Computer simulations of disordering and amorphization kinetics in intermetallic compounds

Molecular dynamics computer simulations on three intermetallic compounds, Cu,Au, Ni,AI and NiAl, have been performed to investigate the kinetics of the disordering and amorphization processes. These systems were chosen because reliable embedded atom potentials were developed for the constituent species and their alloys, and also because extended experimental results are available for them. Previous simulations of collision cascades with 5 keV Cu and Ni primary knock-out atom (PKA) showed a significant difference between the evolution of the short range order (SRO) and the crystalline order (GO) parameters in all of the intermetallics: a complete loss of the crystalline structure and only partial chemical disorder in the core of the cascade [T. Diaz de la Rubia et al., Phys. Rev. B 47 (1993) 11483; M. Spaczer et al., Phys. Rev. B 50 (1994) 13204]. The present paper deals with the simulation of the amorphization process in NiAl by 5 and 15 keV Ni PKAs. The kinetic energy of the atoms in the simulated systems was removed on different time scales to mimic strong or weak coupling between electrons and phonons. No evidence of amorphization was found at the end of the cascades created by the 5 keV recoils. However, the 15 keV PKA events showed that (i) in the no-coupling case the system evolved to a highly disordered state, (ii) an amorphous region with about 100 non-lattice atoms was found in the case of weak coupling, (iii) the locally melted and recrystallized region collapsed to a small dislocation loop when medium coupling was used and (iv) a highly ordered state resulted in the case of strong coupling.

Evidence of Amorphization in Molecular-Dynamics Simulations on Irradiated Intermetallic Nial

Molecular-dynamics (MD) computer simulations on three intermetallic compounds, NiAl, Ni3Al, and Cu3Au have been performed to investigate the kinetics of the disordering and amorphization processes. These systems were chosen because the embedded atom-type potentials work well for these materials and also because they have experimentally different amorphization behavior. Previous simulations of collision cascades with 5-keV Ni and Cu primary knockout atoms (PKA) have shown a complete loss of the crystalline structure but only partial chemical disorder in the core of the cascade. Dynamical melting simulations of the liquid phase provided significant differences in the short-range order between the three intermetallics, namely: (i) Cu3Au is close to an ideal mixture, Ni3Al is the most ordered liquid, and the disordering level of NiAl lies between the two A(3)B intermetallics, and (ii) NiAl has the fastest and Cu3Au has the slowest kinetics in the disordering process after a sudden increase of temperature. For details see Spacer et al. [Phys. Rev. B 50, 13 204 (1994)]. In the present paper we look for the conditions to induce amorphization in MD cascades in NiAl by 5 and 15 keV PKA's. The kinetic energy of the atoms in the simulated systems is removed on different time scales as a way to mimic strong or weak coupling between electrons and phonons. No evidence of amorphization is found at the end of the cascades created by 5 keV recoils. However, the 15 keV PKA events show that (i) in the no-coupling case the system evolves to a highly disordered state, (ii) an amorphous region with about 100 nonlattice atoms is found in the case of weak coupling, (iii) the locally molten and recrystallized region collapses to a small cluster containing 25 atoms when medium coupling is used, and (iv) a highly ordered state results in the case of strong coupling. Amorphization in MD cascades is reported. A 15 keV recoil event with weak electron-phonon coupling is also shown for Ni3Al. The final structure of the Ni3Al system shows no amorphous cluster formation in agreement with experimental results.

1995

Analysis of nano-sized irradiation-induced defects in Fe-base materials by means of small angle neutron scattering and molecular dynamics simulations

Gang Yu

Thermonuclear fusion of light atoms is considered since decades as an unlimited, safe and reliable source of energy that could eventually replace classical sources based on fossile fuel or nuclear fuel. Fusion reactor technology and materials studies are important parts of the fusion energy development program. For the time being, the most promising materials for structural applications in the future fusion power reactors are the Reduced Activation Ferritic/Martensitic (RAFM) steels for which the greatest technology maturity has been achieved, i.e., qualified fabrication routes, welding technology and a general industrial experience are almost available. The most important issues concerning the future use of RAFM steels in fusion power reactors are derived from their irradiation by 14 MeV neutrons that are the product, together with 3.5 MeV helium ions, of the envisaged fusion reactions between deuterium and tritium nuclei. Indeed, exposure of metallic materials to intense fluxes of 14 MeV neutrons will result in the formation of severe displacement damage (about 20-30 dpa per year) and high amounts of helium, which are at the origin of significant changes in the physical and mechanical of materials, such as hardening and embrittlement effects, for instance. This PhD Thesis work was aimed at investigating how far the Small Angle Neutron Scattering (SANS) technique could be used for detecting and characterizing nano-sized irradiation-induced defects in RAFM steels. Indeed, the resolution limit of Transmission Electron Microscopy (TEM) is about 1 nm in weak beam TEM imaging, and it is usually thought that a large number of irradiation-induced effects have a size below 1 nm in RAFM steels and that these very small defects actually contribute to the irradiation-induced hardening and embrittlement of RAFM steels occurring at irradiation temperatures below about 400°C. The aim of this work was achieved by combing SANS experiments on unirradiated and irradiated specimens of RAFM steels with Molecular Dynamics (MD) simulations of main expected nano-sized defects in irradiated pure Fe and Fe-He alloys, as model materials for RAFM steels, and simulations of their corresponding TEM images and SANS signals. In particular, the SANS signal of various types of defects was simulated for the first time. The methodology used in this work was the following: SANS experiments were performed by applying a strong saturating magnetic field to unirradiated and irradiated specimens of three types of RAFM steels, namely the European EUROFER 97, the Japanese F82H and the Swiss OPTIMAX A steels. The available irradiated specimens included specimens which had been irradiated with 590 MeV protons in the Proton IRradiation EXperiment (PIREX) facility at the Paul Scherrer Institute (PSI) at temperatures in the range of 50-350°C to doses in the range of 0.3-2.0 dpa. SANS spectra as well as values of the so-called A ratio, which represents the ratio of the total scattered intensity to the nuclear scattered intensity, were obtained for the various irradiation doses and temperatures investigated. MD simulations of atomic displacement cascades in pure Fe and in Fe-He alloys were performed using Embedded Atom Method (EAM) many-body interatomic potentials. The main nano-sized defects that should be produced in RAFM steels under irradiation were created by means of MD in pure Fe. These included dislocation loops of various types, voids, helium bubbles with various He concentration and Cr precipitates. TEM images of cascade damage and all the defects created by MD were simulated in the dark field/weak beam imaging modes by using the Electron Microscopy Software (EMS) developed by P.A. Stadelmann (EPFL) and analyzed in terms of variations of contrast intensities versus depth inside the specimen. The SANS signal provided by cascade damage and all the defects created by MD was simulated by using a slightly modified version of EMS, accounting for neutrons instead of electrons. The SANS technique has been proven in this work to be a very powerful tool for detecting nano-sized irradiation-induced defects and a tool well complementary to TEM for characterizing such very small irradiation-induced defects. Indeed, TEM appears most adapted to investigate structural defects, such as dislocation loops and helium bubbles with high helium concentration, which yield significant lattice deformation of the surrounding matrix, while SANS is most adapted to investigate phase defects, such as voids, helium bubbles with low helium concentration and Cr precipitates. By combining the results of SANS experiments with those of MD simulations, TEM image simulations and SANS signal simulations, the nano-sized irradiation-induced defects were tentatively identified as small helium bubbles. While the radiation hardening measured for RAFM steels cannot be explained by accounting only for the defects observed in TEM, it could be successfully modeled by accounting also for a reasonable number density of the nano-sized defects evidenced using the SANS technique.

EPFL2008

An Atomistic Simulation Method for Oxygen Impurities in Aluminium Based on Variable Charge Molecular Dynamics

Andreas Elsener

Impurities are known to have a significant impact on materials properties. In particular, the presence of impurities can change mechanical properties and stabilize the microstructure by reducing grain growth and recrystallization processes. In the past atomistic simulations, in particular molecular dynamics, have contributed a lot to the understanding of deformation mechanisms in nanocrystalline metals. Especially, insights into details of dislocation/GB interactions have been gained. Additionally, simulations on coupled grain boundary migration in bicrystalline samples have played an important role in understanding stress assisted grain growth in nanocrystalline metals. However, all these simulations have been performed on monatomic systems. This means that the role of impurities has not yet been considered in these simulations. Some of the important difference between experiments and simulations could be removed by introducing dilute O distributions to computational Al samples. For this purpose, a simulation method is required which can deal with the oxidation of Al atoms around the O impurities and which can simulate the ionic and metallic nature of bonding simultaneously. Here, a method is suggested which fulfills the requirements by modifying the variable charge method of Streitz and Mintmire [Phys. Rev. B 50, 11996 (1994)]. A local chemical potential approach which optimizes the charge on only those atoms expected to be ionic is developed. Additionally the EAM potential for the non-electrostatic interaction given in the publication of Streitz and Mintmire is substituted by empirical potentials which treat the Al-Al interactions with a well-known Al potential. The Al-O and O-O interactions in these EAM potentials are fitted for the simulation of dilute O impurities in Al and additionally for the simulation of corundum in case of a second potential. The aforementioned developments are applied to study the effect of O impurities on the deformation of nc Al. A fully three-dimensional nc sample containing 15 grains of 12 nm grain-size is deformed with a dilute O content in the triple junction lines. The impact of O impurities on dislocation propagation is also considered in a sample which consists of a single grain extracted from a large molecular dynamics simulation. This simulation geometry contains a perfect dislocation, which is pinned at the surrounding grain boundary network. Therefore it allows the study of pinning strength and after unpinning the propagation behavior of the dislocation under various O impurity distributions. Additionally, the role of O impurities on coupled GB migration is investigated by studying two bicrystalline samples with different O distributions in both of them. In the discussion, the emphasis is on the simulation method. Especially, the significance of the samples, simulation conditions and the performance of the method in the different applications are discussed. Additionally, the different developments (local chemical potential approach and EAM potentials) are critically analyzed and improvements for future simulations of impurities are suggested.

EPFL2010