The stepped growth mechanisms of micro-sized hexagonal M7C3 carbides: Self-assembly shell and epitaxial-layered core

This paper gives a new explanation for the growth mechanism and special structural characteristics of the micro-sized hexagonal M7C3 carbides. The in-situ growth process of carbides was observed by high temperature laser scanning confocal microscope (HTLSCM), whose growth rate is measured about 10-fold higher than that predicted by diffusion-controlled growth model. The solidified morphologies indicate that the hexagonal shell of the carbide formed preferentially. The rapid growth of the shell is clarified by the self-assembly mechanism controlled by the entropy at the critical temperature. And the growth of interior is explained by the epitaxial-layered mechanism along the inner wall of the hexagonal shell. The high-density layered structures with different directions and the obvious structural transitions at interfaces are induced by this stepped growth process. In addition, the shell has smaller spacing of layered structures and higher lattice distortions than the interior, which further confirm the difference between them. Finally, after the formation of shell, the limited diffusion of element leads the formation of holes, heterogeneous structures inside and the fan-shaped (funnel-shaped in 3D) diffusion fields at both ends. This study pioneers the self-assembly growth mechanism of metal compounds and has important implications for understanding the growth process of single crystal alloys with special morphologies.