The Effect of Various Additions on the Oxidation Behavior of the γ/γ′ Ni-Based Alloy

The oxidation behavior of four γ/γ′ Ni-based alloys: without additions, and containing: Hf (1.0 wt%), a combination of Hf and Y (0.3 wt% and 0.03 wt%, respectively), and a combination of Hf and Si (1.0 wt% and 1.0 wt%, respectively) was studied in air under isothermal (50 h) and thermal cycling (up to 2225 1-h cycles) conditions. Samples were characterized using SEM, EDX and XRD techniques. The results indicated that all the additions improved the oxidation resistance of the alloy but only in the case of materials containing the (Hf + Y) combination of additions was a long-term effect achieved. Substantial weight losses were observed on the other unmodified and (Hf + Si)-containing materials during thermal cycling after short exposure periods, while on material containing only additions of Hf, they occurred significantly later. Kinetic studies showed the highest oxidation rate in the case of the (Hf + Si)-containing alloys and the fastest initial oxidation, prior to the parabolic law-obeying stage, of alloy with Hf-additions, only. The results indicate the superior effect of simultaneous application of Hf and Y additions at levels not exceeding their solubility limits and that lowering its effectiveness Hf overdoping (1 wt%) cannot be effectively counteracted by the addition of 1% Si.