Wetting and penetration of KCl and NaCl grain boundaries by water and methanol

S c r i p t a METALLURGICA

Vol. 22, pp. 715-719, 1988 P r i n t e d in t h e U.S.A.

Pergamon P r e s s p l c 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 ( R e c e i v e d F e b r u a r y 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 particularly important when the stresses across the two grain interfaces are compressive, since penetration prorides 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 magnitude 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 evidence that deformation is rate controlled by diffusion of the solid through the liquid (1). However, an assumption 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

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Experimental The basic experiment involved immersing polished bicrystals of potassium chloride and sodium c111oride into solutions of water and methanol for prescribed lengths of time, followed by examination of the penetration profiles on polished sections. Because facilities for growing bicrystals were not available, arrangements were made with a single crystal manufacturer (Harshaw/Filtrol) to acquire bicrystals produced when a second grain was accidentally nucleated during the growth of a single crystal. Our original intention was to study potassium chloride only, since results could be directly compared and applied to the known liquid enhanced creep behavior of porous KC1. Unfortunately, only one KC1 bicrystal was available, and it was subsequently found to contain a very low angle boundary (less than I°). Because it is known that the wetting and penetration behavior of low and high angle boundaries can be quite different ($-10), a second bicrystal of a similar salt was sought, and a NaC1 specimen with a relatively high angle grain boundary was obtained (about g°). In both specimens, the plane of the boundary was slightly curved. A stereographic representation of the orientations of the grains relative to the plane of the boundary for the NaC1 specimen, determined by Laue back reflection analysis, is shown in Fig.l. It is seen that the boundary is predominantly of twist character, with a relative rotation of about $°. Curiously, the KCI specimen was also found to contain a twist boundary of almost identical crystallographic habit; however, the relative rotation of the grains was much smaller, less than 1° Specimens for penetration testing with dimensions of about 5 x 5 x 15 mm were cut from each material using diamond and jeweler's saws. The boundary plane was oriented roughly perpendicular to the long axis of each specimen. Specimens were ground and polished to produce smooth external surfaces, rinsed in CC14, and dried with hot air. Solutions of water and methanol were prepared from reagent grade materials. Each solution was saturated with the appropriate salt to prevent bulk dissolution following specimen immersion. The methanol was a special high purity grade containing less than 0.022% water by weight. Penetration was accomplished by immersing the specimens in the solutions for periods of up to 122 days in sealed vessels. Care was taken not to disturb the vessels in order to minimize convection. Following removal from the liquid, the bicrystals were quickly rinsed in CC14 and dried in hot air to preserve the penetration profile. The profile was examined optically after mounting in low viscosity cold setting resin, grinding and polishing. Each specimen was examined at several depths. Quantitative measurements of the depths of penetration and dihedral angles were made directly from micrographs. The values reported here are averages of 10 to 15 measurements on each specimen at different section depths. The depth of penetration was defined as the distance between the midpoint of the line connecting the exterior sides of the groove and its tip.

FIG.1. A stdreographic representation of the NaC1 bicrystal orientation relative to the plane of the boundary. Open and closed symbols represent the two individual grains.

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FIG.2. Depth of penetration as a function of time for NaCl bicrystals exposed to water and methanol,

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TIME (10s u~) FIG.3. Depth of penetration as a function of time for KCI bicrystals exposed to water and methanol.

Results The depths of penetration as a function of time are shown in Figs.2 and 3. An examination of these figures reveals the following: (1) The rate of penetration is extremely slow, irrespective of the nature of the solid or the liquid. Even in the fastest case, the penetration of NaC1 by water, the rate of penetration is only about 4 x 10-s #m/s, and 122 days were needed to reach the largest penetration depth observed in the study, 420 ~m. (2) Water is much more effective at penetrating the boundaries than methanol, though in both solids, some methanol penetration was observed. For KCI, penetration by methanol was immeasurable for the first 43 days. (3) The NaCI specimens were penetrated by water about 5 times faster than the KCI specimens. (4) To a fLrSt approximation, the depth to which the NaCl specimens were penetrated by water is a linear function of time. The rest of the data are too limited in scope to analyze in a similar way with any degree of confidence. Typical penetration prof'fles are shown in Figs.4 and 5. That in Fig.4 is of NaCI exposed to water for 60 days. It is seen that the dihedral angle, defined as the total angle between the faces at the tip of the groove, is quite small, about 5". Fig.5 is a NaCl profile following 101 days of penetration by methanol. The groove is not nearly as narrow at its tip, d u e to the fact that the dihedral angle is much larger (note that the magnifications of the micrographs are different). The average measured value ~ was found to be 42 °. For the KCI specimens, the dihedral angles were much larger - about 120° for both water and methanol. Discussion On the surface, the large dihedral angles and very slow penetration rates observed in the KCI bicrystals suggest that liquid undercutting of grain boundaries cannot be very important during liquid enhanced creep of this material. It should be recalled, however, that the boundary misorientation of the KCI specimens was small, in fact, so small that a precise value could not be determined. All that is really known about the misorientation is that it is less than l °. The reason this is important is that other studies have revealed the degree and rate of grain boundary penetration can depend on boundary misorientation (8-10). Because penetration

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FIG.4. Penetration profile in NaC1 following a 60 day exposure to water.

FIG.5. Penetration profile in NaCI following a 101 day exposure to methanol.

is generally favored by large misorientations, the wetting and penetration behavior observed in the KC1 bicrystais is probably not representative of that which occurs in the majority of grain interfaces in a polycrystalline material. In this regard, the observations in the NaCI bicrystals probably provide a more realistic description of boundary wetting and liquid penetration during liquid enhanced creep of alkali halide salts. The 8* twist boundaries in these specimens were penetrated at greater rates, and the dihedral angles were much smaller. Water was found to penetrate much faster than methanol, but the most significant observation is that the dihedral angles for water and methanol are significantly different (5 ° for water ; 42 ° for methanol). This suggests that the previous conclusion that differences in creep rates are due to differences in solubility of the solid in the liquid is not entirely correct. To illustrate that differences in dihedral angle can affect penetration kinetics, it is useful to consider the mechanism for grain boundary grooving proposed by Mullins (3). It is not our intention to promote this particular mechanism as that which is important in our experiments; in fact, the experimental observation that the depth of penetration increases linearly with time is not consistent with it. Rather, we are interested in qualitatively showing how changes in dihedral angle can effect the rate of penetration. This is quite clear in the Mullins model, and we expect similar behavior for other mechanisms. According to the model, penetration occurs by diffusion of solid through the liquid (or through the solid-liquid interface), driven by the maintenance of surface tension equilibrium. For the case in which penetration is rate controlled by volume diffusion, the quasi-steady-state relation between the depth of the groove, d, and time, t, is d = l . O l m ( A t ) 113

(1)

A = Co~f22D/kT

(2)

Here, A is a kinetic constant defined by

where C o is the solubilty of the solid in the liquid, ? is the solid-liquid interfacial energy, f2 is the atomic or molecular volume, k is Boltzmann's constant, T is the absolute temperature, and m is related to the dihedral angle, ~b, through ~b = n - 2m

(3)

Because d is directly proportional to m, and m decreases as ~ increases, it is seen that large dihedral angles are associated with slower penetration rates. Thus, given that the mechanism of creep involves grain

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boundary penetration and undercutting in a manner similar to that of the Mullins mechanism, it is to be expected that at least part of the reason that creep in porous KC1 in is slower when the porosity is filled with methanol is due to the difference in wetting. Lastly, a comment is warranted concerning the possible mechanisms of penetration. Experimental results of grain boundary penetration studies are frequently interpreted in terms of the Mullins mechanism (11-13), which can be rate controlled by any of three separate processes. It is usual to distinguish among them by identifying the exponent, n, in the relation d ~ tn

(4)

The values consistent with Mullins' theoretical arguments are n = 1/2, 1/3 and 1/4, corresponding to rate controlling processes of dissolution of solid into the liquid (n = 1/2), volume diffusion of solid through the liquid (n ~- 1/3), and surface diffusion of solid through the solid-liquid interface (n --- 1/4) (3). The only data set from which a value of n can be derived with any degree of certainty is that for the penetration of NaC1 by water. The data in Fig.2 show that the rate of penetration is independent of time, or n = I. Thus, the penetration mechanism operating in this system is different from that proposed by Mullins. This is not entirely surprising from the the standpoint that the Mullins analysis is not strictly valid unless dihedral angles are large. It is notable that a similar mechanism appears to operate during the penetration of copper and nickel by liquid bismuth (14,15). In both these materials, the penetration rate is independent of time, and the dihedral angle is very small (effectively zero). However, the exact nature of this mechanism has not been identified (15). Acknowledgements This work was sponsored by the National Science Foundation, Division of Materials Research, under grant number DMR-8618095. The authors are grateful for this support. References 1. 2. 3. 4. 5. 6.

G. M. Pharr and M. F. Ashby, Acta Metall. 31, 129 (1983). Gh. R. Sheikh and G. M. Pharr, Acta Metall. 33, 231 (1985). W. W. Mullins, Trans. Metall. Soc. A I M E 218, 354 (1960). W. W. MuUins, J. Appl. Phys. 28, 33 (1957). P. K. Weyl, J. Geophys. Res. 64, 2001 (1959). W. F. Linke and A. Seidel, Solubilities, Inorganic and Metal-Organic Compounds, American Chemical Society, Washington, D.C. (1965). 7. H. Stephen and T. Stephen, Solubilities oflnorganic and Organic Compounds, Pergamon Press, New York (1964). 8. V. Yu. Traskin, Z. N. Skvortsova, V. I. Kukshev, N. V. Pertsov, and E. D. Shchukin, Colloid Journal USSR 44, 51 (1982). 9. M. L. Gimpl, A. D. McMaster, and N. Fuschillo, Mat. Sci. Res. 3, 259 (1966). 10. N. Fuschillo, M. L. Gimpl, and A. D. McMaster, J. Appl. Phys. 37, 2044 (1966). 1 I. W. M. Robertson, Trans. Metall. Soc. A I M E 242, 2139 (1968). 12. W. M. Robertson, Trans. Metall. Soc. A I M E 233, 1232 0965). 13. C. A. Steidel, Che-Yu Li, and C. W. Spencer, Trans. Metall. Soc. A I M E 230, 84 (1964). 14. E. Scheil and K. E. Schessl, Z. Naturforsch. 4a, 524 (1949). 15. R. F. Cheney, F. G. Hochgraf, and C. W. Spencer, Trans. MetaU. Soc. A I M E 221,492 (1961).