Stochastic simulation of dislocation glide in tantalum and Ta-based alloys

We employ a kinetic Monte Carlo algorithm to simulate the motion of -oriented screw dislocation on a -slip plane in body centered cubic Ta and Ta-based alloys. The dislocation moves by the kink model: double kink nucleation, kink migration and kink-kink annihilation. Rates of these unit processes are parameterized based upon existing first principles data. Both short-range (solute-dislocation core) and long-range (elastic misfit) interactions between the dislocation and solute are considered in the simulations. Simulations are performed to determine dislocation velocity as a function of stress, temperature, solute concentration, solute misfit and solute-core interaction strength. The dislocation velocity is shown to be controlled by the rate of nucleation of double kinks and the dependence of the double kink nucleation rate on stress and temperature are consistent with existing analytical predictions. In alloys, dislocation velocity depends on both the short- and long-range solute dislocation interactions as well as on the solute concentration. The short-range solute-core interactions are shown to dominate the effects of alloying on dislocation mobility. The present simulation method provides the critical link between atomistic calculations of fundamental dislocation and solute properties and large scale dislocation dynamics that typically employ empirical equations of motion.     

Interfacial segregation in Ag-Au,Au-Pd,and Cu-Ni alloys: II. [001] Σ5 twist grain boundaries

Atomistic simulations of segregation to [001] 5 twist boundaries in Cu–Ni, Au–Pd, and Ag–Au alloy systems have been performed for a wide range of temperatures and compositions within the solid solution region of these alloy phase diagrams. In addition to the grain boundary segregation profiles, grain boundary free energies, enthalpies, and entropies were determined. These simulations were performed within the framework of the free energy simulation method, in which an approximate free energy functional is minimized with respect to atomic coordinates and atomic site occupation. For all alloy bulk compositions (0.05 C 0.95) and temperatures (400 T (K) 1,100) examined, Cu and Au segregates to the boundary in the Cu–Ni and Au–Pd alloy systems, respectively; although in the Ag–Au alloys, the majority element segregates to the boundary. The width of the segregation profile is limited to approximately three to four (002) atomic planes. The classical theories for the segregation, and the effects of the relaxation with respect to either the atomic positions or the atomic concentrations, are discussed. The boundary thermodynamic properties depend sensitively on the magnitude of the boundary segregation, and some of them are shown to vary linearly with the magnitude of the grain boundary segregation.     

Grain-boundary grooving and agglomeration of alloy thin films with a slow-diffusing species

We present a general phase-field model for grain-boundary grooving and agglomeration of polycrystalline alloy thin films. In particular, we study the effects of slow-diffusing species on the grooving rate. As the groove grows, the slow species becomes concentrated near the groove tip so that further grooving is limited by the rate at which it diffuses away from the tip. At early times the dominant diffusion path is along the boundary, while at late times it is parallel to the substrate. This change in path strongly affects the time dependence of grain-boundary grooving and increases the time to agglomeration. The present model provides a tool for agglomeration-resistant thin film alloy design.