Additive strengthening in an iron-base superalloy

Abstract

The iron-base superalloy A-286 exhibits primary strengthening by γ′ or Ni 3 (Al, Ti) precipitates. Strengthening also varies with grain size in this alloy. Results for several austenitic stainless steels, in addition to solution-treated A-286, provide a reasonably precise Hall-Petch equation for the grain size strength of the matrix when depleted of Ni, Ti and Al by γ′ precipitation. The strength due to γ′ appears to be additive to the grain size strength but is of considerably greater magnitude. Aging studies were conducted, using TEM to determine γ′ volume fraction and size. These data, together with yield strengths, were used with the approach of Brown and Ham, incorporating the theory of Labusch and Schwarz, to show that the γ′ strength is accounted for by ordering of the γ′, and is large, for the range of particle sizes developed, compared to typical values for nickel-base alloys. This large strengthening increment is consistent with the substantial particle-matrix misfit in A-286, which can be tolerated at the moderate service temperatures for which the alloy was developed. The results illustrate a well-defined case of multiple, additive strengthening in a commercial alloy.