Density functional theory analysis of solute-defect interaction energies in fcc iron: fundamental origins and industrial application, A

Abstract

This project was initiated to develop an understanding of the origin of solute drag on fcc Fe (austenite) grain boundaries and explain differences in the experimentally observed solute drag effects of different solutes in steels, and use this understanding to predict solutes which could be of industrial interest as grain growth inhibitors in austenite. The 3-d and 4-d (period 4 and 5) transition metal elements on the periodic table are considered as substitutional solutes with specific attention given to selected solutes (Nb, Mn, Cr, Mo, and Ni) that are of particular interest to the steel industry. Atomistic modeling using both density functional theory (DFT) and molecular dynamics (MD) are used to achieve these goals. The simulations provided thermodynamic solute-boundary binding energies as well as information about the electronic structure of the system which was accessed using both density of states analysis and direct observation of the calculated charge density. The calculated thermodynamic solute-boundary binding energies correlate strongly with experimental data sets on the effects of solutes on both austenite grain coarsening and austenite recrystallization. The strong correlation with experimental results provides confidence in the modelling work and enables the results to be used to suggest solutes of interest for possible future experimentation as alloying elements. Of particular interest, according to the results, are Y, Zr, and Sc, with possible specialized applications for Pd, Ag, and Cd. The filled kite is identified as a fundamental building block for the grain boundary structure of fcc Fe tilt grain boundaries. This structure is found to function as a chemical sub-structure within the grain boundary. This study characterizes the chemical structure of the filled kite through topological analysis of the charge density and density of states analysis. Further, the solute-defect interaction energy with this structure is relatively independent of the orientation of the filled kite with respect to the grain boundary, leading to the conclusion that understanding the interaction of defects with this structure enables a more complete understanding of the interaction of solutes with general grain boundaries. The most significant finding of this work, presented in Chapter 4, is that the solute-defect interaction energy can be predicted based upon the elemental properties of the solute-solvent system and the structure, both electronic and geometric, of the grain boundary. The chemical and strain interaction energies can be separated by considering changes in the Bader volume and Bader charge, respectively, of the solute and solvent atoms as they move from the bulk to the defect. Through this separation of the chemical and strain energies, it is determined that the chemical hardness, the second derivative of the energy with respect to the electron count, of the solute relative to the solvent is the fundamental elemental property leading to the chemical energy of segregation just as the volume of the solute relative to the solvent is the fundamental elemental property leading to the strain energy of segregation.