Abstract:
Ultra high temperature ceramics (UHTCs), which typically comprise carbides, nitrides, and borides, are a class of materials associated with high melting temperatures and high hardness. These materials offer a range of mechanical responses, from being very brittle to exhibiting significant plasticity as a function of composition and loading temperature. The purpose of this investigation is to characterize the slip mechanisms in ZrB2, where slip has been inferred but not definitively quantified. This work confirmed prior studies that dense dislocation arrays, with straight dislocation lines, exist under room temperature indents and demonstrates that such networks are highly localized to the load region. For elevated temperature deformation, ZrB2 has been reported to have a drop in flexural strength from 390 MPa at 1200 °C to 110 MPa at 1600 °C. Dynamical electron diffraction and image analysis confirmed basal, pyramidal, and prismatic slip which is rationalized by ZrB2’s hexagonal close packed c/a lattice parameter ratio of 1.11, which is less than the ideal ratio of 1.63. The dislocation densities prior to and after this flexural strength drop were 1.3 x 1013 m-2 and 1.0 x 1013 m-2, respectively. This indicates that the reduction in strength was not significantly associated with increased dislocation nucleation but rather stress relaxation.
