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Thermodynamics and Creep Behavior of L12-containing Co-base Superalloys

  • Author(s): Rhein, Robert Karl
  • Advisor(s): Pollock, Tresa M
  • et al.
Abstract

L12-containing Co-base superalloys are of significant interest due to their high temperature strength and oxidation resistance. Previous research has shown that this class of alloys has demonstrated creep performance comparable to second-generation Ni-base superalloys. Like Ni-base superalloys, Co-base superalloys derive their strength from a two-phase γ-γ’ microstructure consisting of cuboidal ordered L12 precipitates embedded within a solid solution matrix. The thermal stability of this L12 precipitate phase is therefore of great importance if these alloys are to retain their mechanical properties in high-temperature applications. Topics explored in this study include: (i) thermal stability of the L12 γ’ precipitate phase, (ii) the creep performance of quinary Co-base superalloys, (iii) the deformation mechanisms present in these alloys, and (iv) oxidation behavior of these alloys.

The primary challenge in developing Co-base superalloys is to stabilize the L12 γ’ phase. It has been shown that Co-base superalloys exhibit lower γ’ solvus temperatures than their Ni-base counterparts. A comprehensive first-principles study was performed for the Co-Al-W ternary system in order to determine the stability of the L12 γ’ phase field. This analysis was first performed on the 75 at% Co pseudobinary line and it was found that the L12 γ’ phase is indeed stable at W-rich compositions at temperatures above 600 ◦C. This was then extended to the full Co-Al-W ternary system where a small phase field of L12 was also found at high temperature.

Higher-order d-block alloying additions were identified using subsequent first-principles calculations and single crystals of quinary Co-base superalloy compositions were cast using the conventional Bridgman process. Creep deformation mechanisms were characterized, with lower 900 ◦C temperature deformation dominated by antiphase boundary (APB) formation and higher 982 ◦C temperature deformation dominated by superlattice intrinsic stacking fault (SISF) formation. Additional simulations were performed using generalized stacking fault (GSF) surfaces as input to further investigate deformation mechanisms during creep. A parametric study was conducted in which different dislocation configurations interacted with different precipitate morphologies. The formation of SISFs is a dominant creep deformation mechanism in Co-base superalloys, and the solute segregation around them is an important consideration. To this end, experimental and modeling efforts were performed in order to qualitatively predict the changes in composition near these faults. Oxidation behavior of several CoNi-base superalloy compositions were additionally investigated to determine which compositions form a continuous, protective α-Al2O3 scale.

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