Interatomic bonding and plastic deformation in iridium and molybdenum disilicide

Interatomic bonding in the refractory transition metal iridium and transition metal-based intermetallic compound MoSi2 is of mixed metallic and covalent character. Such interatomic bonding is associated with the strong angular dependencies of covalent bonds and these affect significantly the structures and properties of the extended defects, such as dislocations and grain boundaries, that control plastic deformation and fracture in these materials. Bond-order potentials (BOPs) have been developed for iridium and the molybdenum silicides that capture accurately the angular character of bonding. A many-body repulsive term in the expression for the total energy allowed the negative Cauchy pressures of both iridium and MoSi2 to be fitted. The environmental dependencies of bond integrals in the open C11b crystal structure of MoSi2 were introduced explicitly via analytic screening functions derived using non-orthogonal tight-binding theory. The BOPs were constructed by fitting to a small set of experimental and ab initio calculated data. Rigorous testing of the BOPS showed that they are very accurate and transferable to environments substantially different from those used in their construction. Hence, they are eminently suitable for the atomistic simulation of extended defects. Atomistic simulation of the screw dislocation in iridium found a metastable, non-planar configuration for the core in addition to a glissile planar core that corresponds to dissociation into Shockley partials. Stress applications showed that transformations between these two core structures are driven primarily by applied stress and give rise to a mechanism for cross-slip that does not require thermal activation. Such athermal cross-slip then leads to an unusually high rate of dislocation multiplication and extensive plasticity owing to the unusually high frequency of the formation of Frank-Read sources. It is proposed that this process accounts for the high dislocation densities accumulated homogeneously in plastically deformed iridium and the associated strong work hardening found experimentally. This strong work hardening is linked intimately with the tendency of iridium to undergo brittle transgranular cleavage since the mean free path of dislocations can become so small that the material cannot relax stress concentrations around cracks by dislocation mediated plasticity.