THE CRITICAL RESOLVED SHEAR-STRESS OF GAMMA'-STRENGTHENED NICKEL-BASED SUPERALLOYS WITH GAMMA'-VOLUME FRACTIONS BETWEEN 0.07 AND 0.47

Gamma'-strengthening of four nickel-based superalloys has been investigated theoretically and experimentally. The critical resolved shear stress (CRSS) of under- and of peak-aged single crystals has been measured as a function of the radius and of the volume fraction f of the gamma'-particles. f ranged from 0.07 to 0.47. The observed variation of tau(p) (= contribution of the gamma'-particles to the CRSS) with f cannot be described by current models of gamma'-strengthening. The reason is an apparent increase of the gamma'-antiphase boundary energy gamma with f. This variation of gamma with f is tentatively interpreted as being due to the recently reported inhomogeneity of the composition of the gamma'-particles: their outer layers are relatively poor in Ni and Al and relatively rich in Co and Cr. If current models of gamma'-hardening are modified to allow for the variation of gamma with f, these models lead to satisfactory descriptions of the experimental data.