On the solid-state dendritic growth of M₇C₃ carbide at interfaces in an austenitic system

Solid-state growth of M₇C₃ carbide at different interfaces in an austenitic Ni–Cr–Fe alloy (Alloy 600) was studied in-situ by high temperature confocal laser scanning microscopy during continuous cooling from 1140 ◦C. The carbide develops a dendritic morphology and grows laterally across the free surface via a diffusion-assisted mechanism. Primary dendrite arms can extend beyond 100μm, with some dendrites also developing secondary arms with spacing approximately 3.0μm. By using focused ion beam tomography, some of the surface carbides are found to be directly connected to dendritic carbide at grain boundaries in the bulk confirming they originated from the same nucleus. Matrix deformation is induced in the vicinity of a carbide during growth. This work provides significant insight into the important topic of carbide development by a rare combination of both in-situ microscopy and a 3D tomographic technique; the new knowledge discovered is expected to apply to other austenitic systems.