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 Pt-Aluminised 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.
Highlights
The 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) [6,7,8] 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
Summary
The 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]. Understanding how the individual alloy elements affect SRZ morphologies in coated fourth generation Ni-base superalloys contributes to the development of improved Ni-base superalloys
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