Dispersion hardening of Ni-(Co)-Cr-Nb alloys

Conclusions

• 1. Alloys of the Ni-Co-Cr-Nb system containing 15% Cr, 10% Nb, and 6.8–33.5% Co exhibit a distinct dispersion hardening effect, with high strength and elastic characteristics and satisfactory relaxation resistance at 500°.

• 2. Cobalt (12.6–21.5%) increases the effect of precipitation hardening on the strength and elastic properties of Ni-15Cr-10Nb. The largest increase of these properties and the relaxation resistance is obtained with 21.5% Co.

• 3. Transmission electron microscopy showed that the basic hardening phase in Ni-(Co)-Cr-Nb alloys is metastable γ″ phase (Ni₃Nb) with a bct lattice (superstructure of the D₀₂₂ type) that precipitates during aging at 700° throughout the bulk of the grains in the form of finely dispersed disk-shaped particles. Precipitation of γ′ phase (fcc lattice) in the form of globules and platelets is observed in the boundaries and intersections of grains and also near precipitates of primary β phase (Ni₃Nb) with an orthorhombic lattice.

• 4. The addition of 21.5% Co to the Ni-15Cr-10Nb alloy increases the dispersity and volume of precipitates of γ″ phase and also favors its transformation to β phase with an orthorhombic lattice in the process of overaging.