R.C. Pasianot

Ternary Fe–Cu–Ni many-body potential to model reactor pressure vessel steels: First validation by simulated thermal annealing

In recent years, the development of atomistic models dealing with microstructure evolution and subsequent mechanical property change in reactor pressure vessel steels has been recognised as an important complement to experiments. In this framework, a literature study has shown the necessity of many-body interatomic potentials for multi-component alloys. In this paper, we develop a ternary many-body Fe–Cu–Ni potential for this purpose. As a first validation, we used it to perform a simulated thermal annealing study of the Fe–Cu and Fe–Cu–Ni alloys. Good qualitative agreement with experiments is found, although fully quantitative comparison proved impossible, due to limitations in the used simulation techniques. These limitations are also briefly discussed.

Fe?Ni many-body potential for metallurgical applications

A many-body interatomic potential for the Fe–Ni system is fitted, capable of describing both the ferritic and austenitic phase. The Fe–Ni system exhibits two stable ordered intermetallic phases, namely, L10 FeNi and L12 FeNi3, that are key issues to be tackled when creating a Fe–Ni potential consistent with thermodynamics. A procedure, based on a rigid lattice Ising model and the theory of correlation functions space, is developed to address all the intermetallics that are possible ground states of the system. While controlling the ground states of the system, the mixing enthalpy and defect properties were fitted. Both bcc and fcc defect properties are compared with density functional theory calculations and other potentials found in the literature. Finally, the potential is thermodynamically validated by constructing the alloy phase diagram. It is shown that the experimental phase diagram is reproduced reasonably well and that our potential gives a globally improved description of the Fe–Ni system in the whole concentration range with respect to the potentials found in the literature.

A many body potential for α-Zr. Application to defect properties

An embedded atom model (EAM) interatomic potential that reproduces room temperature elastic behavior for α-Zr is developed. At variance with previous potentials from the literature, the present one predicts a self-interstitial relaxation volume consistent with the relatively low measured value. A critical review of results for static and dynamic properties of point defects is undertaken by comparing with predictions from other potentials and with experimental findings. This survey allows us to conclude that most well fitted EAM potentials give low vacancy migration energies and a basal crowdion as the stable interstitial configuration with fast 1D migration.

Iron chromium potential to model high-chromium ferritic alloys

We present an Fe–Cr interatomic potential to model high-Cr ferritic alloys. The potential is fitted to thermodynamic and point-defect properties obtained from density functional theory (DFT) calculations and experiments. The developed potential is also benchmarked against other potentials available in literature. It shows particularly good agreement with the DFT obtained mixing enthalpy of the random alloy, the formation energy of intermetallics and experimental excess vibrational entropy and phase diagram. In addition, DFT calculated point-defect properties, both interstitial and substitutional, are well reproduced, as is the screw dislocation core structure. As a first validation of the potential, we study the precipitation hardening of Fe–Cr alloys via static simulations of the interaction between Cr precipitates and screw dislocations. It is concluded that the description of the dislocation core modification near a precipitate might have a significant influence on the interaction mechanisms observed in dynamic simulations.

Point defects diffusion in α-Ti

Abstract A research on the statics and dynamics of vacancies and self-interstitials in model α-Ti lattices is carried out by means of computer simulation techniques. A comprehensive study beginning with the development of an appropriate interatomic potential up to the final evaluation of the anisotropy of the self-diffusion by both vacancy and self-interstitial mechanisms is undertaken. Experimental results on self-diffusion in α-Ti single-crystals are analyzed within the framework of the calculated diffusion constants for a vacancy mechanism. A strongly dominating basal diffusion for self-interstitials is predicted.

Computer simulation of SIA migration in bcc and hcp metals

A Molecular statics and dynamics study of self-interstitial diffusion mechanisms in model Fe, Mo (bcc) and Zr (hcp) is performed. Embedded-atom-method type interatomic potentials developed by the present authors are employed. Molecular dynamics simulations are carried out at constant energy and volume for different temperatures. Defect diffusion coefficients are computed and the migration jumps at both, low and relatively high temperatures, are qualitatively identified by simple visualization techniques. The relevance of crowdion-type interstitials is demonstrated in both hcp and bcc structures. Highly non-Arrhenius behavior is predicted for the basal diffusion in Zr. Also, the dynamically computed migration energies result roughly in half of the values computed using static techniques. This points to the difficulties of a straight application of Transition State Theory under conditions of moderately complex defect and/or energy barrier structures.

Interatomic potentials for alloys: Fitting concentration dependent properties

A detailed analysis of the embedded atom method and Finnis–Sinclair formalisms is performed, showing their limitations to fit concentration dependent properties of alloys. Two empirical extensions of the former methods, so-called two-band model and concentration dependent model, are analysed in depth, and their heuristic equivalence is shown. An algorithm is proposed for the two-band model, capable of fitting concentration dependent properties of the alloy, such as mixing enthalpy and bulk modulus. The algorithm is then applied to the Fe-Cr system, deriving two interatomic potentials that closely reproduce Fe-Crs complex mixing enthalpy.

Fitting interatomic potentials consistent with thermodynamics: Fe, Cu, Ni and their alloys

In computational materials science, many atomistic methods hinge on an interatomic potential to describe material properties. In alloys, besides a proper description of problem-specific properties, a reasonable reproduction of the experimental phase diagram by the potential is essential. In this framework, two complementary methods were developed to fit interatomic potentials to the thermodynamic properties of an alloy. The first method involves the zero Kelvin phase diagram and makes use of the concept of the configuration polyhedron. The second method involves phase boundaries at finite temperature and is based on the cluster variation method. As an example for both techniques, they are applied to the Fe–Cu, Fe–Ni and Cu–Ni systems. The resulting potentials are compared to those found in the literature and are found to reproduce the experimental phase diagram more consistently than the latter.

Kinetics versus thermodynamics in materials modeling: The case of the di-vacancy in iron

Monte Carlo models are widely used for the study of microstructural and microchemical evolution of materials under irradiation. However, they often link explicitly the relevant activation energies to the energy difference between local equilibrium states. We provide a simple example (di-vacancy migration in iron) in which a rigorous activation energy calculation, by means of both empirical interatomic potentials and density functional theory methods, clearly shows that such a link is not granted, revealing a migration mechanism that a thermodynamics-linked activation energy model cannot predict. Such a mechanism is, however, fully consistent with thermodynamics. This example emphasizes the importance of basing Monte Carlo methods on models where the activation energies are rigorously calculated, rather than deduced from widespread heuristic equations.

Computer simulation of (100) dislocation core structure in NiAl

A detailed study of the structure of (100) dislocations in NiAl is presented. Atomistic computer simulation is carried out with embedded atom interatomic potentials and the results are analysed using invariants of the strain tenser in the dislocation core region. No dislocation dissociation is observed in the simulations. The spreading of the cores in various crystallographic planes is identified and correlated with the strain fields given by anisotropic elasticity theory. Although the relaxed structures are closely related to those given by elasticity, non-elastic effects, originating mainly from the discrete nature of the lattice, are evident. Among them, asymmetries of the twinning-antitwinning type are found.