Papers by Author: Wolfgang Pfeiler

Abstract: Kinetic Monte Carlo (KMC) simulation is a valuable tool to investigate configu-ration changes in intermetallic compounds. The elementary process is the jump of an atomfrom a lattice site to a neighboring vacancy. In classical transition state theory the jump ratecontains the energy difference between the original equilibrium state and the saddle point (=transition) state. In traditional KMC the saddle point has mostly received rather careless treat-ment, setting it constant or relating it to the type of jumping atom. In the present work, saddlepoint heights were considered explicitly. Taking L1₂ ordered Ni3Al as an example, jump energyprofiles for various atom environments were calculated ab initio in relaxed configurations ofa 3x3x3 supercell, employing the Nudged Elastic Band method where necessary. From theseresults, effective ’pure’ saddle point heights were extracted. To show the effect on kinetics,simulations of order-order transitions were done with jump probabilities based on these results.When compared to the old assumption of constant saddle point heights, both overall kineticsand detailed jump statistics result considerably changed.

Abstract: “Order-order” relaxations driven by atomic migration in superstructures proceed in nonsteady- state of a system, which relaxes to the equilibrium atomic configuration. Hence, the corresponding studies are complementary to standard steady-state diffusion investigations. Two time scales operating in “order-order” relaxations in L12-ordered (Ni3Al) and L10-ordered (FePd, FePt) binary intermetallics were experimentally observed. On the other hand, in B2-ordered NiAl – known of a giant vacancy concentration, “order-order” relaxations appeared surprisingly slow. Definite relationships between the activation energies for diffusion ( ) D A E and “order-order” relaxations ( ) O O A E − were revealed: ( ) D A E < ( ) O O A E − in L12-type superstructure; ( ) D A E ³ ( ) O O A E − in L10- and in B2-type superstructures. Corresponding simulation studies elucidated the specific atomistic mechanism of the processes. It has been shown that different time scales active in “order-order” relaxations in L12 and L10-ordered systems follow from specific atomic-jump correlations, which result from non-steady-state conditions and particular superlattice geometries: the availability of easy diffusion channels. A model of “order-order” kinetics in NiAl as controlled by a triple-defect mechanism is proposed.