A Qualitative Study of the Oxidation Resistance of PBF-LB/M High-Ta Ni-Based Superalloys with Hf Additions

Recent studies have shown that a new family of Ni-based superalloys with high Ta and high Hf contents exhibits a promising property profile and may be able to fill the gap between creep-resistant alloys and those processable by PBF-LB/M. The effect of simultaneously high Ta and Hf contents on oxidation resistance has not yet been investigated and is addressed qualitatively in this study. Isothermal oxidation tests were conducted in air at 950 °C for 100 h, 300 h, and 500 h. After cooling, the weight change and cross-sections of the specimens were examined. The study shows that the Hf-free alloy exhibits severe spallation of the Al-oxide and Cr-/Ni-oxide layer. The Hf-containing alloys exhibit improved oxide layer adhesion and a promoted formation of a continuous Al-oxide layer, which is attributed to the early formation of Hf-oxide particles. Furthermore, the addition of Hf influences the morphology of internally oxidized Al, which grows preferentially parallel to the surface rather than perpendicular to it. This behavior leads to effective protection of the alloys by an Al-oxide layer, either external or internal, which is remarkable considering the moderate Al content of only 3 wt.%.