Abstract: In aerospace, BBC-Nb alloys confront notable challenges in thermal stability and toughness under cyclic fatigue at varying temperatures. Insufficient thermal stability and expedited coalescence of precipitates substantially accelerates the degradation of alloys at elevated temperatures. Here, a Nb alloy with impressive thermal stability and mechanical properties was designed using theoretical calculations and a two-step graded heat treatment process. The superlative properties of the Nb alloy are primarily associated with the NbC hierarchical structures, i.e., stable nanoparticles in Nb-BCC grains and discontinuous microparticles at grain boundaries (GBs). The hierarchical carbides configuration avoids continuous precipitation of carbides at GBs and preferential coarsening within the grains. The process involves precipitating ZrC nanoparticles at 1350 °C, then stabilizing NbC at 1800 °C by replacing Zr with Nb. Nb-FCC nanophases enveloping NbC prevent coarsening and have strong relationships with both NbC nanoparticles and matrix. The concept of fine-tuning NbC precipitation within grains and introducing NbC at GBs with a substitution method offers a strategy for high-strength, heat-resistant materials.

Key words: Toughness, Thermal stability, Nb superalloy, Carbides, Hierarchical structure