Dislocation slip systems in pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX)

An exam ination has been made of the orientation dependence of microhardness determined using a Knoop indenter on the principal habit faces of RDX ({210}) and PETN ({110}). The variations found (RDX , 32-44 kg mm ^{- 2} ; PETN, 13-24 kg mm ^{- 2} ) were consistent with the crystallographic symmetry of the solids and confirm previous estimates obtained using a Vickers indenter. On the basis that the principal deformation mechanism is dislocation slip and that the variation in hardness reflects the orientation of the dominant slip systems to the surface and hence to the faces of the indenter, a theoretical assessment was made using the models of F. W. Daniels and C. G. Dunn and C. A. Brookes, J. B. O’Neill and B.A.W. Redfern of the expected variation in hardness due to this cause. Satisfactory agreement with experiment was obtained using slip systems of the types (010) [001] and {021} [100] for RDX and (110) [111]-type for PETN . This is the first time that this type of analysis has been attempted for such highly anisotropic systems. These assignments were confirmed by etching and by X-ray topographic analysis of the distribution of dislocation alignments around the hardness impressions. The difference in mechanical behaviour of these solids and its possible role in sensitivity to detonation is discussed on the basis of the relative ease of migration of these dislocation systems.