JOURNAL OF MATERIALS SCIENCE l0 (1975) 427-435 The influence of pressurization-induced dislocations on the plastic deformation of LiF and NaCI monocrystals R. A. EVANS*, A. S. WRONSKI, B. A. W. REDFERN School of Materials Science, University of Bradford, West Yorkshire, UK Single crystals of LiF containing voids and of NaCI containing Na2SO, precipitates were pressurized to introduce dislocations in the vicinities of the discontinuities and subsequently compressed along (1 00) at room temperature. The yield stress was raised in both materials; additionally, in LiF discontinuous yielding and easy glide were suppressed and work hardening rate increased by the pressurization-induced dislocations. Following pressurization at 0.85 GN m -=, for example, the 0.1% shear flow stress of LiF was doubled to ~ 4 MN m-2 and stage II work hardening rate quadrupled to N 180 MN m -2, Pressurization of NaCI above 0.6 GN m-~ resulted in an increase in the 0.1% flow stress from ~ 1.2 to ~ 2.0 MN m-L If the slip bands in LiF were initiated by a precompression, pressurization prevented the broadening of these fresh slip bands during subsequent plastic flow. Deformation now took place at a higher stress both in LiF and NaCI. These effects resemble in some ways latent hardening in that oblique as well as conjugate dislocation intersections must take place to continue the deformation. In contrast to latent hardening data, the strain hardening rate was increased in LiF and was approximately proportional to the pressurization-induced dislocation density. This ratio, 5 to 6 dyne per dislocation, is in fair agreement with two sets of independent calculations reported by Gilman and Johnston. The results suggest, therefore, that in the present case also hardening may be due to defects left in the wakes of pressurization-induced moving dislocations. 1. Introduction When single crystals possessing the rock-salt structure are compressed along (100), four <110) {110} slip systems are equally stressed [1-3]. Usually, however, slip commences on two orthogonal systems - the primary pair - and when one of these becomes inactive a linear stress-strain curve results [1]. The two inactive systems, oblique to the first pair, are nevertheless hardened by the primary slip; this phenomenon is called latent hardening. It has been investigated in the alkali halides [3-7] simply by compressing single crystals along one <100) direction and then measuring the flow stress on the previously inactive, i.e. latent, oblique system by com- pressing in a second (hard) cube direction parallel to the primary planes. The increase in flow stress, by factors of up to 14 in LiF and 9 in NaC1 [3], has been associated with the interaction of glide dislocations in the latent slip system with dipoles and dislocations in the primary system, tentatively according to the reaction [6, 7]: a [1101 + a [T0i]__ a [01i] (1) ~ ~ 9 The work hardening rates on the latent system in both NaC1 and LiF were approximately zero and Li [6] associated the propagation of a dislocation band in these circumstances to that of a Liiders band. It was found, however, that an enhanced rate of hardening, at a reduced flow stress, results if the latent hardened crystal is annealed. He associated this phenomenon with the dislocation distribution after recovery: reduced dislocation density and dispersed dis- location bands. Surprisingly for LiF, uniform *Present address: Synthetic Fibres Division, Courtaulds Ltd, Coventry CV6 5AE, UK. 9 1975 Chapman and Hall Ltd. 3 427