On the early stages of scale development on Ni–22Al–30Pt with and without Hf additions at 1150°C

Abstract

Platinum modified Ni –Al-based alloys play an important role in the aeronautical industry as materials applied as Bond Coats (BCs) in thermal barrier coating systems (TBC), which provide protection against high temperature oxidation. Therefore, it is crucial for the assessment of the performance of the TBC to understand the oxidation behaviour of the Bond Coat material and the properties of the thermally grown oxide developing on it.

This paper reports on the scale growth mechanism of Pt-modified γ′-Ni₃Al-based alloys with and without Hf additions at the early stages of oxidation. Samples were oxidized at 1150°C for up to 50 min. The consecutive stages of the scale evolution were followed by a systematic approach using a two-stage oxidation treatment in atmospheres containing different amounts of ¹⁸O₂ oxygen isotope. The scale growth mechanism was followed with the aid of the in-depth analysis of elemental distribution, in particular of the oxygen isotopes, across the scale using secondary ion mass spectrometry (SIMS). The surface morphology of the scales was observed using scanning electron microscopy, while its local phase composition was determined using photoluminescence spectroscopy. In addition, the imaging SIMS was used to generate distribution maps of the oxygen isotopes. Because of the limited lateral resolution of the SIMS technique (down to 0.5 μm) they were conclusive only for the most prolonged exposures (50 min).

Typical stages of scale evolution were observed for the growing scales: initially flat layers were formed, followed by more or less developed blade-like surface grains, phase-transformation-related cracks and round patches and, finally, ridges. The latter were only found on the Hf-free material. The scale growth mechanism evolved from an initially predominant outward growth mechanism towards an increasing contribution of the inward mechanism. The relative extents of the outward and inward growth mechanisms depended strongly on the surface fraction of patches and through-scale cracks. The phase composition analyses showed that cracks and patches are regions where the transient aluminas were preferentially transformed into the α-Al₂O₃ phase which is the required protective scale. The ridges formed in cracks essentially consisted of α-Al₂O₃. For the most of studied exposure periods both types of phases: transient aluminas and α-Al₂O₃ co-existed in the scales.

The results obtained indicate that: (i) phase transformations occur locally and not simultaneously in the entire scale; (ii) Hf additions retard the phase transformation; (iii) the phase transformation is completed only on Hf-free material and not after oxidation for 50 min; (iv) the two-stage oxidation approach should be carefully applied to study the growth mechanism of evolving scales in which regions of different phase compositions develop.

Keywords:

• γ/γ′ bond coats
• early oxidation stages
• Hf-effect
• Ni-22Al-30Pt