  Grain boundary tracer diffusion of Ni, Fe and Cr was studied in a Ni-base
602CA coarse-grained alloy. A co-existence of several short-circuit
contributions was distinguished at higher temperatures in Harrison's B-type
regime (773-873 K), which were related to different families of high-angle
grain boundaries with distinct coverages by precipitates and segregation levels
as revealed by HAADF-STEM combined with EDX measurements. Annealing at 873 K
for 18 hours resulted in Cr23C6-type carbides coexisting with an
\alpha-Cr-Mn-enriched phase in addition to sequential segregation layers of Al,
Fe and Ni around them. Curved and hackly grain boundaries showed a high density
of plate-like carbides, whereas straight grain boundaries were composed of
globular carbides with similar chemical composition variations and additionally
with alternating layers of Cr and Ni in between the carbides, similar to
microstructures after a spinodal decomposition. At lower temperatures,
discontinuous interfaces with Cr and Cr-carbide enrichment dominated and the
alloy annealed at 403 K for 72 hours contained plate-like Cr23C6-type carbides
surrounded by a Ni-rich layer around them. The Ni grain boundary diffusion
rates at these relatively low temperatures (formally belonging to C-type
kinetics) showed an anomalous character being almost temperature independent.
This specific diffusion behaviour was explained by a concomitant relaxation of
transformation-induced elastic strains occurring on a longer time scale with
respect to grain boundary diffusion. Thermodynamic insights into the probable
mechanism of decomposition at grain boundaries are provided.
