Analytical TEM and SEM characterisation of aluminide coatings on nickel based superalloy CMSX-4

Abstract

Aluminide coatings are applied on gas turbine components to improve oxidation and corrosion resistance or as bond coats for thermal barrier coatings (TBC). In the present study, coatings produced on a CMSX-4 superalloy by pack gas phase aluminising and subsequently annealed at 1050°C for 2 h were investigated. Microstructure, morphology and phase composition of the coating were characterised using light microscopy, X-ray diffraction, scanning electron microscopy and analytical transmission electron microscopy (TEM). Scanning electron microscopy (SEM) investigations of the 39 µm thick coating revealed a complex multilayered microstructure produced by diffusion at 1050°C at the surface of CMSX-4 alloy. Detailed TEM phase analysis was performed on cross-section thin foils by electron diffraction and energy dispersive X-ray spectrometry (EDS). The outer zone (1) consisted of β-NiAl phase. The interdifusion zone (2) contained two sublayers consisting of a Ni₅Al₃ matrix with precipitates of topologically close packed phases, most probably σ and µ. Close to the surface of the CMSX-4 superalloy, a further zone (3) consisting of γ+γ′ with large precipitates (possibly µ and P phases) was observed.