Sigma phase formation in conventional and PM nickel-base superalloys

Abstract

Ageing treatments of up to 5000 h at 705–980°C have been carried out on five chemically modified nickelbase superalloys based on Astroloy (No. I), MAR-M432 (No.2), IN-100 (No.3), PAI0l(No. 4), and AF-2-IDA (No.5). The kinetics and morphology of a-phase formation were examined in two unstable heats (Alloys 3 and 5) processed by powder metallurgy methods and by conventional casting and extrusion procedures. The morphologies of σ were similar in both forms of a given alloy; however, σ precipitation occurred sooner in the powder metallurgyform of Alloy 3 than in the cast and extruded form. Numerical methods based on electron-vacancy concepts predicted σ formation correctly when experimentally determined γ-phase (austenite) compositions were used. Standard methods for computing γ and γ′ contents and compositions were unsuccessful with these modified alloys, and therefore a new regression equation was developed for γ′ contents using a 71 alloy data base. Using this new equation, more accurate γ and γ′ contents and compositions were obtained, and closer correlation was obtained between the observed and predicted formation of σ-phase.