Growth mechanism of primary needles during the solidification of chromium carbide overlays

A mechanism for modelling the growth of faceted primary M₇C₃ needles during solidification of chromium carbide overlays is presented for the first time. In contrast with existing models that consider diffusion-controlled growth over the length of the needle, this work disaggregates the flow of solute at the tip (diffusion-controlled) and the faceted sides (interface-controlled). The proposed solution predicts an initial transient followed by a steady state longitudinal growth. The outward growth of the needle prism faces is linear and slow, consistent with interface-dominated kinetics. Abrupt decreases in longitudinal growth rate was observed when the tip of the carbides approached other carbides, consistent a soft-impingement effect when the diffusion field of two carbides a overlap. The model was compared to in-situ visualisation of the solidification of an alloy with primary chromium carbide needles, and was found to match closely to observations.