Influence of point defects on grain boundary mobility in bcc tungsten.

Atomistic computer simulations were performed to study the influence of radiation-induced damage on grain boundary (GB) sliding processes in bcc tungsten (W), the divertor material in the ITER tokamak and the leading candidate for the first wall material in future fusion reactors. In particular, we calculated the average sliding-friction force as a function of the number of point defects introduced into the GB for a number of symmetric tilt GBs. In all cases the average sliding-friction force at fixed shear strain rate depends on the number of point defects introduced into the GB, and in many cases introduction of these defects reduces the average sliding-friction force by roughly an order of magnitude. We have also observed that as the number of interstitials in the GB is varied, the direction of the coupled GB motion sometimes reverses, causing the GB to migrate in the opposite direction under the same applied shear stress. This could be important in the microstructural evolution of polycrystalline W under the harsh radiation environment in a fusion reactor, in which high internal stresses are present and frequent collision cascades generate interstitials and vacancies.

Effects of substrate rotation in oblique-incidence metal(100) epitaxial growth.

The effects of substrate rotation on the surface morphology in oblique-incidence metal(100) epitaxial growth are studied via kinetic Monte Carlo simulations of a simplified model, and compared with previous results obtained without rotation. In general, we find that substrate rotation leads to two main effects. At high deposition angles with respect to the substrate normal, rotation leads to a significant change in the surface morphology. In particular, it leads to isotropic mounds and pyramids with (111) facets rather than the anisotropic ripples and rods observed in the absence of rotation. Due to the existence of rapid transport on these facets, the lateral feature size increases approximately linearly with film thickness. Due to the fact that substrate rotation tends to reduce the effects of shadowing, the surface roughness is also decreased compared to the roughness in the absence of rotation. While this leads to a moderate reduction in the roughness for the case of ballistic deposition, the effect is significantly larger in the case of deposition with attraction. In the case of ballistic deposition, we also find that the surface roughness increases with rotation rate Omega for Omega<1 rev/monolayer (ML) before saturating at larger rotation rates ( Omega>1 rev/ML). In contrast, for the case of attraction the surface roughness exhibits a negligible dependence on rotation rate for finite rotation rate.

Vacancy formation and strain in low-temperature Cu/Cu(100) growth.

The development of compressive strain in metal thin films grown at low temperature has recently been revealed via x-ray diffraction and explained by the assumption that a large number of vacancies were incorporated into the growing films. The results of our molecular dynamics and parallel temperature-accelerated dynamics simulations suggest that the experimentally observed strain arises from an increased nanoscale surface roughness caused by the suppression of thermally activated events at low temperature combined with the effects of shadowing due to off-normal deposition.