Microstructure characterisation of solution treated (ST) and solution treated and aged (STA) Incoloy 909

Abstract

A microstructural analysis has been carried out on the commercially solution treated and aged (STA) Incoloy 909 superalloy to evaluate the phases present using optical, SEM, TEM and differential scanning calorimetry. Comparison is made where possible with non-standard STA analyses reported in the literature. In the STA condition, the main phase present besides the γ matrix was a deliberately produced Laves phase to control grain growth. Increasing the solution temperature above that recommended for the alloy resulted in eventual solution of the Laves phase and concomitant grain growth. In the STA condition, in addition to the Laves phase, a copious precipitation of a γ′ phase (Ni₃TiNb) was observed. In addition, a limited observation was made of the early stages of the transition of some of the γ′ phase to the ϵ phase for the over aged condition. There was no ϵ phase observed in the STA condition. A final observation was the presence of ‘black grain’ previously reported in the literature, in both the ST and STA conditions in the authors’ research. The present view is that the phenomenon is an etching artefact and not a genuine microstructural feature.