Creep modeling and first-principles investigation of high-temperature alloys

Abstract: Stainless steels and nickel-based superalloys are materials that have been widely used to manufacture components servicing at high temperatures. Creep strength is one of the most important properties in such conditions. High creep strength generally comes from a combination of solid solution hardening, precipitation hardening and dislocation hardening. However, some details of the mechanism of solid solution hardening are still not fully understood. The present thesis can be separated into two parts.In the first part, fundamental creep models are used in an investigation of creep rate of nickel-based alloys. The fundamental models are based on dislocation theories without using any adjustable parameters to describe the creep data. All parameters in the models have been derived from experimental data or using computational approaches such as ab initio methods. In the models, the effects of stacking faults, strain-induced vacancies and pipe diffusion have been taken into account. W is a vital element to create solid solution hardening and improve creep strength of nickel-based alloys. W readily dissolves in nickel to form a solid solution and to provide a significant effect on the creep strength. Moreover, many ternary and more complex systems of nickel-based alloys are developed starting from Ni-W solid solutions. The developed models can describe the dramatic reduction of the creep rate due to W, which has not been possible in the past. The reduction has a close correlation with the stacking fault energy and drag stress.In the second part, the exact muffin-tin orbital method combined with the coherent potential approximation has been interfaced with a quasi-harmonic Debye model to predict the elastic and other thermodynamic properties of selected metallic alloys at high temperature. The knowledge of such properties is very useful in modeling the behavior of materials servicing at high temperatures. However, few experimental studies have been focused on measurements of thermo-mechanical properties at such temperatures. Ab initio methods based on density functional theory is an alternative way to obtain information about thermo-mechanical properties. Therefore, in the present work, ab initio based studies of the elastic and thermodynamic properties of pure nickel, nickel-based solid solutions and Fe25Cr20NiMnNb austenitic stainless steel have been performed. Although the modeling technique cannot fully reproduce the temperature dependencies in some of the considered cases where experimental data are available, the computed values of such properties as shear moduli, thermal expansion coefficients and entropy are close to the experimentally derived values.