The effects of Fe and Si on the phase equilibria in a γ′-strengthened Co–Al–W-based superalloy

Abstract

The effects of Fe and Si alloying additions (by replacing Co) in a base alloy composition (expressed as mole fraction times 100) Co–20Ni–10Cr–10Al–7W-0.1B on the stability of the microstructure were investigated at 800 °C and 900 °C. The two-phase γ+γ′ microstructure was found to be greatly destabilized with Fe-additions in a mole fraction of 5% or greater, exhibiting a decrease in γ′ solvus temperature by about 8 °C for each 1% Fe added, and a significantly higher volume of secondary phases μ and B2 compared to the base alloy at both temperatures. In contrast, Si additions of up to a mole fraction of 1% did not decrease the γ′ stability by an appreciable amount. Characterization of the Si additions showed no decrease in γ′ solvus temperature, no change in the fractions of secondary phases in the annealed microstructure after annealing for 168 h at 800 °C with 0.5% Si, and only a slight increase in the secondary phase fractions for a mole fraction of 1% Si. Annealing at 900 °C for 1001 h, however, did show an increase in secondary phase formation, suggesting a decrease in the stability of the two-phase γ+γ’ microstructure at higher temperature. The present results show that it may be possible to add up to a mole fraction of 1% Si for increased oxidation resistance without significantly degrading the high temperature capability of the base Co-base superalloy.