Morphological evolution of TiC from octahedron to cube induced by elemental nickel†

TiC particles with various morphologies from octahedron to cube were synthesized and investigated by Field emission scanning electron microscopy (FESEM) in three-dimensional space. The morphological evolution of TiC grain and its growth mechanism were also discussed. TiC particles prefer an octahedral morphology (equilibrium shape) enclosed by eight {111} facets with minimized total surface free energy in Al–Ti–C alloy, while they tend to form a cube enclosed by six {100} facets under the influence of Ni in Al–Ni–Ti–C alloy. Due to the strong interaction between Ni–3d and C–2p orbitals, Ni atoms in the melt selectively absorb on {100} faces of the growing TiC crystal rather than on the polar {111} faces and reduce the specific surface energy of {100}. According to Wuff's theorem, the growth rate of {100} is lowered correspondingly, while the relative growth rate of {111} is accelerated. Thus, the higher growth rate along <111> direction will lead to the shrinkage of {111} faces gradually, while six {100} faces are reserved to form a TiC cube because of their lower growth rates. Furthermore, a similar morphology evolution to TiC crystals can also be found in Fe- and Co-containing melts. It is revealed that the crystal growth of TiC follows the same model under the effect of group VIII elements (Fe, Co and Ni).