Development and characterization of W-free Co-based superalloys

Abstract

Superalloys show outstanding high-temperature properties such as creep, oxidation, and hot corrosion resistance and are therefore the key materials in jet propulsion gas turbines. Recently, Co-based superalloys have attracted a great deal of attention for high-temperature applications by the discovery of a γ/γ′ microstructure in the ternary Co-Al-W system. The main drawbacks of Co-Al-W-based superalloys are a limited γ′ phase stability and a high density. Co-Ti system, as the only known binary Co system which forms thermodynamically stable γ/γ′ microstructure, is used as the base system of alloy development instead of Co-Al-W. There are also several challenges associated with Co-Ti alloys which must be overcome such as a low γ′ volume fraction and γ′ solvus temperature. Therefore, more advanced Co-Ti-based alloys forming the desired γ/γ′ microstructures with optimized properties must be developed. In this dissertation, the effects of the alloying elements on the microstructure and mechanical properties of Co-Ti alloys were investigated. The atomic-scale elemental partitioning behavior of alloys were studied in terms of 3D atom probe tomography.