Cross slip and the plastic deformation of NaCl single and polycrystals at high pressure

The pressure dependence of the sigma-epsilon curve for NaCl single crystals deformed in compression has been determined for pressures ranging to 10 kb at room temperature. Under pressure the stress for the onset of the third stage of deformation, sigma(III), decreases linearly by 7 to 10% per kb, while the stress for stage II, sigma(II), is pressure independent; stage II is expected to vanish in the vicinity of 10-13 kb. If stage III is due to the onset of abundant, thermally activated cross slip of screw dislocations, then pressure evidently enhances cross slip. This is believed to result from a pressure-induced increase in the stacking-fault energy, gamma(110), for a/2 < 110 > dislocations on {110} planes. The ductility of NaCl polycrystals deformed in compression at room temperature is substantially increased under pressure and the flow stress, sigma, for a given strain is reduced by about 20% at 10 kb. It is believed that the increased ease of cross slip at high pressure accounts for the reduction of flow stress of the polycrystals. As only two of the primary {110} < 110 > slip systems in NaCl are independent, easier cross slip at high pressure will also contribute to increased ductility.