Defects in boron carbide before and after neutron irradiation

Abstract

Many crystalline compounds contain regular geometrical arrangements of atoms which are so stable that the presence and motion of defects such as dislocations are expected to leave these units more or less intact. In boron and most of its compounds, the stable units are icosahedral groups of twelve boron atoms and in boron carbide, which is rhombohedral with rhombohedral angle 65° 36′, the B 12 icosahedra are arranged in an array which is almost a f.c.c. lattice, as a result of which, fault surfaces analogous to stacking faults in close-packed metals are observed. The small departure from cubic symmetry is, however, sufficient to impose a distinction between rhombohedral planes, which are hosts for translation twin boundaries created by the dissociation of glide dislocations, and the basal plane, which is believed to contain Frank partial dislocation loops resulting from the agglomeration of point defects introduced during neutron irradiation. Another important consequence of the rhombohedral distortion is that the atomic displacements produced by the leading and trailing partials in an extended dislocation are different. In physical terms, both refinements of the close-packed cubic case arise from the presence of the carbon atoms. The analogy to close-packed metals is carried over to the relationship between translation twins and orientation twins; the dissociation of a glide dislocation in boron carbide is accompanied by rotation of C 3 chains which thread the slip plane and this rotation both may and may not be sufficient to render the translation twin boundary region identical to a thin orientation twin.