Role of TiAl3 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. TiAl 3 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. © 2011 The Japan Institute of Metals.