Grain boundary precipitation behavior of δ-Ni₃Nb (D0_a) phase in a Ni-Nb-Fe ternary model alloy

Aiming at covering grain boundary (GB) with a geometrically close-packed (GCP) type intermetallic phase in Ni-based superalloys, GB precipitation behavior of the δ-Ni₃Nb phase with the D0_a structure in the A1 matrix is investigated using a model alloy Ni-12Nb-3Fe. The D0_a phase precipitates on GB prior to grain interior (GI). The precipitation of D0_a phase tends to take place in a continuous manner above its nose temperature (1373 K) and in a discontinuous manner below the temperature. The area fraction of a GB covered by the precipitate (ρ) significantly differs by GB in aging at 1423 K, i.e. between 0 and 80% in short time aging and between 30 and 100% in long time aging, while their average area fraction (ρ¯) increases up to about 75% after long time aging. The observed large difference in ρ is found to be enhanced in the growth stage of the precipitation. A crystallographic orientation analysis indicates that the difference may be caused by the geometry of the habit planes, {111} in the matrix phase, with respect to the GB plane; that is, in the case that one of the habit planes is nearly parallel to the GB plane, the D0_a phase grows along the GB, resulting in high ρ, while the phase grows towards GI when any habit planes are inclined to the GB plane, resulting in low ρ. A multi-step heat treatment is proposed to suppress the growth of the precipitates towards GI.