Effect of γ’ volume fraction on the precipitation kinetics of the σ phase in wrought Ni-base superalloys

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The precipitation of topologically close-packed (TCP) phases, among which the σ phase is one of its most important representatives in nickel-base superalloys, is a matter of increasing significance because new generations of alloys are designed with higher concentrations of refractory element to meet the ever-increasing requirements of high-temperature materials. As significant volume fractions of TCP phases are known to have negative effects on key properties of superalloys (e.g. creep strength, fracture toughness, fatigue and oxidation resistances), research efforts are being made to either destabilize these phases or decelerate their precipitation kinetics. For the latter reason, it is vital to have a better understanding of the factors that affect the precipitation kinetics of TCP phases in superalloys. The present proposal aims to investigate how the presence of the γʹ phase and its volume fraction influence the precipitation kinetics of the σ phase in wrought superalloys. The motivation to study this behavior is based on a simple observation, i.e., the formation of TCP phases in superalloys is typically accompanied with the formation of γ’ envelops around TCP particles. Since the γ’ phase has a low solubility for TCP formers such as Cr, Mo, W, etc., the γ’ envelops are expected to strongly decelerate the growth kinetics of TCP particles. However, to the best of our knowledge, this important microstructural feature has not been considered so far in simulations of TCP-phase precipitation kinetics in superalloys. The investigation of this issue is complex because there are other factors that also affect the precipitation kinetics of TCP phases in superalloys (e.g. driving force, elemental diffusivities, misfit, mean grain size, etc.). To overcome this difficulty, the originality of the present project consists in investigating model alloys with different γ’ volume fractions in which these factors are kept constant. To reach this goal, alloy design is guided by thermodynamic calculations. It is proposed to investigate metastable alloys in the precipitation hardened state with an initial γ/γ’ microstructure that have similar grain sizes and the same γ composition (at a given temperature). The latter will ensure that the driving force for TCP-phase formation, the misfit between TCP phases and γ matrix, and the diffusivities remain constant. In the present project, atom probe tomography will be employed to characterize the precipitation hardened alloys. These will be then aged between 800 and 925 °C for various times followed by chemical and microstructural investigations at several length scales to investigate how time-temperature-transformation diagrams for the σ-phase precipitation kinetics are delayed with increasing γ’ volume fraction. Overall, the systematic understanding of relationships between phase stability and transformation kinetics will provide the basis for developing new generations of wrought superalloys with optimized properties.