The growth process of He bubbles in W is investigated using molecular dynamics and parallel replica dynamics for growth rates spanning 6 orders of magnitude. Fast and slow growth regimes are defined relative to typical diffusion hopping times of W interstitials around the He bubble. Slow growth rates allow the diffusion of interstitials around the bubble, favoring the biased growth of the bubble towards the surface. In contrast, at fast growth rates interstitials do not have time to diffuse around the bubble, leading to a more isotropic growth and increasing the surface damage.
Propagation of the Wigner function is studied on two levels of semiclassical propagation: one based on the Van Vleck propagator, the other on phase-space path integration. Leading quantum corrections to the classical Liouville propagator take the form of a time-dependent quantum spot. Its oscillatory structure depends on whether the underlying classical flow is elliptic or hyperbolic. It can be interpreted as the result of interference of a pair of classical trajectories, indicating how quantum coherences are to be propagated semiclassically in phase space. The phase-space path-integral approach allows for a finer resolution of the quantum spot in terms of Airy functions.
By calculating free energies, several published interatomic interaction potentials for iron are investigated with respect to the stability of the low-temperature bcc phase and the high-temperature fcc phase. These are empirical many-body potentials for use in atomistic simulation. We find that in all of these potentials—except one—the bcc phase is the stable crystal structure for all temperatures up to the melting point. However, several potentials exhibit a metastable fcc phase in the sense that the fcc structure corresponds to a local minimum of the free energy.
By means of classical molecular-dynamics simulations, we investigate solid-solid phase transitions in cylindrical iron nanowires. The interatomic potential employed has been shown to be capable of describing the martensite-austenite phase transition in iron. We investigate the dependence of the transition temperature on the wire diameter, the heating/cooling rate, and a tensile stress applied in axial direction. We observe that the phase transition temperature is inversely proportional to the wire diameter during heating and depends linearly on an applied axial tensile stress. The transition temperature becomes independent of the heating/cooling rate for the smallest rates investigated. The time the wire needs for completing the structural change is found to be independent of the diameter, the tensile loading, and the heating/cooling rate for the range of parameters considered. Finally, we find that there exists a maximum tensile stress above which the nanowire can no longer recover its initial structure after cooling.
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