Bond order potentials for atomistic studies of dislocations and other extended defects in titanium aluminide.

摘要

The main theme of this thesis is development of potentials that are a necessary precursor for computer simulations of lattice defects in TiAl at atomic level. Extended defects, such as dislocations and stacking-fault type defects, play an important role in controlling the overall mechanical behavior of crystalline materials. Understanding their key structural properties at the atomistic level is a necessary prerequisite for gaining a deeper insight into the macroscopic scale deformation processes. The main focus of the computer simulation presented in this work is on studying the core structure of dislocations in L1₀ TiAl. Results of atomistic simulation of dislocation cores provide then a framework for analyzing the experimentally observed deformation modes in this material. An appropriate description of atomic interactions is needed for physically meaningful computer simulation studies. For this purpose a substantial part of this thesis was devoted to the development of the Bond order potentials (BOP) for TiAl. BOP represents a semi-empirical real-space tight-binding based scheme that is computationally efficient and at the same time capable to capture directionality of bonding arising from the unfilled d band in this material. The new BOP for TiAl was extensively tested and its ability to describe different bonding environments in both TiAl and Ti₃Al demonstrated. Using this potential the core structures and possible dissociations of both the ordinary 1/2⟨101] dislocation and the ⟨101] superdislocation were investigated together with the energies of stacking fault type defects participating in dislocation splitting. Our results for the ordinary 1/2⟨110 ] dislocations indicate the existence of a non-planar core for screw and 60 degrees mixed dislocations. This core structure renders these dislocations sessile. In the case of the $101$ it is observed to dissociate into partial dislocations and the two following configurations were found in our simulations 101=1/6 112+SISF+1/210 1+CSF+1/621 1 101=1/6112 +SISF111 +1/3201+S ISF111 1/6112 A planar configuration and a configuration spreading into two intersecting {lcub}111{rcub} type planes, which is, presumably, sessile.