Nucleation of the primary al phase on TiAl3 during solidification in hot-dip Zn-11%Al-3%Mg-0.2%Si-coated steel sheet

The solidification structure of a hot-dipZn-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 At 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 At phase. The latter was revealed to have perfect lattice coherency with the nucleus TiAl3 phase. The crystal orientation relationships between TiAl3 and the primary At are (001)(TiAl3) // (001)(Al) and [100](TiAl3) // [100](Al), (100)(TiAl3) // (001)(Al) and [001](TiAl3) // [100](Al), (102)(TiAl3) // (110)(Al) and [201](TiAl3) // [110](Al), (110)(TiAl3) // (110)(Al) and [110](TiAl3) // [110](Al), indicating that the primary Al phase grows in an epitaxial manner from the nucleus TiAl3 phase. The planar disregistry delta 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.