Atomic-scale modeling of the deformation of nanocrystalline metals

Nanocrystalline metals, i.e. metals with grain sizes from 5 to 50 nm, display technologically interesting properties, such as dramatically increased hardness, increasing with decreasing grain size. Due to the small grain size, direct atomic-scale simulations of plastic deformation of these materials are possible, as such a polycrystalline system can be modeled with the computational resources available today. We present molecular dynamics simulations of nanocrystalline copper with grain sizes up to 13 nm. Two different deformation mechanisms are active, one is deformation through the motion of dislocations, the other is sliding in the grain boundaries. At the grain sizes studied here the latter dominates, leading to a softening as the grain size is reduced. This implies that there is an ``optimal'' grain size, where the hardness is maximal. Since the grain boundaries participate actively in the deformation, it is interesting to study the effects of introducing impurity atoms in the grain boundaries. We study how silver atoms in the grain boundaries influence the mechanical properties of nanocrystalline copper.Comment: 10 pages, LaTeX2e, PS figures and sty files included. To appear in Mater. Res. Soc. Symp. Proc. vol 538 (invited paper). For related papers, see http://www.fysik.dtu.dk/~schiotz/publist.htm

DEFECT CONFIGURATION AND ENERGY COMPUTATION IN SIMPLE METALS

L'application de la méthode de Hartree et de la théorie des pseudo-potentiels permet d'écrire l'énergie des métaux normaux sous la forme : E = [MATH] Ei + [MATH] W(|ri – rj|) ri et rj désignant les positions de deux atomes. A volume constant, Ei est constant et les variations de l'énergie du cristal dépendent seulement de la contribution des interactions de paires. Ce résultat est utilisé pour le calcul des énergies de fautes d'empilement et des configurations de cœur des dislocations dans les métaux normaux.By using the Hartree method and the results of pseudo-potentials theory, the energy of simple metals can be put as follows : E = [MATH] Ei + [MATH] W(|ri – rj|) ri and rj characterizing the positions of two atoms. At constant volume, Ei is constant and variations of the crystal energy depend only on the pair interaction contribution. This result has been used to compute stacking fault energies and dislocations core configurations in simple metals

Atomic force microscopy investigation of buckling patterns of nickel thin films on polycarbonate substrates

The evolution of buckling patterns of nickel thin films have been studied in situ by atomic force microscopy during cyclic tests composed of uniaxial compression followed by release of the external applied stress. After the first strain cycling, buckling structures evolve from straight-sided wrinkles to varicose patterns characterized by a debuckling of some parts of the film. Further cycling tests reveal that rebonding of the film on its substrate does not occur once decohesion has taken place.Anglai

STACKING FAULT ENERGY CALCULATIONS IN THE FLUORITE STRUCTURE

Des calculs d'énergies de faute d'empilement ont été effectués dans la structure fluorine. Plusieurs méthodes sont développées dans lesquelles différents potentiels à courte portée sont utilisés. La contribution de la polarisation électronique est introduite par l'emploi d'un modèle de la coquille. Il est montré que l'énergie de polarisation électronique est faible quand les fautes d'empilement sont relaxées. Les résultats obtenus par différentes méthodes sont comparés. Généralement, les énergies de faute d'empilement trouvées sont fortes.Computer calculations of stacking fault énergies have been carried out in ionic crystals having the fluorite structure. Several methods are developped in which different short range potentials are used. In order to include the polarization a shell model is introduced. It is shown that the polarization energy is low when the stacking faults are relaxed. The results obtained by the different methods are compared. Generally, the stacking faults are found high

Interactive study of straight-sided buckling patterns in thin films under compressive stress

In situ atomic force microscopy observations have been carried out of thin films under external compressive stress. Straight-sided buckling patterns arise perpendicular to the compression axis which tend to attract one another during propagation a few hundred nanometers apart. The mechanisms whereby these debonding patterns interact have been investigated taking into account the elastic energy of both the film and the substrate. The equilibrium distance between two straight-sided wrinkles has been determined; good agreement has been obtained between the experimental results and the mechanics involved