Secondary Reaction Zone Formations in Pt-Aluminised Fourth Generation Ni-Base Single Crystal Superalloys

Abstract

Abstract. Fourth generation superalloys are characterised by the addition of Ru which contributes to improved creep resistance whilst improving the microstructural stability. However, Ru additions have a negative effect on coated Ni-base superalloys, promoting Secondary Reaction Zone (SRZ) formation. Formation of a layer of SRZ beneath an aluminised or Pt-aluminised coating has the potential to reduce the effective cross section of a blade by in excess of 100 µm or 10% of the wall thickness. In this paper the effects of alloy composition on the formation of the SRZ in PtAluminised fourth generation alloys were investigated systematically. A series of experimental fourth generation alloys was used having two distinct compositions of Co, Mo, W and Ru and conforming to a four factorial 'Design of Experiments' model. These alloys showed significant and consistent changes in the SRZ depending on alloy composition. These were in distinct contrast to the effects of these elements on stability in the bulk. Mo was demonstrated to be by far the most effective element suppressing SRZ formation, followed by Co. In contrast, both W and Ru enhance SRZ formation. IntroductionThe latest fourth generation Ni-base superalloys, contain between 2 and 5% Ruthenium (Ru), which improves the mechanical properties in part by suppressing the formation of deleterious intermetallic Topologically Close Packed (TCP) phases [1,2]. This comes at the cost of the degradation of the oxidation resistance and coatings, such as Platinum Aluminide coatings (PtAl) and MCrAlY, are necessary to protect the turbine blades during service, details of these coatings are given elsewhere [3][4][5].The formation of the secondary reaction zone (SRZ) [6-8] is a major problem associated with coated Ni-base superalloys leading to the loss of the coating, and degradation of the properties of coated Ni-base superalloys, and is potentially life-limiting to turbine blades. The SRZ is an intermediate layer formed between an aluminised or Pt-Aluminised coating and the substrate by a discontinuous precipitation reaction similar to recrystallisation. It transforms the metastable aluminium-enriched substrate microstructure into an equilibrium mix of ',  and TCPs. The TCPs coarsen and align perpendicular to the growth direction facilitated by the rapid diffusion path of the high angle boundary. Therefore, the SRZ is associated with poor mechanical properties and provides boundaries allowing a high diffusivity path into the substrate facilitating oxidation and spallation. Traditionally alloys have been developed principally for mechanical performance and any deficit in environmental performance has been remedied by the application of a coating. However the realisation that the longevity and properties of coated Ni-base superalloys are significantly correlated with alloy composition has placed optimisation of the coating performance as a critical part of alloy development. Understanding how the individual alloy elements affect SRZ morphologies in coated fourth generation Ni...

Comparison of microstructural evolution in Pt aluminised Ni based superalloys with and without Ru

“…This effect is observed to kick in later with SRR300D-ref due to the lower prior exposure. A similar phenomenon was observed previously 15,21…”

“…A similar phenomenon was observed previously. 15,21 Secondary reaction zone growth rates in both alloys with and without Ru were very similar up to 50 h exposure, regardless of morphology, and the growth rate in both alloys decreased after 100 h. After 100 h, the growth rate in SRR300D saturated and remained stable through 500 h, while the growth rate in SRR300Dz3Ru continued to increase and showed a slightly higher growth than the Ru free alloy. Up to 100 h, the highest penetration depth in SRR300D in areas where the SRZ had precipitated was very similar to that in SRR300Dz3Ru (Fig.…”

Formation of Diffusion Zones in Coated Ni-Al-X Ternary Alloys and Ni-Based Superalloys

“…Many of the X-rich (X: Re or Ta) precipitates within the diffusion zone also grow perpendicular to the substrate in Fig. 2, and this is a consistent propensity of TCPs growing in SRZ formed in coated Ni-based superalloys [1][2][3][4]. However, the interfacial morphology between β-NiAl and the substrate (γ/γ' phases) formed in Ni-Al-Cr was very similar to that in CMSX-4, and therefore was classified as an inter-diffusion zone.…”

“…Previous studies have shown both positive and negative effects of different alloying elements in the case of Pt-aluminized fourth generation Ni-based superalloys, e.g. Ru encourages SRZ nucleation [5], and Mo is an effective element in impeding SRZ formation [4]. However, these effects have been observed from various multicomponent Ni-based superalloys, with complex alloy compositions, and are not always consistent across alloy systems.…”

“…As the operating temperature increases and the environment becomes more aggressive, the oxidation and corrosion resistance of turbine blade materials becomes more crucial than before. Furthermore, inter-diffusion of alloy elements between a coating and a substrate becomes more important, as this will be very significant at higher temperature, and will accommodate microstructure changes during service, such as the formation of Secondary Reaction Zones (SRZs) [1][2][3][4][5][6] and Topologically Close-packed (TCP) phases [7]. The term 'SRZ' was suggested by Walston [1], and they were first observed in coated third generation Ni-based superalloys.…”