Two procedures were developed to fit interatomic potentials of the embedded-atom method (EAM) form and applied to determine a potential which describes crystalline and liquid iron. While both procedures use perfect crystal and crystal defect data, the first procedure also employs the first-principles forces in a model liquid and the second procedure uses experimental liquid structure factor data. These additional types of information were incorporated to ensure more reasonable descriptions of atomic interactions at small separations than is provided using standard approaches, such as fitting to the universal binding energy relation. The new potentials (provided herein) are, on average, in better agreement with the experimental or first-principles lattice parameter, elastic constants, point-defect energies, bcc–fcc transformation energy, liquid density, liquid structure factor, melting temperature and other properties than other existing EAM iron potentials.

http://homepages.ed.ac.uk/graeme/Fe_PhilMag.pdf

Development of new interatomic potentials appropriate for crystalline and liquid iron

Nanometre-scale contact experiments1,2,3,4,5,6 and simulations7,8,9,10 demonstrate the potential to probe incipient plasticity—the onset of permanent deformation—in crystals. Such studies also point to the need for an understanding of the mechanisms governing defect nucleation in a broad range of fields and applications. Here we present a fundamental framework for describing incipient plasticity that combines results of atomistic and finite-element modelling, theoretical concepts of structural stability at finite strain, and experimental analysis. We quantify two key features of the nucleation and subsequent evolution of defects. A position-sensitive criterion based on elastic stability determines the location and character of homogeneously nucleated defects. We validate this stability criterion at both the atomistic and the continuum levels. We then propose a detailed interpretation of the experimentally observed sequence of displacement bursts to elucidate the role of secondary defect sources operating locally at stress levels considerably smaller than the ideal strength required for homogeneous nucleation. These findings provide a self-consistent explanation of the discontinuous elastic–plastic response in nanoindentation measurements, and a guide to fundamental studies across many disciplines that seek to quantify and predict the initiation and early stages of plasticity.

Atomistic mechanisms governing elastic limit and incipient plasticity in crystals

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...

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

We present the derivation of an interatomic potential for the iron–phosphorus system based primarily on ab initio data. Transferability in this system is extremely problematic, and the potential is intended specifically to address the problem of radiation damage and point defects in iron containing low concentrations of phosphorus atoms. Some preliminary molecular dynamics calculations show that P strongly affects point defect migration.

http://iopscience.iop.org/article/10.1088/0953-8984/16/27/003/pdf

Development of an interatomic potential for phosphorus impurities in α-iron

We report a detailed ab initio study of the stability and migration of self-interstitial atoms (SIAs) and di-interstitials (di-SIAs) in $\ensuremath{\alpha}$-Fe. The $⟨110⟩$ dumbbell is confirmed to be the most stable SIA configuration, 0.7 eV below the $⟨111⟩$ dumbbell. The lowest-energy migration path corresponds to a nearest-neighbor translation-rotation jump with a barrier of 0.34 eV. The most stable configuration for di-SIAs consists of $⟨110⟩$ parallel dumbbells. Their migration mechanism is similar to that for SIAs, with an activation energy of 0.42 eV. These results are at variance with predictions from existing empirical potentials and allow one to reconcile theory with experiments.

Stability and Mobility of Mono- and Di-Interstitials in α-Fe

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

A molecular dynamics study of displacement cascades in α-iron

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.

Computer simulation of defect production by displacement cascades in metals

Displacement cascades with wide ranges of primary knock-on atom (PKA) energy and mass in iron were simulated using molecular dynamics. New visualisation techniques are introduced to show how the shock-front dynamics and internal structure of a cascade develop over time. These reveal that the nature of the final damage is determined early on in the cascade process. We define a zone (termed ‘spaghetti’) in which atoms are moved to new lattice sites and show how it is created by a supersonic shock-front expanding from the primary recoil event. A large cluster of self-interstitial atoms can form on the periphery of the spaghetti if a hypersonic recoil creates damage with a supersonic shock ahead of the main supersonic front. When the two fronts meet, the main one injects atoms into the low-density core of the other: these become interstitial atoms during the rapid recovery of the surrounding crystal. The hypersonic recoil occurs in less than 0.1 ps after the primary recoil and the interstitial cluster is form...

https://hal.archives-ouvertes.fr/hal-00581022/document

A.F. Calder

Papers

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

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

A molecular dynamics study of displacement cascades in α-iron

Computer simulation of defect production by displacement cascades in metals

On the origin of large interstitial clusters in displacement cascades