Plastic anisotropy and cracking at hardness impressions in single crystal ammonium perchlorate

Abstract

Diamond pyramid (Vickers) microindentation hardness testing was used to investigate the plastic deformation and cracking behaviors of single crystal ammonium perchlorate (AP). Traces of slip planes and cracks formed at hardness impressions in ( 210) and (001) cleavage surfaces were analyzed on a stereographic projection basis. Indentation-forming (primary deformation) and volume-accommodating (secondary deformation) slip systems were identified, including a number of systems not reported previously. Cleavage cracks were observed to form in the regions of greatest plastic deformation and are explained in terms of specific dislocation pile-up reactions at intersecting slip systems that are strongly surface orientation dependent. Also polarized light microscopy observations have revealed far-reaching residual strains extending beyond crack tips at impressions put into the ( 210) surface, thus giving additional evidence of the wedge-opening action of dislocation pile-ups. Prominent two-fold symmetric troughs extending from impressions put into the (001) surface are attributed to the unique characteristic of outward movement by indentation-forming dislocations. An important connection is made between the identified orthorhombic slip systems and those representative of the higher-temperature rocksalt (cubic) structural form of AP.