MICROSTRUCTURAL CHARACTERIZATION OF GALVANNEAL COATINGS BY TRANSMISSION ELECTRON-MICROSCOPY

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) and 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 Gamma(1) phase, clear-cut interface between Gamma and Gamma(1) phases, a layer of single Gamma(1) phase, and a layer of a mixture of the Gamma(1) and delta 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.