Pengting Li

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 Wuffs theorem, the growth rate of {100} is lowered correspondingly, while the relative growth rate of {111} is accelerated. Thus, the higher growth rate along 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).

The dispersive orientated-precipitation of AlP on alumina film and its effect on the primary Si gathering behavior in the Al–Si alloy surface layer

In this paper, the dispersive orientated-precipitation of AlP on γ-Al2O3 film and its effect on the primary Si gathering behavior in the near-eutectic Al–Si alloy surface layer was investigated. Experimental results show that (111) orientated AlP can nucleate on the γ-Al2O3 film, which is attributed to the high tendency for P to be adsorbed on γ-Al2O3 and the good lattice matching relationship between AlP and γ-Al2O3. It was also found that the pre-precipitated AlP particles on the γ-Al2O3 film could induce Si atoms to precipitate into numerous primary Si particles, leading to an extraordinary high volume fraction of primary Si in the near-eutectic Al–Si alloy surface layer. What is more, the X-ray diffraction experiment shows that the intensity of the Si (111) peak increases significantly and other Si peaks decrease to a very low level, which means that the primary Si particles inherited the (111) orientation characteristic from the orientated AlP grains.