On stabilizing an α/α′/α″ microstructure in ferritic superalloys

Ferritic superalloys with an α/α′ (A2/B2) microstructure constitute an auspicious materials system for high-temperature application and could help satisfy the need for more sustainable creep resistance materials. An exciting way to improve their creep resistance further through alloy development and microstructure engineering is introducing a hierarchical network consisting of α, α′, and α″ (L2 ₁ ). This work investigates the formation and evolution of this microstructure in Fe-Al-Ni-Co(Cr-Mo-Ti-Cu) alloys. It has been identified that the substitution of Fe by Cr leads to the stabilization of γ (fcc, A1) at intermediate temperatures. This amount of γ was found to decrease in the presence of Mo, and the transformation is entirely avoidable by carefully adjusting the α - and γ-stabilizing elements. The observations in quinary and senary alloys led to the development of two promising hierarchical α/α′/α″ alloys with additions of Co and Cu. Both solutes are enriched in the precipitates and are expected to act as strengthening elements. Microstructural analysis of the particle size evolution in the range of 800–950 °C indicates that the precipitates undergo a non-classical ripening process at early stages with coarsening exponent deviating from the t ^1/3-relationship predicted by the Lifshitz-Slyozov-Wagner (LSW) theory for multicomponent alloys. Furthermore, after furnace cooling from 950 °C, a pronounced reprecipitation phenomenon was found to occur not only in the matrix but also inside the primary precipitates.