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Grain boundary diffusion and grain boundary structures of a Ni-Cr-Fe-alloy: evidences for grain boundary phase transitions
Grain boundary structure-property relationship was studied in a Ni-base 602CA coarse-grained alloy using a novel correlative tracer diffusion-analytical microscopy approach. Co-existence of several short-circuit contributions to tracer diffusion was distinguished at higher temperatures. These contributions 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 and a detailed atom probe tomography analysis. Annealing at such conditions resulted in Cr23C6-type carbides co-existing with an α-Cr-Mn-enriched phase in addition to sequential segregation layers of Al, Ni, and Fe around them. Curved and hackly grain boundaries showed a high density of plate-like carbides. In contrast, 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 spinodal microstructures. At lower temperatures, hackly interfaces with Cr and Cr-carbide enrichment dominated and the alloy annealed at 403 K 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 showed an anomalous character being almost temperature independent. This specific diffusion behaviour corresponds to a metastable grain boundary transition which is explained by a concomitant relaxation of transformation-induced elastic strains occurring on a longer time scale in comparison to those for grain boundary diffusion. Thermodynamic insights into the probable mechanism of decomposition at grain boundaries are provided. The paper provides solid evidences towards the existence of grain boundary phase transitions in the Ni-base multi-component alloy. It underlines the capabilities of the correlated kinetic-structure measurements as a tool to probe such changes.