Nucleation of the Primary Al Phase on TiAl<SUB>3</SUB> 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, TiAl 3 was observed by a scanning electron microscope, which suggests that TiAl 3 acts as a heterogeneous nucleation site of the primary Al phase. The latter was revealed to have perfect lattice coherency with the nucleus TiAl 3 phase. The crystal orientation relationships between TiAl 3 and the primary Al are ð001Þ TiAl3 == ð001Þ Al and ½100 TiAl3 == ½100 Al , ð100Þ TiAl3 == ð001Þ Al and ½001 TiAl3 == ½100 Al , ð102Þ TiAl3 == ð110Þ Al and ½ 2 201 TiAl3 == ½ 1 110 Al , ð110Þ TiAl3 == ð110Þ Al and ½ 1 110 TiAl3 == ½ 1 110 Al , indicating that the primary Al phase grows in an epitaxial manner from the nucleus TiAl 3 phase. The planar disregistry between the two phases was calculated to be less than 5%, owing to this good lattice coherency. The TiAl 3 phase is considered to decrease the degree of undercooling necessary for the nucleation of the primary Al phase.

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

“…Moreover, the existence of Cu on the same EDS chart is due to the generation of a characteristic X-ray from the Cu support stand carrying specimens. The crystals could be identified crystallographically as TiAl 3 intermetallic compounds; 8) however, when these EDS analysis results are taken into account, it may be more correct to state that the intermetallic compounds are identified as Ti (Al, Si) 3 , in which part of the Al was substituted by Si.…”

“…8) This process has high reproducibility, and an example of the refinement obtained is shown using the optical micrograph in Fig. 2(a).…”

“…8) As for the solidification structure, coating layers were chemically polished to a depth of several mm from the surface before analyzing the crystal orientations of the TiAl 3 and its surrounding Al, in order to provide a specific region for TEM observation. For the transmission electron microscope, a FE-TEM (the HF-200, from Hitachi Co., Ltd.), equipped with a field-emission electron gun carrying an energy dispersive spectroscopy (EDS) system, was used under the conditions of an accelerating voltage of 200 kV.…”

“…On the other hand, it emerged that the dendrite structure of the primary Al phase was refined when a small amount of Ti was added to the hot-dip Zn-11 mass%Al-3 mass%Mg-0.2 mass%Si coating. 8) It is important when improving product properties to refine the coating structure, and the basic background of this study lies in examining the control factors for the solidification structure refinement. Based on the analysis results of crystal orientations by electron back scattering diffraction pattern (EBSD) presented in a previous paper, 8) TiAl 3 , formed by the Ti addition, was expected to act effectively as a nucleation site for the primary Al phase, since the TiAl 3 (D0 22 ) and Al phases (FCC) have excellent lattice coherency.…”

Heterogeneous nucleation of α-Al primary crystal on L12modified Al2.7Ni0.3Ti phase

“…The plane disregistry δ, 15,16) which is often adopted to discuss favorable heterogeneous nucleation sites, is calculated from the equation ðhklÞ s ðhklÞ n ¼ 1 3…”