Emily Carter - Academia.edu (original) (raw)
Papers by Emily Carter
Physical Review B, 2003
We perform density-functional theory ͑DFT͒ calculations of carbon dissolution and diffusion in ir... more We perform density-functional theory ͑DFT͒ calculations of carbon dissolution and diffusion in iron, the latter being a typical example of interstitial diffusion. The Kohn-Sham equations are solved with periodic boundary conditions and within the projector-augmented-wave formalism, using the generalized gradient approximation for electron exchange and correlation. With the solution enthalpy as an indication of cell size convergence, we find a supercell with 128 Fe atoms and one C atom is sufficient for describing dilute concentrations of carbon in bcc Fe. The solution enthalpy of carbon in an octahedral site in ferrite is predicted to be 0.74 eV, i.e., the dissolution of carbon in bcc ferromagnetic ͑FM͒ Fe is an endothermic process. Using the Fe128C1 periodic cell, we find that the minimum-energy path ͑MEP͒ of carbon diffusion from one octahedral site to another ͑via a tetrahedral site͒ has a barrier of 0.86 eV, in excellent agreement with the experimental value of 0.87 eV. This encouraging benchmark result prompted us to investigate carbon diffusion in austenite, whose electronic structure is less well characterized experimentally. Cell size convergence results show that a supercell with 32 Fe atoms and one C atom is sufficient. The calculated solution enthalpy is Ϫ0.17 eV, which indicates that the dissolution of carbon in fcc Fe is exothermic, consistent with the known greater solubility of C in austenite compared to ferrite. The MEP shows that carbon moves linearly from an octahedral site to another, contrary to the common notion of an off-plane diffusion path. The diffusion barrier is calculated to be 0.99 eV. Since we model austenite with the FM high-spin phase, the diffusion barrier we obtain is not directly comparable to the experiments in which austenite is usually paramagnetic. However, this prediction is relevant for C incorporation into Fe thin films, since FM high-spin fcc Fe can be obtained by epitaxial growth of thin Fe films on a Cu substrate.
Surface Science, 2003
We report results of gradient-corrected pseudopotential-based density functional theory calculati... more We report results of gradient-corrected pseudopotential-based density functional theory calculations on bulk Fe 3 C in the cementite structure and its (0 0 1), (1 1 0), (0 1 1), (1 0 0), (1 0 1), (0 1 0), and (1 1 1) surfaces. Bulk properties are in reasonable agreement with available experimental data. The cementite local density of states shows predominantly metallic character, along with some polar covalent bonding contributions (charge transfer from iron to carbon) for both bulk and surfaces. We predict cementite surface energies in the range of 2.0-2.5 J/m 2 , most of which are lower than all pure Fe surface energies. In particular, we predict the Fe 3 C (0 0 1) surface to be the most stable and the Fe 3 C (1 0 0) surface to be the least stable. We show that greater stability is associated with localized Fe-C bonding at the surface, smoother surfaces created, e.g., by large C atom relaxation into the bulk, and more uniform coordination at the surface. The relatively greater stability of Fe 3 C surfaces is suggested to provide the driving force for cementite to form at the surfaces of bcc iron. Implications for the carburization erosion mechanism for steel, such as cracking and melting, are discussed.
Physical Review B, 2004
We report periodic spin-polarized density functional theory (DFT) predictions of hydrogen adsorpt... more We report periodic spin-polarized density functional theory (DFT) predictions of hydrogen adsorption, absorption, dissolution, and diffusion energetics on and in ferromagnetic (FM) body-centered cubic (bcc) iron. We find that H prefers to stay on the Fe surface instead of subsurfaces or in bulk. Hydrogen dissolution in bulk Fe is predicted to be endothermic, with hydrogen occupying tetrahedral (t) sites over a wide range of concentrations. This is consistent with the known low solubility of H in pure Fe. In the initial absorption step, we predict that H occupies the deep subsurface t-site for Fe(110) and the shallow subsurface t-site for Fe(100). Diffusion of H into Fe subsurfaces is predicted to have a much lower barrier for Fe(100) than Fe(110). For H diffusion in bulk Fe, we find that H diffuses through bcc Fe not via a straight line trajectory, but rather hops from one t-site to a neighboring t-site by a curved path. Moreover, we exclude a previously suggested path via the octahedral site, due to its higher barrier and the rank of the saddle point. Quantum effects on H diffusion through bulk Fe are discussed.
Physical Review B, 2003
We perform density-functional theory ͑DFT͒ calculations of carbon dissolution and diffusion in ir... more We perform density-functional theory ͑DFT͒ calculations of carbon dissolution and diffusion in iron, the latter being a typical example of interstitial diffusion. The Kohn-Sham equations are solved with periodic boundary conditions and within the projector-augmented-wave formalism, using the generalized gradient approximation for electron exchange and correlation. With the solution enthalpy as an indication of cell size convergence, we find a supercell with 128 Fe atoms and one C atom is sufficient for describing dilute concentrations of carbon in bcc Fe. The solution enthalpy of carbon in an octahedral site in ferrite is predicted to be 0.74 eV, i.e., the dissolution of carbon in bcc ferromagnetic ͑FM͒ Fe is an endothermic process. Using the Fe128C1 periodic cell, we find that the minimum-energy path ͑MEP͒ of carbon diffusion from one octahedral site to another ͑via a tetrahedral site͒ has a barrier of 0.86 eV, in excellent agreement with the experimental value of 0.87 eV. This encouraging benchmark result prompted us to investigate carbon diffusion in austenite, whose electronic structure is less well characterized experimentally. Cell size convergence results show that a supercell with 32 Fe atoms and one C atom is sufficient. The calculated solution enthalpy is Ϫ0.17 eV, which indicates that the dissolution of carbon in fcc Fe is exothermic, consistent with the known greater solubility of C in austenite compared to ferrite. The MEP shows that carbon moves linearly from an octahedral site to another, contrary to the common notion of an off-plane diffusion path. The diffusion barrier is calculated to be 0.99 eV. Since we model austenite with the FM high-spin phase, the diffusion barrier we obtain is not directly comparable to the experiments in which austenite is usually paramagnetic. However, this prediction is relevant for C incorporation into Fe thin films, since FM high-spin fcc Fe can be obtained by epitaxial growth of thin Fe films on a Cu substrate.
Surface Science, 2003
We report results of gradient-corrected pseudopotential-based density functional theory calculati... more We report results of gradient-corrected pseudopotential-based density functional theory calculations on bulk Fe 3 C in the cementite structure and its (0 0 1), (1 1 0), (0 1 1), (1 0 0), (1 0 1), (0 1 0), and (1 1 1) surfaces. Bulk properties are in reasonable agreement with available experimental data. The cementite local density of states shows predominantly metallic character, along with some polar covalent bonding contributions (charge transfer from iron to carbon) for both bulk and surfaces. We predict cementite surface energies in the range of 2.0-2.5 J/m 2 , most of which are lower than all pure Fe surface energies. In particular, we predict the Fe 3 C (0 0 1) surface to be the most stable and the Fe 3 C (1 0 0) surface to be the least stable. We show that greater stability is associated with localized Fe-C bonding at the surface, smoother surfaces created, e.g., by large C atom relaxation into the bulk, and more uniform coordination at the surface. The relatively greater stability of Fe 3 C surfaces is suggested to provide the driving force for cementite to form at the surfaces of bcc iron. Implications for the carburization erosion mechanism for steel, such as cracking and melting, are discussed.
Physical Review B, 2004
We report periodic spin-polarized density functional theory (DFT) predictions of hydrogen adsorpt... more We report periodic spin-polarized density functional theory (DFT) predictions of hydrogen adsorption, absorption, dissolution, and diffusion energetics on and in ferromagnetic (FM) body-centered cubic (bcc) iron. We find that H prefers to stay on the Fe surface instead of subsurfaces or in bulk. Hydrogen dissolution in bulk Fe is predicted to be endothermic, with hydrogen occupying tetrahedral (t) sites over a wide range of concentrations. This is consistent with the known low solubility of H in pure Fe. In the initial absorption step, we predict that H occupies the deep subsurface t-site for Fe(110) and the shallow subsurface t-site for Fe(100). Diffusion of H into Fe subsurfaces is predicted to have a much lower barrier for Fe(100) than Fe(110). For H diffusion in bulk Fe, we find that H diffuses through bcc Fe not via a straight line trajectory, but rather hops from one t-site to a neighboring t-site by a curved path. Moreover, we exclude a previously suggested path via the octahedral site, due to its higher barrier and the rank of the saddle point. Quantum effects on H diffusion through bulk Fe are discussed.