The Role of the γ′ Precipitate Dispersion in Forming a Protective Scale on Ni-Based Superalloys at 750 °C

Abstract

Fourth generation Ni-based superalloys are being developed for use as single crystal turbine blades in aero-engines. They contain between, approximately, 2–5 wt% Ru together with ~6 wt% Re and differing amounts of other refractory elements such as W, Ta and Mo. These alloys experience sub-surface pitting attack when oxidised in air at intermediate temperatures (~750 °C). The pits consist of γ′ particles (nominally, Ni3Al) which are oxidised in situ and separated by unoxidised γ channels. Earlier generations of alloys do not show this form of attack and, in contrast, form a protective surface layer of NiAl2O4. The reasons for this difference are explored in this paper. It is shown that the dispersion of the strengthening γ′ particles is much more ordered in the fourth generation, Ru-bearing alloy than in a third generation (CMSX10-K, also known as RR3000), Ru-free variant. It is considered that this is the principal reason why pit formation is observed in the later alloy. Pits arise because the long γ channels between the arrays of γ′ particles have insufficient aluminium content to form the protective oxide and remain open to inward diffusion of oxygen over extended periods of time. The third generation alloy, on the other hand, has a more random arrangement of γ′ particles which is conducive to the early formation of a protective oxide layer.