Abstract

Accelerated Design and Characterization of

Oxidation Resistant Co-Based Gamma / Gamma-Prime Superalloys

by

Colin Alistair Stewart

Superalloys are metallic materials comprising predominantly an FCC matrix (γ) with an L12 intermetallic reinforcing precipitate (γ’), capable of load-bearing applications at high temperatures such as the hot section components of gas turbine engines. Recently discovered Co-based γ/γ’ alloys offer a potential avenue for improving engine performance beyond the current limit of commercial Ni-base alloys. To exploit their potential, Co-base alloys must be able to grow a protective, continuous α-Al2O3 scale after brief (minutes) exposure to an oxidizing environment at temperatures on the order of 1100°C. Simultaneously, an alloy must also maintain high thermal stability of the strengthening γ’ phase. The composition space available to achieve such behavior is vast, with commercial Ni-base superalloys typically containing 11+ elements. Instead of a more traditional alloy design process by incremental trial and error, this dissertation employs modern computational modeling in combination with high-throughput experimental techniques, pursuant to the mission statement of the NSF DMREF program.

Combinatorial ion plasma deposition was used to systematically synthesize 234 distinct alloy samples in Co-Ni-W-Al-Cr-Ta space. The alloy phase constitution and oxidation behavior after 1 h in air at 1100°C were investigated. Photostimulated luminescence spectroscopy (PSLS) was employed to rapidly screen for the presence of α-Al2O3 in the oxide scales. While PSLS can adequately detect the formation of α-Al2O3, its ability to do so is moderated by the presence of extraneous, non-desirable oxides with varying thickness and composition. Notably, it was concluded that overlaying CoO was more opaque to the PSLS signal than NiO. Nevertheless, it was shown that a large number of positive PSLS spectra was often related to the overall thickness of the scale, with thinner scales usually characterized by an underlying continuous α-Al2O3 scale. The merging of these synthesis and characterization techniques identified the compositional space of promising oxidation behavior, and the effects of Al, Cr, and to a lesser extent W and Ni, on this behavior.

The experimental results were combined with calculations of the senary phase equilibria using available thermodynamic databases. In this manner, the design space for Co-base γ/γ’ alloys with suitable stability of the reinforcing phase and desirable oxidation resistant were identified. New compositions designated as “DMREF” alloys were synthesized, and were found to achieve a superior combination of properties relative to other Co-base alloys reported to date. Thus, this dissertation has established the oxidation behavior and phase-constitution in composition space relevant to Co γ/γ’ superalloys, designed and generated a novel alloy with markedly improved properties, and demonstrated proof of concept for the strategy of accelerated exploration, design and characterization of new alloy systems.