Oxidation & coating evolution in aluminized fourth generation blade alloys

Abstract

Oxidation tests have been undertaken in air on two aluminized experimental fourth-generation Ni-based superalloys containing, respectively, 3 and 5 wt.% Ru, in order to detail the effect of Ru on oxidation characteristics and coating evolution. A Ru-free, aluminide coated third generation alloy (CMSX-10K) was also included in the test programme which used temperatures covering the range 750-1100°C Aluminization has been shown to effectively minimize the effect of alloy chemistry on oxidation rate constants over the timescales studied, when compared to the uncoated substrates. Aluminization also prevents the formation of internal oxidation pits in the coated Ru-bearing alloys after oxidation at 750°C Increasing Ru marginally decreased the mass-gain due to oxidation of the three aluminized alloys at all temperatures. At 750°C the rate of oxidation of aluminized specimens was controlled not by the formation of aluminas but by that of the spinel phase Ni(Al,Cr)2O4. Interestingly, each of the three aluminized alloys produced a different coating - substrate microstructural interaction after exposure at 1100°C, with Ru increasing the propensity for secondary reaction zone (SRZ) formation. SRZs are characterized by a weak high angle boundary which can lead to localized coating loss (and hence increased section loss). Ru also underwent long range diffusion, concentrating in the β-NiAl phase in solution, and where SRZ was present resulted in the precipitation of β2-RuAl within the γ′ SRZ matrix in addition to the topologically close-packed precipitates (TCPs).