Nucleation of the Primary Al Phase on TiAl₃ during Solidification in Hot-Dip Zn-11%Al-3%Mg-0.2%Si-Coated Steel Sheet

Abstract

The solidification structure of a hot-dip Zn-11%Al-3%Mg-0.2%Si coated steel sheet with a slight Ti addition was investigated by EBSD. In every center of the primary Al phase of the alloy-coating layer, TiAl₃ was observed by a scanning electron microscope, which suggests that TiAl₃ acts as a heterogeneous nucleation site of the primary Al phase. The latter was revealed to have perfect lattice coherency with the nucleus TiAl₃ phase. The crystal orientation relationships between TiAl₃ and the primary Al are (001)_TiAl3||(001)_Al and [100]_TiAl3||[100]_Al, (100)_TiAl3||(001)_Al and [001]_TiAl3||[100]_Al, (102)_TiAl3||(110)_Al and [\\bar201]_TiAl3||[\\bar110]_Al, (110)_TiAl3||(110)_Al and [\\bar110]_TiAl3||[\\bar110]_Al, indicating that the primary Al phase grows in an epitaxial manner from the nucleus TiAl₃ phase. The planar disregistry δ between the two phases was calculated to be less than 5%, owing to this good lattice coherency. The TiAl₃ phase is considered to decrease the degree of undercooling necessary for the nucleation of the primary Al phase.