A computational investigation of the effect of alloying elements on the thermodynamic and diffusion properties of fcc nickel alloys, with application to the creep rate of dilute nickel-X alloys

In this dissertation, two computational techniques have been employed to understand the alloying effects of various transition elements in Ni and its alloys.;To aid in the process of narrowing down the large composition space for the design of future Ni-base superalloys, a thermodynamic model using the CALPHAD approach is developed, where Gibbs energy functions of individual phases are parameterized based on fittings to experimentally measured phase equilibria or thermochemical data and computationally predicted thermochemical data. Multi-component Ni-base superalloys can be accurately described within the CALPHAD approach through the extrapolation of the Gibbs energy functions of the simpler sub-systems which are modeled where experimental and computational data is usually more abundant. The Re-Y and Re-Ti systems, integral binary alloy systems in the Ni-base superalloy database, are modeled in the present work.;In addition to studying thermodynamic and phase stability properties of Ni-base superalloys, this thesis also highlights the importance of the kinetic properties of these materials through their diffusion coefficients. Vacancy mediated self-diffusion coefficients are calculated on ferromagnetic and non-magnetic fcc Ni as a function of temperature. Within Eyring’s reaction rate theory, minimum energy pathways for the diffusing atom is calculated using the Nudged Elastic Band method.;The present work demonstrates that the mid-row 5d transition row element impurities have the highest activation barriers for impurity diffusion, and subsequently are the slowest diffusers in Ni. The fastest diffusers in Ni coupled with the lowest activation barriers for impurity diffusion are demonstrated to be at the far left of the 3d and 4d transition element rows on the periodic table. The present work also demonstrates that the primary mechanism driving the variation in the impurity diffusion coefficient from element to element is the migration barrier for impurity diffusion. In addition, the correlation of the impurity diffusion coefficients is not found to be to the size of the element as previously predicted in the literature, but rather, that the impurity diffusion coefficients have a much stronger correlation to the compressibility of the associated Ni-X dilute alloy. A charge density analysis on the transition state of six of the twenty-six systems shows how the impurity affects surrounding Ni atoms.;Assessments of the validity of the five-frequency model and the relaxation techniques for the treatment of the maximum energy point along the diffusion pathway are discussed using the Ni-Al system as a model case. First, an analysis of the assumptions made for the relaxation scheme of the three saddle configurations are made. It is shown that by using a new relaxation scheme, the calculated impurity diffusion coefficient can be improved with respect to experimental data. Additionally, an alternate calculation of the correlation factor for the impurity diffusion coefficient calculation is performed that assumes interactions of the solute and vacancy go beyond the first nearest neighbor shell. The alternate method includes the jump frequency associated with the migration of the host atom in the presence of an impurity at a second nearest neighbor position, as opposed to the original method which assumes this type of jump is analogous to self-diffusion in the host system. The result shows an increased agreement with the experimental data in the case of the Ni-Al system.;The goal of the present thesis is to provide a better understanding of the thermodynamic and kinetic parameters of Ni-base alloys to aid in the future development of more advanced Ni-base superalloy systems. The methodology, results, and analysis presented in this thesis provide a better understanding of the effects of alloying elements on the diffusion properties of dilute and non-dilute Ni alloys, and establish a benchmark for effects and trends of impurity diffusion in other magnetic alloy systems, such as bcc Fe as the host matrix. (Abstract shortened by UMI.).