On the morphology of grain boundary discontinuous reactions and phase identification in an advanced Cr–Fe–Ni alloy

Grain boundary (GB) precipitation phenomena in Alloy 33 that occurred during isothermally aging at 800 °C for periods of 1–10 h have been studied by scanning electron microscopy, and analytical electron microscopy (including scanning/transmission electron microscopy (STEM/TEM), X-ray energy-dispersive spectroscopy (XEDS), and electron diffraction). The type and morphology of phases formed during discontinuous precipitation (DP) resulting from the combined effect of GB migration, diffusion, and precipitation processes have been investigated. STEM-XEDS maps and electron diffraction data have shown that the DP reaction products resulted in the precipitation of 3 different phases within the same colony: (1) face-centred cubic Cr-rich M23C6, (2) diamond-cubic Si-enriched M6N, and (3) lamellar body-centred cubic α-Cr phase. In general, DP colonies in this alloy system develop as follows: at early stages of the aging process intergranular precipitation of M23C6 and M6N occurs at original GB; with increasing aging time, some GBs migrate with the concomitant precipitation of lamellar α-Cr-rich phase, thereby developing DP colonies. The necessary solute partitioning is operated by interface diffusion mechanism through the GB acting as reaction front. Besides the occurrence of three different precipitated phases within the same DP colony, the most remarkable characteristic in the overall the precipitation process in this alloy system refers to consistent evidence that the original intergranular precipitates are different from the lamellar phase that precipitates in association with the migrating GB. This study provides the first report of this observation in a complex multiphase discontinuous GB precipitation reaction and evidence for DP phenomena involving three different precipitated phases.