Effect of Surface Treatment and Crystal Orientation on Microstructural Changes in Aluminized Ni-Based Single-Crystal Superalloy

Abstract

The effect of pre-surface treatment and crystal orientation on microstructural changes in the aluminized 4th generation Ni-based singlecrystal superalloy TMS-138 was investigated. The substrate superalloy was cut along the {100} and {110} planes on which three kinds of surface finishing, such as grit blasting, mechanical polishing and electro-polishing were conducted prior to the conventional high-activity aluminizing process. A thermal cycling test at 1373 K revealed that heavy deformation of the substrate surface by grit blasting gave rise to the formation of a secondary reaction zone (SRZ) in the vicinity of the interdiffusion zone/substrate interfaces. When the surfaces were finished by electropolishing, voids were formed in the vicinity of the interdiffusion zone/substrate interfaces. It was also found that accelerated formation of SRZ and voids was observed along the h110i directions rather than the h100i directions during the thermal cycling test. The difference in morphological changes of substrates can be related to the residual stress introduced by the surface finishing.

Effect of Thermal History on Microstructural Changes in Aluminized Nickel-Based Single-Crystal Superalloy

“…On the other hand, when the substrate surface was not deformed, numerous voids were formed. 19) In this study, SDZ formation was observed in the isothermally heated aluminized specimen, while SRZ formation was observed in the Pt-aluminized specimen, even though there was no substrate surface deformation. The reason for such different changes in the microstructure will be discussed below.…”

“…This mechanism seems to be another factor causing void growth. 19) On the other hand, in the isothermally heated specimen, an SDZ was formed. In this specimen, the outward diffusion from the substrate is faster than the inward diffusion from the coated layer, as for the cyclically heated specimen.…”

“…It has also been reported that the Al concentration is higher in the SRZ than in the areas other than the SRZ. 19) However, in this study, the surface strains in the substrates were removed by electropolishing, so it is unlikely that the surface residual strain promoted SRZ formation.…”

Isothermal Oxidation of Aluminized Coatings on High-Entropy Alloys

“…That is the method for HEAs to confront temperature up to 1400 K and even higher. The same matter is also resolved for superalloys, since they are widely used for blades in turbine engines [10][11][12][13]. In aircraft, the gas turbines have been equipped with single-crystal blades.…”

“…The aluminizing process is an effective coating method frequently used for the protection of metal surfaces against oxidation, via the formation of an α-Al 2 O 3 layer. This technique is provided with a simple apparatus set-up, high cost performance and uniform formation of coatings [12][13][14].…”