On the role of dislocations in heavily strained materials

As a tribute to professor Hans Weertman upon the occasion of his 70th birthday this paper deals with the role of dislocations in materials deformed at very high strain rates. In contrast to classical crystal deformation experiments which occur typically at strain rates of 10(-5) s(-1) to 10(-1) s(-1), ballistic deformation experiments involve strain rates of 10(4) s(-1) to 10(6) s(-1). These high strain rates yield completely different deformation mechanisms since defects can not move the same way at high rates as they do at low rates. Although the main focus will be on shock wave compaction of high T-c ceramic mater:als some comparison will be made also to metals and other cere lies deformed at intermediate high strain rates of the order of 1. s(-1). Because of the brittle nature of the high T-c superconducting ceramic materials plastic deformation of these materials is difficult. Its brittle character may however be suppressed by performing shock loading techniques. This paper describes TEM and HREM study of the dynamic compacted samples at various E/M values, i.e. the ratio between the mass density of the explosives and the mass density of the material to be compacted, ranging between 0.7 and 2.1. Apart from the well established {100} glide system, the role of a novel [110](1-1 0) and [010](1 0 0) glide systems are studied. All glide systems are found to interact with the ferroelastic domains of the material, each in a different way. Because a higher dislocation density is produced by dynamic compaction, one would expect that this technique also contributes to an increment in the pinning capacity of high T-c superconducting properties in an external magnetic field due to the generation of point defects. Here a comparison is made with in-situ NMR experiments on the deformation-induced generation of point defects in ceramic materials and metals. The spin-lattice relaxation rate provides information about the production rate of point defects during deformatio.