Role of TiAl<SUB>3</SUB> Fine Precipitate in Nucleation of the Primary Al Dendrite Phase during Solidification in Hot-Dip Zn-11%Al-3%Mg-0.2%Si Coated Steel Sheet

Abstract

The primary Al phase crystallizes preferentially on TiAl3 fine precipitate during the solidification process of hot-dip Zn-11 mass%Al-3 mass%Mg-0.2 mass%Si coating, which leads to the formation of fine dendrite structures by the small amounts of Ti added to the coating bath. TiAl3 is thought to act as heterogeneous nucleation sites of the primary Al phase. Detailed TEM observation of the sectioned specimen by a micro-sampling unit equipped with an FIB system revealed that the TiAl3 fine precipitate is not a single crystal but polycrystals. However, a crystallographic relationship exists between the TiAl3 phase and the surrounding Al phase, namely, [001]TiAl3||[001]Al, [010]TiAl3||[010]Al and [100]TiAl3||[100]Al. High-resolution TEM observation of the interface indicates a good coincident relationship between the Al matrix and the precipitate. Therefore, the primary Al phase nucleated heterogeneously on the surface of the TiAl3 phase. Furthermore, detailed TEM observation revealed a monotectoid reaction by the primary Al phase to the extremely fine Al and Zn phases.