Microstructure Evolution of a Platinum-Modified Nickel-Aluminide Coating During Thermal and Thermo-mechanical Fatigue
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MATERIALS used to produce hot components of aero- and land-based gas turbine engines have to possess high strength, oxidation, and corrosion resistance, as they are subjected to different thermo-mechanical loadings at elevated temperatures close to 1373 K (1100 C) and higher. Ni-based superalloys are particularly well designed to resist these severe conditions leading to high temperature creep. Nickel-aluminide and MCrAlY coatings are known to increase the oxidation resistance of the superalloys providing supplementary Al and/or Cr to form a stable a-Al2O3 (or Cr2O3) scale on the surface,[1] which then protects the superalloy from further oxidation. Moreover, Pt is added to increase the adherence, as well as to decrease growth kinetics of the a-Al2O3 scale for nickel-aluminide coatings.[2]
PIERRE SALLOT, Advanced Materials R&D Engineer, is with Safran Tech, Rue Genevie`ve Aube´ 78117, Chateaufort, France. Contact e-mail: [email protected] VINCENT MAUREL, Research Professor, LUC RE´MY, Senior Research Professor, and FRANCK N’GUYEN, Ph.D. Researcher, are with the PSL Research University, MAT - Centre des Mate´riaux, CNRS UMR 7633, MINES ParisTech, Evry Cedex, France. ARNAUD LONGUET, Material & Process Leader, is with Snecma, Rond Point Rene´ Ravaud, Re´au, 77550, Moissy-Cramayel, France. Manuscript submitted November 27, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
Platinum-modified nickel-aluminide coatings are produced through several stages: (i) Pt electroplating on the superalloy substrate, (ii) diffusion treatment under vacuum, followed by (iii) aluminization process.[3] After such a procedure, the coating consists of either an external single-phase b-(Ni,Pt)Al layer or a dual-phase b-(Ni,Pt)Al + PtAl2 layer depending on process parameters.[3,4] Moreover, during the coating deposition the diffusion of elements from substrate to the external coating and vice versa leads to the formation of an interdiffusion zone (IDZ).[3–6] Basuki et al.[7] have also shown that for a typical nickel-aluminide coating deposited on Rene´ 80H, this IDZ was initially composed of a b-NiAl matrix that was progressively transformed into c phase (nickel-based solid solution) during high temperature exposure. Therefore, coating/substrate interdiffusion is an important phenomenon, as it affects coating microstructure and thus both oxidation resistance and mechanical properties of the substrate. For example, Nesbitt[8] clearly illustrated the detrimental effect of diffusional degradations on the (Ni,Co)CrAl coating lifetime. In this case two mechanisms are responsible for coating degradation: Al loss due to the Al2O3 formation on the surface and Al loss due to the diffusion of Al from the coating to the superalloy substrate. Therefore, the coating becomes depleted in Al and is not able to supply enough Al to the surface for continuous growth of the Al2O3 scale, which finally leads to a catastrophic oxidation behavior. Such effects are observed as well for
both plain[7,9,10] and Pt-modified nickel aluminide used as bond coats for typical th
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