Hf affecting growth stress and adhesion of pre-oxidized film on AFA alloys with two-step process under low oxygen pressure

The effect of Hf on formation, residual stress, and adhesion of the pre-oxidation film on the surface of Fe-45Ni-25Cr-4Al-1Nb-0.3Si-0.45C-xHf (x = 0, 0.05, 0.25 wt.%) austenitic stainless steel was studied using a two-step process under low oxygen pressure. In comparison with Hf-free alloy, Hf segregated at columnar Al₂O₃ grain boundaries, effectively sup-pressing both outward Al³⁺ and inward O^2- diffusion, which led to a reduction in film thickness. The thinner film resulted in lower growth stress, inhibiting wrinkling, and minimized elastic strain energy accumulation during cooling. Further-more, Hf addition hindered interfacial pore formation and promoted internal oxides near the surface. However, with only 0.05 wt.% Hf, spallation was not completely inhibited. When Hf content was raised to 0.25 wt.%, the columnar grains were refined, thereby improving the film elasticity. Simultaneously, the internal Hf-rich oxide pegs formed at the film/matrix interface, along with the enriched HfO₂ pinning phases located between the columnar grains adjacent to the substrate, collectively improving the adhesion of Al₂O₃ film and significantly enhancing its resistance to cracking and spallation.