1995 Volume 35 Issue 5 Pages 494-502
A technique has been developed so that a number of cross sections of galvanneal coatings can be examined efficiently and reproducibly by transmission electron microscopy, and applied to study the microstructures of a series of simulator-made galvanneal coatings on interstitial-free (IF) interstitial-free rephosphorized (IFP) steel sheets. It is demonstrated in this report that all intermetallic phases observed have been unambiguously identified by electron diffraction and that each intermetallic phase is associated with characteristic microstructure. It is possible to establish a microstructural atlas which enables one to make quick and ready identification of phases only from the appearance in TEM. The present study has clarified several unresolved questions regarding the microstructure and phases of galvanneal coatings, including the existence of the Γ1 phase, clear-cut interface between Γ and Γ1 phases, a layer of single Γ1 phase, and a layer of a mixture of the Γ1 and δ phases. The grain structures of the Fe-Zn intermetallic phases formed by hot-dip galvannealing are compared with those formed by annealing of Fe-Zn diffusion couple and those by electrodeposition.