Olivier Hardouin Duparc

Carbon-rich icosahedral boron carbide designed from first principles

The carbon-rich boron-carbide (B11C)C-C has been designed from first principles within the density functional theory. With respect to the most common boron carbide at 20% carbon concentration B4C, the structural modification consists in removing boron atoms from the chains linking (B11C) icosahedra. With C-C instead of C-B-C chains, the formation of vacancies is shown to be hindered, leading to enhanced mechanical strength with respect to B4C. The phonon frequencies and elastic constants turn out to prove the stability of the carbon-rich phase, and important fingerprints for its characterization have been identified.

On the atomic structure of an asymmetrical near Sigma=27 grain boundary in Copper

The atomic structure of an asymmetrical near Σ = 27 {525} tilt grain boundary (GB) in copper is determined by coupling high-resolution transmission electron microscopy and molecular dynamics simulation. The average GB plane is parallel to {414} in crystal (1) and {343} in crystal (2). The detailed GB structure shows that it is composed of facets always parallel to {101} and {111} in crystals (1) and (2), respectively. The atomic structure of one facet is described using the structural units model. Each facet is displaced with respect to its neighbours by a pure step, giving rise to the asymmetry of the GB plane orientation. The energy of this asymmetrical GB is significantly lower than that of both the {525} symmetrical and the {11,1,11}/{111} asymmetrical Σ = 27 GBs. One GB region displays another atomic structure with a dislocation that accounts for the misfit between interatomic distances in the {414} and {343} GB planes.

On the origins of the Finnis–Sinclair potentials

I trace back the origins of the famous Finnis-Sinclair potentials. These potentials mimic the results of tight binding theory through their use of the square root embedding function. From the tentative beginnings of tight binding in the 1930s up to 1984 or so, some of the famous names involved are Bloch, Seitz, Montroll, Friedel, Cyrot-Lackmann, Ducastelle, to name just a few. The application of the method of moments to the description of densities of states and its connexion to the physics of closed paths linking nearest neighbours interacting atoms helped to formalize Friedels rectangular band model for the d electrons in transition metals. Extension from perfectly periodic structures to defective ones could not be but a slow process due to the change of paradigm for solid state scientists and to the necessary caution to be paid to self-consistency. The British scientists school also contributed significantly in the 80s. Computer progress and pragmatism helped to go from mainly analytical developments to numerical experiments (another change of paradigm). I also digress on various not so well known historical points of interest to this story.I trace back the origins of the famous Finnis–Sinclair potentials. These potentials mimic the results of tight binding theory through their use of the square root embedding function. From the tentative beginnings of tight binding in the 1930s to 1984 or so, Bloch, Seitz, Montroll, Friedel, Cyrot-Lackmann and Ducastelle are some of the famous names involved, to name just a few. The application of the method of moments to the description of densities of states and its connection to the physics of closed paths linking nearest neighbours interacting atoms helped to formalise Friedels rectangular band model for the d electrons in transition metals. Extension from perfectly periodic to defective structures could not be but a slow process due to the change of paradigm for solid state scientists and to the necessary caution to be paid to self-consistency. British scientists also contributed significantly in the 1980s. Computer progress and pragmatism helped to go from mainly analytical developments to numerical experiments (another change of paradigm). I also digress on various not so well-known historical points of interest to this story.

Atomic structures of Si and Ge Σ = 13 [0 0 1] tilt grain boundaries studied by high-resolution electron microscopy and atomistic simulations

By combining high-resolution electron microscopy and atomistic simulations, the atomic structures of several interfaces, {5 1 0}, {2 3 0} and {8 1 0}/{7 4 0}, in germanium and in silicon Σ = 13 [0 0 1] tilt grain boundaries (TGBs) are studied using bicrystals prepared in two different ways from the melt. The interfaces are characterized by either transmission electron microscopy or scanning transmission electron microscopy (STEM). The Si TGB shows only one interface, {1 5 0} with one interfacial structure. The Ge TGB contains many facets. In Ge, observations performed in two perpendicular directions, [0 0 1] and [  5 0], confirm that the {5 1 0} interface has two different structures. One structure, called M-structure, is periodic along [0 0 1] and has tetracoordinated atoms. The other structure, called U-structure, is more peculiar as it contains a fixed part surrounding a variable complex core. High-resolution STEM, realised in modern microscopes equipped with a probe Cs-corrector, is a very effective technique for structure determination of grain boundaries (GBs). However, current limitations for high-resolution study of GBs are the structural changes under the electron beam and the limited number of crystallographic axes suitable for atomic-resolution imaging. The structures of GB atomistic models can be ordered according to their calculated energies. It appears that energies calculated using empirical potentials, like Tersoff or Stillinger-Weber potentials, do not give the same classification as ab initio calculations and cannot be used to determine the structure of lowest energy. This structure is the M-structure, the structure observed in the Si bicrystal.

The Effect of Pressure on Martensitic Phase Transformations

Stability of the crystal structure is determined by the competition between attractive and repulsive interatomic forces. Using many-body exponential potentials it can be shown that the bcc structure corresponding to austenitic phases is more stable for low values of the q-parameter characterising the attractive forces for a fixed value of the p-parameter describing the repulsive forces. The structural stability can be changed with the acting pressure that may alter the martensitic transformations from the bcc-austenite to a close-packed structure. The effect of pressure is examined in a generic model employing many-body potentials and the results are compared with ab initio calculations for zirconium representing a monoatomic material with displacive phase transformation.

Alloying behaviour of binary transition metal systems

Abstract The properties of A – B binary alloys are controlled by the mixed AB interaction potentials. The interfaces between two different metals are investigated for fictive mixed potentials spreading from A-type to B-type nature. The elemental AA and BB potentials are fixed. The tendencies towards formation of ordered structures on one side, and separation of phases on the other side are discussed. A description of interatomic forces respecting metallic bonding character in many-body potentials is used. This approach can be helpful for applications of alloy potentials in models with complex structures of extended defects such as dislocations and grain boundaries and in the assessment of their behaviour.