Superalloys strengthened by γ′(L12)-precipitates in γ(f.c.c.)-matrix exhibit superior high temperature mechanical properties and environmental resistance over long periods of operation, making them ideal candidates for aerospace and energy conversion applications. The emerging class of superalloys based on Co-Al-W ternary system was identified with a melting temperature 50-100 ˚C higher than conventional Ni-based superalloys, which after decades of incremental development, are operating at their temperature limit. However, before Co-based superalloys may be considered for practical applications, numerous materials related challenges, such as the inferior mechanical properties, need to be addressed. Studies are performed to investigate the effect of lattice misfit on microstructural and mechanical properties of Co-based superalloys, where Cr alloying is used to adjust the lattice misfit. The evolution of microstructure is studied using scanning electron microscopy (SEM) to characterize the stability of γ-γ′ microstructure and morphology of γ′-precipitates. Atom-probe tomography (APT) provides accurate measure of elemental distribution between the γ′-precipitate and γ-matrix phase. The elevated temperature mechanical properties are assessed by creep experiments. The effect of alloying addition and lattice misfit are then correlated to the observed microstructure and mechanical properties. Additions of Cr have shown to gradually alter the lattice parameter of γ- and γ′-phases leading to a decreased lattice misfit and a transition from cuboidal to spherical γ′-morphology. The Cr-containing alloy has demonstrated a superior environmental resistance due to an improved ability to form a passivating chromium and aluminum oxide upon exposing to an oxygen atmosphere. Chromium was found to deteriorate the compressive creep resistance in quaternary (Co-9Al-9W-xCr) and quinary (Co-30Ni-10Al-7W-xCr) alloys but poses minimal impact in multicomponent systems with more than 7 alloying components. The discrepancy in creep behavior is attributed to the combined weakening effect of a reduced lattice misfit and a strengthening effect from additions of Ni, Ti and Ta. Through both phase-field simulation and experimental observation, lattice misfit was shown to alter the stress induced directional coarsening (rafting) behavior, where tensile stress results in p-type rafts while compressive stress leads to a n-type rafted microstructure. After creep, the alloy with a higher lattice misfit exhibit an extensively rafted microstructure while alloys with a low lattice misfit maintain its initial microstructure. APT analysis suggests that compositional re-distribution occur along the γ-γ′ interface and within the γ-matrix to facilitate the rafting driven diffusion during creep. The impact of γ′-raft orientation on the creep resistance was also assessed via tensile creep, and show that a n-type rafted microstructure demonstrates a creep resistance that is twice of p-type rafted microstructure. However, the instability of n-type morphology under prolonged tensile load prompts additional work to maintain the microstructure during creep.

Creator

• Chung, Ding-Wen

DOI

• https://doi.org/10.21985/n2-tgjh-b426

Subject

• Materials Science

Language

• en

Alternate Identifier

• etdadmin_upload_717986
• http://dissertations.umi.com/northwestern:14989

Keyword

• Microstructural evolution
• Co-based superalloys
• Atom-probe tomography
• Phase-field
• Mechanical properties
• Creep

Date created

• 2020-01-01

Resource type

• Dissertation

Rights statement

• In Copyright