Papers by Keyword: Dislocation Nucleation

Abstract: Mechanical properties of (001) Mo and (001) Mo – 1.5 at.% Ir single crystals have been studied by nanoindentation. It has been found that the iridium addition to molybdenum leads to an increase in both hardness and elastic modulus. An abrupt elasto-plastic transition (pop-in) at a depth of about 20 - 40 nm caused by dislocation nucleation in previously dislocation-free volume has been observed in the initial portion of the loading curve. It has shown that the Ir addition essentially affects the dislocation nucleation. Mean shear stress required for the dislocation nucleation increased from 10.8 GPa (G/12) for a Mo single crystal to 18.2 GPa (G/8) for the Mo – 1.5 at% Ir solid solution. Thus, the Ir solution in a Mo single crystal affects not only the resistance to the motion of dislocations (hardness) but the nucleation of them as well. The latter is likely to occur as a result of an increase in the structure perfection of the Mo – 1.5 at% Ir solid solution as compared to the pure Mo single crystal.

Abstract: Epitaxial group-III nitride films, although in single crystalline form, contain still a large number of threading dislocations. These set limits to performance and lifetime of devices, notably to high power structures like lasers. The strategy in material development was and will be (at least until lattice-matched substrates become available) to reduce the dislocation densities. The present contribution elaborates on possible dislocation origination mechanisms that determine the population of dislocations in the epitaxial layers. These mechanisms can be controlled to a certain degree by proper deposition procedures. The achieved dislocation populations then determine the processes that can reduce the dislocation densities during growth of the epitaxial layers. The mutual annihilation of threading dislocations is rather efficient although affected by the glide properties of the growing epitaxial crystal and the thermal stresses during the cooling down after growth.

Abstract: Molecular dynamics simulation of nanoindentation on Al(111) surface is presented. The simulation is performed using the Ercolessi-Adams glue potential and the Berendsen thermostat. Boundary conditions of 'pseudo' thin film are imposed in order to focus on the dislocation motion in ultra-thin film. Nucleation and development of defects underneath the indenter tip are visualized, and the gliding patterns of dislocation loops are investigated with particular emphasis on the effect of film thickness. Simulation results show that the early emission of dislocation loop is highly dependent on the film thickness.