Defect formation in L1 2 intermetallics

Intermetallic alloys play an important role in today`s material science and engineering. Their attractive and unusual high temperature behavior is based on the. existence of long-range order. Physical and mechanical properties of these materials depend strongly on the occurrence of lattice defects. It is by far not straightforward to derive important physical parameters for such defects from experiment, This information is needed for a fundamental understanding of these processes and optimization of these materials. State-of-the-Art methods will be applied and adopted to gain information needed by experimentalists and engineers about thermal effects which are out of reach with simplified model calculations. With the advent of powerful computing facilities and suitable theoretical methods it is now possible to tackle these basic problems on a atomistic level with first-principles methods. These methods are based on fundamental Quantum Mechanics. Physical properties can be derived without empirical parameters and compared to real materials. Thermodynamical models are needed for a realistic, temperature dependent description of defect formation utilizing the resuls of first-principles calculations. Such models have to be adopted or developed for the present project in combination with the performance of large-scale computations for the first-principles data bases.

In this project we investigated the energies needed for atom jumps in certain ordered alloys. Intermetallic compounds (intermetallics) are a technologically important class of alloys. Contrary to what is usual, their can get stronger as temperature rises; Another of their favourable properties is enhanced corrosion resistance. These qualities depend, however, on an ordered configuration of the atoms. Going along a row of atoms, there is for instance a sequence of Ni-Al-Ni-Al-. and so on. The amount of order changes with temperature. For the industry it is necessary to know the stability of a material at a given temperature, and the characteristics of transformation in a production process. For the materials physicist, this translates into the kinetics of ordering and disordering, which in these alloys happens by the jumps of single atoms into neighbouring vacant lattice sites. For this reason, we want to know how much energy is needed to create a vacant lattice site, and how much energy (activation energy) is needed for an atom to jump into the vacancy . These energies can be calculated by first-principle (= ab initio) calculations. This procedure, originally developed by Nobel prize winner W. Kohn, is based on quantum mechanics and needs substantial computer resources. Ideally nothing is needed besides what atoms, and in what proportions, are involved. In the framework of this project all calculations were done by well-established ab initio methods (e.g. VASP, developed in Vienna in the group of J. Hafner), In the first stage of the project, the energies of vacancies and antisites (which are sites occupied by the "wrong" atom in the ordered pattern) in Ni3 Al were computed. Next, the energy cost of an atom moving to an adjacent vacancy was investigated for different situations. As expected from atom size considerations, an Al atom was found to be in an enhanced state of energy after having jumped from its "regular" position among smaller Ni atoms to an antisite position where it is surrounded by larger Al atoms. This spawned a cooperative model of atom jumps: Blocking the back jump by a Ni atom entering into the vacancy retains the Al in a state of higher energy. An antisite pair has thus been created. This kind of cooperative (simultaneous) movement of atoms was considered in a separate calculation. Comparatively high activation energies were found for this process corresponding those found in experiments (Ordering kinetics observed by measuring the electrical resistance of the sample). The investigation was subsequently extended so as to include a set of chemically analogous ordered alloys. Finally, the influence of different atomic surroundings on the atom jump rates was studied.