Amir A. Farajian, Ph.D. (Advisor); Raghavan Srinivasan, Ph.D., P.E. (Committee Member); H. Daniel Young, Ph.D. (Committee Member); Dallas R. Trinkle III., Ph.D. (Committee Member); Christopher Woodward, Ph.D. (Committee Member)
Doctor of Philosophy (PhD)
Vacancy-mediated diffusion along dislocations, often referred to as pipe diffusion, can contribute to creep deformation of metals in many engineering applications. This process is studied along an a/2⟨1 -1 0⟩ screw dislocation in fcc Ni using a density functional theory approach. An accurate geometrical configuration of the screw dislocation core, dissociated into Shockley partial dislocations and separated by a stacking fault, was previously derived using a lattice Green's function technique. Activation energies and frequencies are calculated for atom-vacancy exchanges that contribute to diffusion around and along one of the partial cores. This analysis reveals the significant role of the sites within the compressive component of the dislocation, the dominant contribution of the hops around the screw geometry rather than directly along the dislocation line, and the importance of including the stacking fault sites. Kinetic Monte Carlo simulations use these frequencies to generate diffusion coefficients that account for correlation effects. Near 80% of the melting temperature, these pipe diffusivities are an order of magnitude higher than those found in fcc regions, and they are eight orders higher at room temperature. Calculations are compared to experimental results and the differences are discussed. While pipe diffusion is unlikely to contribute to isotropic mass flux at low dislocation densities, it will accelerate dislocation mechanisms controlling creep, particularly when alloying elements are involved. To quantify the effects of chemistry on diffusivity in addition to geometry, the described techniques also analyze the pipe diffusivity of several important alloying elements to Ni-base superalloys: Co, Cr, Al, Ti, Mo, W, and Re. Calculations quantify how the partial core enhances diffusivities of each element relative to isotropic diffusion in fcc Ni. Advances in first principles methods provide a means to explore the rate constants of mechanisms that constitute diffusion with molecular dynamics. We apply these to select atom-vacancy exchanges near the same screw dislocation partial core in fcc Ni at 1000 and 1400 K. Intrinsic properties of this dynamic approach contrast limitations of the widely used harmonic transition state theory (HTST) approximations that were previously applied, revealing direction-dependent free energy profiles and temperature-dependent correlation effects. In contrast to static HTST, the dynamic approach reveals an asymmetry in mass transport that is related to the edge character of the partial dislocation. This arises from different philosophies underlying the two methodologies. The absolute values of the rate constants, however, are within the same order of magnitude, with the HTST method generally underestimating rates by a factor of 2-5. Combined with a twofold increase in rates due to thermal expansion at high temperatures, this partially explains why our kinetic Monte Carlo simulations yield lower diffusivities for pipe diffusion than experiments observe.
Department or Program
Ph.D. in Engineering
Year Degree Awarded
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