Solid State Phenomena Vol. 129

Abstract: First principles density functional calculations have been performed for a number of Al(100) twisted interfaces with the purpose to investigate the most stable atomic structures. Orientation dependence of interface energy normalized to bulk is established and discussed. The obtained results can be used to model grain growth in molecular dynamics studies.

Abstract: At uniform excitation of semiconductors by laser radiation with pre-threshold power, locally melted regions are formed on irradiated surfaces. This is induced by thermo diffusive instability of a distribution of uniformly generated electron-hole plasma. The shapes of locally melted regions give rise to a great variety of interesting surface patterns. A mathematical model of the surface dynamics, when the instability of the melt front arises along a chosen wave vector, is proposed. The results of computer simulation of interface dynamics of solitary melted region are compared with experimental data.

Abstract: In this study a series of 3D models for curved [100] grain boundaries (GBs) in pure α-iron have been constructed. Each model consisted of a spherical grain, with an initial size of about 9 nm, surrounded by a large single-crystal. Different orientations have been assigned to the grain and the matrix in order to obtain interfaces with misorientation angles in the range of 5-45 degrees in steps of 5 degrees. The molecular dynamics with Embedded Atom Method (EAM) potential have been performed for investigation of the temporal changes in GB migration and grain rotations at temperature of 1000 K. The relationship between GB misorientation and its mobility has been found. It was also discovered that the density of the material decreases with a reduction of GB area. The effect of a triple junction on the interface motion has been also studied by introducing a bi-crystal matrix instead of a singlecrystal one. The results are discussed in terms of grain growth investigations in nanometals.

Abstract: After an overload was imposed during a constant amplitude fatigue experiment, a retardation period was observed. The deformation in the vicinity of a crack tip was studied using neutron and x-ray microbeam-diffraction techniques, which provide millimeter and submicrometer spatial resolutions, respectively. From the neutron-diffraction measurements, compressive lattice strains and higher dislocation densities at the macroscale were observed in front of the crack tip, which indicates a plasticity induced crack-closure phenomenon. Furthermore, Laue patterns obtained from the microbeam diffraction at different locations near the crack tip show alternating regions with high and low dislocation densities at the mesoscale.

Abstract: Nanomaterials, due to their fine grain sizes, exhibit enhanced mechanical properties. However, their low stability at also relatively low temperatures might limit their future applications. In the present work, a statistical model has been proposed in order to study grain growth processes in nanomaterials. The Hillert’s approach has been extended by incorporating two mechanisms of growth for an individual grain: grain boundary migration – GBM - (diffusion based - continuous) and grain-rotation coalescence – GRC - (discontinuous). The influence of the grain size distribution on the grain growth process has been studied. The results show that the inclusion of GRC mechanisms results in a departure from the parabolic law of grain growth. Such a deviation has also been observed experimentally, especially in nanomaterials. The results reveal that grain growth rate increases with higher dispersion of the fine grains and the rotation mechanism can initiate growth even with low dispersion. This causes a steady increase in the coefficient of variation which, after some time interval, decays to homogeneity. This paper also demonstrates that the average rotation mobility which is a consequence of the varying misorientation angle contributes up to about 50% of the overall average boundary mobility.