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.

Overaged metallography of alloy 909, a low coefficient of expansion superalloy

“…It has been previously reported in the literature that e phase (platelet) precipitates were present in the aged condition but not in commercially aged heat treatment condition. 1,[3][4][5] The previous investigation by the authors 6 did not confirm the presence of e phase in the STA condition, though there were some similar evidence to that of Guo et al, 3 showing c9 particles precipitating in a line type fashion, likely at an early stage in the transformation to the e platelets. Guo et al 3 have also concluded that when aged at higher temperatures (770uC), the e platelets precipitate directly from the supersaturated c matrix.…”

“…The black grains were discussed in a previous paper by the authors, and the reader is referred to Ref. 6 for further details. Figure 2 shows a SEM image of alloy 909 in STOA condition showing the globular phase as well as copious amounts of the acicular or a platelet-like phase.…”

“…The black grains were discussed in a previous paper by the authors, and the reader is referred to Ref. 6 for further details.…”

“…A previous publication by the authors6 evaluated the microstructure of alloy 909 in the solution treated and aged condition (STA), which consisted of solution treatment at 982°C, air cooling, followed by aging at 712°C for 8 h, furnace cooling to 621°C, holding at 621°C for 8 h and air cooling. This treatment was shown by Smith and Holderby7 to give the best combination of mechanical properties.…”

Effects of Elevated Withdrawal Rate on the Microstructure and Segregation Behavior of a Nickel-base Single Crystal Superalloy

“…Therefore, it is very important to investigate the effect of withdrawal rates on the microstructure of nickelbase single crystal superalloys. Moreover, the multi-alloying of single crystal superalloys can lead to severe segregation in the as-cast microstructure and ultimately influences the heat treatment process and the performance of alloy [12][13][14] . In the present paper, the relationships between growth rates of the nickel-base single crystal superalloys, dendrite arm spacings (λ 1 , λ 2 ), γ' phase, γ/γ' eutectic and composition segregation were investigated.…”