STRUCTURE OF MISFIT DISLOCATIONS IN NIOBIUM- SAPPHIRE INTERFACES AND STRENGTH OF INTERFACIAL BONDING: AN ATOMISTIC STUDY

摘要

The formation of networks of misfit dislocations is investigated at the (0001)Al_2O_3, ‖(111)_Nb inter- face using a recently proposed approach (Vitek et al., Phil. Mag. A, l995, 71, 1219) which employs a very simple pair-potential to describe interaction between the metal and the substrate that contains the strength of interfacial adhesion as a parameter. The calculations demonstrate how the strength of bonding between the two materials decides both the form of the network and the atomic structure of the cores of these dislo- cations. At the same time it reveals that diffusion is essential for the formation of the observed triangular network of 1/2〈111〉 dislocations. The calculated structures are then used to investigate related high resol- ution electron microscope (HREM) images using a multislice technique. In these simulations translational symmetry along the electron beam was not assumed but for each slice of material along the beam different sub-structures were used. This allowed us to investigate fully the effect of the dislocation intersections upon the images of the dislocation cores. Their effect is, indeed, considerable if an intersection is in the region producing the image but if not, the images of the cores of misfit dislocations are affected only marginally and HREM can capture fine details of the core structure. A direct comparison of an experimental obser- vation in Mayer and co-workers (Acta metall. mater., 1992, 40, S217; Scripta metall. mater., 1994, 31, 1097) with the present simulations demonstrates this ability.