Atomic structure of short range ordered alloys

Short range chemical order is a condition that often prevails in alloys - notably at temperature higher than the critical ordering temperature (Tc) for the formation of long range ordered (LRO) structures, or at lower temperatures as a precursor to the formation of LRO structures or as a consequence of irradiation. In most cases of ordering, the occurrence of LRO can be explained on the basis pair-wise interactions between the two atomic species. Structure determination is also a fairly straightforward process for LRO structures. However, thermodynamic models, as well as techniques like HRTEM, often fall short of explaining the origin and the nature of SRO in alloys. Monte Carlo simulations of the atomic structures of SRO states in Ni-Mo-based alloys and Al-richγ-TiAl alloys will be focus of this presentation. The role of k-space thermodynamic quantities like V(k) in the evolution of SRO and the attendant diffuse intensity distribution in the SAED patterns will be emphasized. The diffuse intensity distribution itself serves as an invaluable aid in the choice of pair interaction parameters for the simulation. Of particular interest in this context is the occurrence of the paradoxical short range ordered long period and quasiperiodic superlattice structures in Al-rich γ-TiAl alloys. (author)