration is large and equal in magnitude for each liquid. Whether or not this is actually the case is not clear, and because of this, it is conceivable that the differences in liquid enhanced creep behavior in porous KCI produced by the various liquids are not solely a result of differences in solubility. To explore this possibility, experiments were recently performed to measure the equilibrium dihedral angles and the extent to which water and methanol wet and penetrate the grain boundaries of alkali halide salts. This paper summarizes the results of those studies. 715 0036-9748/88 $3.00 + .00 Convri~ht (c) 1988 Pergamon Press plc