Effect of Precipitation on Residual Stresses in Alloy 693

Abstract

Alloy 693 is a precipitation hardenable nickel base superalloy containing about 30% Cr, 4% Fe, 4% Al, 1.5% Nb and 0.3% Ti (in wt.%). At elevated temperatures, alloy precipitates out particles of an ordered γ′ phase (L12 crystal structure) with Ni3(Al,Ti,Nb) stoichiometry. These particles remain coherent and maintain a cube-to-cube orientation relationship with the face-centered cubic matrix. The alloy also has a tendency to form chromium-rich α phase (body-centered cubic crystal structure) particles at temperatures above 800 °C. The α phase particles maintain a Kurdjumov–Sachs orientation relationship with the matrix. This paper reports the effect of the precipitation of γ′ and α phases on the residual stresses developed in the alloy, and correlates them with the misfit strains due to the precipitation of the two phases. Residual stresses have been measured using the blind hole drilling method, while the misfit strains have been calculated based on the lattice parameters of matrix, γ′ and α phases determined using neutron diffraction. The precipitation of the γ′ phase introduces tensile nature of stresses in the matrix, while that of the α phase introduces compressive stresses. The overall nature of the residual stresses in the alloy is governed by the volume fraction of the two precipitates.