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

  • Abstract: 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 O2- 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.

     

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