Phase transformation pathways in BCC-B2 superalloys

The development of high-temperature materials requires a deep understanding of phase transformations governing microstructural stability and mechanical properties. BCC+B2 superalloys, inspired by the γ/γ′ microstructure in Ni-based superalloys, feature a ductile BCC matrix with coherent B2 precipitates that enhance high-temperature strength and creep resistance. This article examines the thermodynamics and phase transformation pathways leading to BCC+B2 microstructures in ferritic and refractory superalloys. Their stability depends on miscibility gaps, lattice misfit, elastic modulus contrast, and site occupancies. Ferritic superalloys (Fe-Al-based) are suited for applications below 1000 °C, while refractory superalloys target extreme temperatures (1300 °C–1800 °C). However, challenges remain in phase stability, oxidation resistance, and creep performance. Strategies for microstructural control through heat treatment, phase-field modeling, and CALPHAD-based design are discussed. Enhancing these aspects will optimize BCC+B2 superalloys for aerospace, energy, and defense applications, providing a pathway for next-generation materials designed for extreme environments.