The role of cobalt in B2-NiAl precipitation and stability for a ferritic heat-resistant alloy

The coarsening of B2-NiAl precipitates severely limits the application of NiAl-strengthened ferritic heat-resistant alloys. In this study, diffusion couple experiments producing a cobalt-concentration gradient were employed to systematically investigate the effect of Co on the microstructure of a Fe-12.5Ni-4.5Al-10Cr-2Mo alloy. With increasing Co content, B2 precipitate nucleation is promoted and the coarsening tendency at 700 °C is effectively reduced, decreasing the average precipitate diameter from 225 nm to 161 nm after ageing for 240 h, while increasing both the number density and volume fraction. Co partitions into α-Fe matrix and B2 precipitates, lowering the α/B2 interface misfit from 0.94 ± 0.14% to 0.69 ± 0.15%, and the formation of Co-Al and Co-Al-Ni clusters with lower bonding energies provides an energetically favorable nucleation pathway. Meanwhile, a higher fraction of rod-shaped precipitates forms with Co addition. Regarding the matrix, increasing Co content progressively stabilizes austenite at 1200 °C and suppresses martensitic transformation. The alloy with 12.5 wt.% Co exhibits a nearly full austenitic structure after furnace cooling and subsequently transforms into submicron α-Fe grains with uniformly distributed B2 precipitates upon ageing, resulting in superior microhardness retention. These findings demonstrate that Co addition significantly improves microstructural stability and enables morphological control of precipitates in NiAl-strengthened ferritic alloys.