LAUSR

Abstract 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 D0a structure in the A1 matrix is investigated using a model alloy Ni-12Nb-3Fe. The D0a phase precipitates on GB prior to grain interior (GI). The precipitation of D0a 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 D0a 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.