Abstract
The rapid growth of modern industries and technologies has raised an urgent demand for advanced materials with a higher energy efficiency and engineering reliability. Ni-based superalloys have long been widely employed under extreme environments of both high temperatures and stresses, such as gas turbine engines for the aircraft propulsion and power generation. However, further improvements in operating temperatures via Ni-based superalloys are approaching their limits, which are hindered by their melting temperatures. By comparison, Co-based alloys demonstrate a promising potential to surpass Ni-based superalloys with higher operating temperatures due to the inherent higher melting temperature of Co over Ni. Conventional Co-based alloys rely on carbide formation and solid-solution for strengthening. However, such strengthening mechanisms appear to be less effective at elevated temperatures. The discovery of ordered L12-type precipitates among Co-based alloys provides a new approach to design high-temperature structural materials with broader temperature capabilities. In this thesis work, a series of L12-strengthened Co-rich multicomponent alloys are carefully formulated and evaluated for enhanced metallurgical and mechanical properties. Phase equilibria, alloying effects, microstructural evolutions, and mechanical properties are all addressed in this thesis, with the aim of advancing their high-temperature applications.In the first part of the thesis, we started off with a compositionally simple Co- based ternary alloy and eventually attained multicomponent Co-rich alloys with improved γ′ stability by carefully tunning alloying additions. L12-type γ′-Co3(Al, Nb) precipitates were experimentally discovered in the Co-Al-Nb ternary alloy system upon an isothermal heat treatment at 700 °C, whose presence provided a substantial hardening effect. However, the precipitates dissolved into the γ-Co matrix as the temperature increased to 800 °C, leaving B2-CoAl phase in equilibrium with the γ-Co matrix. Given the insufficient thermal stability of the L12 strengthener, the subsequent research focused on the stabilization of the γ′ phase via alloying additions, such as Ni, Ti, and Ta. These alloying additions were found to substantially enhance the thermal stability of the γ′ phase by significantly increasing the γ′-solvus temperature. Multicomponent Co-Al-Nb-Ni-Ti-Ta alloys were designed and developed with a stable γ/γ′ dual-phase microstructure. High-density γ′ precipitation with an improved thermal stability resulted in a remarkable high-temperature strength (984 MPa at 750 °C).
A multicomponent Co-rich alloy was designed in the second part of the thesis based on the following considerations: (1) introducing Ni to the γ-Co matrix for the broadening of the γ-γ′ dual-phase region and suppressing detrimental intermetallic phases formation; (2) alloying with Al and Cr additions to impart oxidation resistance; (3) using Ti, Mo, Ta, and Nb as substitution for the high-density W to reduce the overall mass density without destabilizing the γ-γ′ microstructure. The multicomponent Co-rich alloy maintained a stable γ/γ′ dual-phase microstructure without any detrimental intermetallic phase formation neither at grain boundaries nor grain interior upon the long-term thermal exposure in the temperature range between 800 and 1100 °C. The γ′ precipitates coarsened via coalescence and coagulation mechanism, instead of the classic evaporation and condensation mechanism. The temporal evolutions of the L12 precipitation were also quantitatively evaluated for the multicomponent Co-rich alloy. The multicomponent Co-rich alloy exhibited an excellent thermal stability for the precipitates (1125 °C as the γ′-solvus temperature) together with a low mass density (8.28 g cm-3). More surprisingly, a remarkable yield strength at elevated temperatures (664 MPa at 900 °C) was achieved in the multicomponent Co-rich alloy, which is comparable with well-developed commercial Ni-based superalloy Waspaloy.
Our present findings demonstrate the feasibility of designing L12-strengthened high-temperature materials based on cobalt. The investigations on the effects of alloying additions on the microstructural evolutions and mechanical performance, elemental partitioning behavior, and coarsening kinetics lay the foundations for understanding these emerging L12-strengthened Co-based alloy systems. Based on these results, we offer an essential paradigm for the future development of advanced structural materials with broader temperature capabilities.
| Date of Award | 28 Jan 2021 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Chain Tsuan LIU (Supervisor) |