UBC Theses and Dissertations
Atomistic simulations of solute-interface interactions in iron Jin, Hao
The kinetics of the recrystallization and austenite-ferrite (fcc-bcc) phase transformation in steels are markedly affected by substitutional alloying elements. Nevertheless, the detailed mechanisms of their interaction with the grain boundaries and interfaces are not fully understood. Using density functional theory, we determine the segregation energies of commonly used alloying elements (e.g. Nb, Mo, Mn, Si, Cr, Ni) in the Σ5 (013) tilt grain boundary in bcc and fcc Fe, and the bcc-fcc interfaces. We find a strong interaction between large solutes (e.g. Nb, Mo and Ti) and grain boundaries or interfaces that is consistent with experimental observations of the effects of these alloying elements on delaying recrystallization and the austenite-to-ferrite transformation in low-carbon steels. In addition, we compute the solute-solute interactions as a function of solute pair distance in the grain boundaries and interfaces, which suggest co-segregation for these large solutes at intermediate distances in striking contrast to the bulk. Besides the prediction of solute segregation, the self- and solute-diffusion in Fe-based system are also investigated within a framework combining density functional theory calculations and kinetic Monte Carlo simulations. Good agreement between our calculations and the measurements for self- and solute diffusion in bulk Fe is achieved. For the first time, the effective activation energies and diffusion coefficients for various solutes in the α-Fe Σ5 (013) grain boundary are determined. The results demonstrate that grain boundary diffusion is significantly faster than for lattice diffusion, confirming grain boundaries are fast diffusion paths. By contrast, the effective activation energy of self-diffusion in a bcc-fcc Fe interface is close to the value of fcc bulk self-diffusion, and the investigated bcc-fcc interface provides a moderate "fast diffusion" path.
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