Yu.N. Osetsky

The classical transition state theory for calculating complex dissociation rates requires separation of time scales. It is shown here that this condition is satisfied for complexes with long-range interaction, such as an edge dislocation and a cluster of self-interstitial atoms in metallic materials. Hence, one can apply the equations for first-order reactions with a rescaled mean dissociation time and the cross-section of complex formation. The rescaling coefficient is the Eyring transmission coefficient. A general expression for this coefficient through the first two moments of the distribution function of dissociation times is derived. It is shown that it is equal to unity if the dividing surface between ‘bound’ and ‘free’ states is defined as that where the interaction energy is equal to the thermal energy.

Molecular dynamics (MD) simulation has been used to study the thermally activated mobility of clusters of self-interstitial atoms (SIAs) in Fe and Cu. Such clusters are formed in metals during irradiation with energetical particles and, according to the cascade production bias model, they play an important role in the microstructure evolution of metals under irradiation. An extensive simulation of clusters from 2 to 30 interstitials has been carried out for the temperature range ≍360-1200K using long-range interatomic pair potentials. The results show that clusters bigger than two SIAs are one-dimensionally mobile. Di-interstitials have two migration mechanisms depending on the temperature. At low temperature the mechanism is one-dimensional whereas at high temperature the probability of rotation and three-dimensional migration increases. It was found that in both metals the effective migration energy of clusters estimated via their jump frequency does not depend on the cluster size, although the cluster jump frequency decreases as the cluster size increases. The mechanism of cluster migration and problems of the treatment of one-dimensional mobility are discussed.

Diffusion of self-interstitial atoms (SIAs) has been studied in bcc-Fe and fcc-Cu using molecular dynamics and interatomic potentials of different types. The Fe potentials describe SIA configurations of different stability. The temperature dependence of the SIA diffusion mechanisms is qualitatively similar for both potentials. At high temperature the diffusion is three-dimensional via the dumbbell mechanism. The contribution of one-dimensional mechanism via the crowdion increases when temperature decreases. At low temperature (<300K) the diffusion mechanism depends on the stable configuration of the SIA.In fcc-Cu all the potentials reproduce the same stable configuration, namely the dumbbell. The migration mechanism is mainly a three-dimensional random walk via this dumbbell with small contributions from the crowdion at high temperature and a two-dimensional caging mechanism at low temperature.

The evolution of a copper precipitate with a diameter of 3·5 nm containing up to 2·0 at.% of vacancies at different temperatures has been studied by molecular dynamics. The diffusion coefficients were calculated for a single vacancy and vacancy concentrations inside the precipitate of 0·5, 1·0 and 2·0 at.%. The diffusion coefficients for each set of vacancies change with time. During the initial stage, the diffusion of populations of vacancies is very close to that of single vacancy diffusion. The initial stage is interpreted in terms of arrangements of vacancies which contain some clusters and are temperature dependent. For low temperatures (about 600 K) these clusters contain few vacancies whereas for high temperatures (about 1100 K) they are mainly formed by divacancies. These clusters are not stable and change in time quite rapidly. The evolution of larger precipitates with vacancies was followed under ageing at constant temperature followed by cooling at one of several constant speeds. For a precipiate of 7nm diameter containing 6·0 at.% of vacancies and cooled from 1000 K with a speed of 5·0 K ps⁻¹, small nuclei of fcc phase were formed. However, the complete bcc → fcc transformation of the precipitate was not obtained, and possible mechanisms of the phase transformation are discussed.