D.J. Bacon

Computer simulation of point defect properties in dilute Fe-Cu alloy using a many-body interatomic potential

Abstract The behaviour of copper atoms in dilute solution in α-iron is important for the microstructural changes that occur in ferritic pressure vessel steels under fastneutron irradiation. To investigate the properties of atomic defects that control this behaviour, a set of many-body interatomic potentials has been developed for the Fe—Cu system. The procedures employed, including modifications to ensure suitability for simulating atomic collisions at high energy, are described. The effect of copper on the lattice parameter of iron in the new model is in good agreement with experiment. The phonon properties of the pure crystals and, in particular, the influence of the instability of the metastable, bcc phase of copper that precipitates during irradiation are discussed. The properties of point defects have been investigated. It is found that the vacancy has lower formation and migration energy in bcc copper than in α-iron, and the self-interstitial atom has very low formation energy in this phase of coppe...

A comparison of displacement cascades in copper and iron by molecular dynamics and its application to microstructural evolution

Abstract The use of molecular dynamics simulation and improved many-body potentials make it possible to compare displacement cascade evolution in different materials. However, the extreme variability between individual cascades requires multiple simulations at nominally identical conditions of temperature and energy in order to assure that the comparison is statistically valid. We describe such a comparison of copper and iron in this paper. Over 600 cascades have been investigated, with simulation energies in the range 60 eV to 10 keV and temperatures from 100 to 900 K. The evolution of the cascades is similar in both materials, with the development of a highly disordered core and the emission of focusons and replacement collision sequences during the collisional phase of the cascade. The majority of vacancy-type defects are found in the cascade core when in-cascade recombination is complete, while the interstitial-type defects tend to be distributed around the periphery of this region. The final defect structure has been characterized by the total surviving defect fraction, and the number and size of the point defect clusters produced. Since these parameters have significant implications for the nuclear industry in its assessment of radiation damage, we show how they depend on cascade energy and temperature. To illustrate their importance, we provide an example of how the molecular dynamics results can be used in a rate theory model of ferritic steel embrittlement.

An atomic-level model for studying the dynamics of edge dislocations in metals

A model for simulating the dynamic behaviour of edge dislocations in metals at the atomic level is presented. The model extends an earlier approach based on an array of edge dislocations periodic in the Burgers vector direction and allows the external action (either shear strain or resolved shear stress), crystal energy, plastic displacement and dislocation position and velocity to be determined unambiguously. Two versions of the model for either static or dynamic conditions, i.e. zero or non-zero temperature, are described. The model is tested for elastic response of a perfect crystal and the atomic properties of a ½111 edge dislocation in a model of bcc Fe. Several examples of dislocation glide behaviour and dislocation–obstacle interactions at zero and non-zero temperature are presented and discussed.

A molecular dynamics study of displacement cascades in α-iron

Abstract The mechanisms of defect production in displacement cascades in α-iron have been investigated by computer simulation. Cascades of up to 5 keV in energy have been simulated by molecular dynamics in crystals with atomic interactions described by a many-body potential. The effects of lattice temperature have been studied by using block temperatures of either 100 or 600 K. 80 cascades have been modelled overall. The morphology of cascades during the collisional phase changes at about 1–2 keV, due to the collective nature of atomic displacements at higher energy. This transition is reflected in the relaxation time during the subsequent recombination phase, and it also decreases the efficiency factor for defect production. This factor is similar in size to that obtained from recent modelling of copper, an fcc metal. Although the cascade zone contains a large number of displaced atoms, true melting was not observed in α-Fe, and vacancy clustering did not occur in the thermal spike phase. Interstitial clustering has been analysed, and found to be less pronounced than in copper. One large cluster was observed to grow by interstitial movement during the thermal spike, and visual analysis has shown that it formed a perfect dislocation loop: it was not nucleated by the Eyre-Bullough mechanism, however. Statistics on the cascade parameters are presented, and comparisons with work on other crystal structures are drawn where possible.

The primary damage state in fcc, bcc and hcp metals as seen in molecular dynamics simulations

Recent progress in the use of molecular dynamics (MD) to investigate the primary state of damage due to displacement cascades in metals is reviewed, with particular emphasis on the influence of crystal structure. Topics considered include the effect on defect formation in pure metals and alloys of primary knock-on atom (PKA) energy and irradiation temperature. An earlier empirical relationship between the production efficiency of Frenkel pairs and cascade energy is seen to have wide validity, and the reduction in efficiency with increasing irradiation temperature is small. Crystal structure has little effect on the defect number. In terms of the development of models to describe the evolution of radiation damage and its role in irradiation-induced changes in material properties, the important parameters are not only the total number of Frenkel defects per cascade but also the distribution of their population in clusters and the form and mobility of these clusters. Self-interstitial atoms form clusters in the cascade process in all metals, and the extent of this clustering does appear to vary from metal to metal. Vacancy clustering is also variable. The mobility of all clusters depends on their dislocation character and thus on the crystal structure and stacking fault energy. It is shown that computer simulation can provide detailed information on the properties of these defects.

Molecular dynamics computer simulations of displacement cascades in metals

Abstract MD simulations of displacement cascades have become important in the study of atomic-scale processes in radiation damage. Here, we review recent advances and discuss results on a variety of processes in pure metals and alloys. The MD procedures, and the many-body interatomic potentials on which most recent simulations are based, are described, and then the general features of cascades in metals are reviewed. The ways in which the cascade state during the thermal-spike phase can be investigated are presented, and it is found that a liquid-like core is generated for cascade energies above 1 to 2 keV. This is shown to have important consequences for atomic mixing, disordering in ordered alloys, and vacancy clustering. Frenkel-pair production efficiency in the primary damage state at the end of the cascade process is found to be well below the NRT theoretical value in all the metals and alloys modelled to date. Clustering of interstitials is a persistent feature of cascade simulations for all pure metals. The mechanisms underlying these results are discussed, and it is observed that the simulations are in generally good agreement with what has been found by experiment.

One-dimensional atomic transport by clusters of self-interstitial atoms in iron and copper

Atomic-scale computer simulation has been used to study the thermally activated atomic transport of self-interstitial atoms (SIAs) in the form of planar clusters in pure Cu and f-Fe. There is strong evidence that such clusters are commonly formed in metals during irradiation with high-energy particles and play an important role in accumulation and spatial distribution of surviving defects. An extensive study of the mobility of SIA clusters containing two to 331 interstitials has been carried out using the molecular dynamics simulation technique for the temperature range from 180 to 1200 K. The results obtained show that clusters larger than three to four SIAs are one-dimensionally mobile in both Cu and Fe. Large clusters of more than 100 SIAs in Cu and 300 SIAs in Fe have significantly reduced mobility. The problem of describing one-dimensional (1D) motion in three-dimensional space is discussed. An attempt is made to describe the mobility of SIA clusters within the approximation of 1D diffusion. For clusters in both metals the effective migration energy of 1D diffusion as estimated via the jump frequency of the cluster centre of mass is found to be independent of the number of SIAs in the clusters, although the cluster jump frequency decreases with increasing cluster size. Mechanisms of 1D mobility of interstitial clusters are discussed.

A new model for {1012} twin growth in hcp metals

Abstract Computer simulation has been used to study the interaction of a perfect, basal dislocation with a {1012} twin boundary in a hcp metal for the situation where the 1/3{1120} Burgers vector is inclined at 60° to the interface. It is found that slip is not transferred from one crystal to the other with a residual dislocation left at the interface. Instead, the matrix dislocation decomposes into interfacial defects. We show that as a result of this decomposition the matrix dislocation becomes a new source of twinning dislocations that produce twin growth when the appropriate stress is applied to the crystal. The mechanism described does not require twinning dislocations to multiply by a pole process.

Computer simulation of the structure and mobility of twinning disclocations in H.C.P. Metals

Abstract The properties of twinning dislocations in [10 1 1}, {11 1 2}, {11 2 2} twin boundaries in hexagonal-close-packed metals have been investigated by atomic-scale computer simulation. Care has been exercised (by use of the concept of bicrystal structure maps) to ensure that all possible stable interface structures have been modelled and the work extends the research reported earlier by us [6] in several ways. First, we have now treated the important case of the {10 1 1} twin Second, the dependence of dislocation energy on the atomic structure of the core has been investigated. Third, the mobility of these interfacial dislocations has been examined by computing the critical resolved shear strain for glide. It has been found that the twinning dislocations corresponding to the {10 1 2} and {11 2 1} twins observed in practice are highly glissile, wheras those for the {10 1 1} and {11 2 2} modes are not. These effects are deduced to be related to the atomic structure of the core (and, in particular, to its width rather than height), and are found to be consistent with the nature of deformation twinning reported for the h.c.p. metals. For {10 1 1} twinning, however, the dislocation of lowest energy and highest mobility does not correspond to the mode observed in practice, implying that twin nucleation may possibly be a controlling factor in that case.

Computer simulation of defect production by displacement cascades in metals

Abstract MD simulations of displacement cascades in a variety of pure metals and alloys of different crystal structure are reviewed. For low recoil energies, these simulations have provided extensive results on the orientation-dependence and mean value of the displacement threshold energy in different crystal systems, and this information is tabulated. Large numbers of recoils have been simulated at true cascade energies, and the results show that Frenkel-pair production at the end of the cascade process is well below the NRT theoretical value in all metals and alloys. A new empirical relationship between Frenkel-pair number and damage energy is proposed. In contrast with this, antisite production efficiency in ordered alloys increases with increasing energy. Clustering of interstitials is a feature of cascade processes for all metals, but the degree of clustering is material-dependent. Atomic mixing in cascades is strongly dependent on cascade energy and is shown to be independent of crystal structure. The mechanisms underlying these results are discussed, particularly in relation to the highly disordered zone formed at the end of the thermal spike.