The iron-zinc system has been studied frequently due to the fact that zinc inhibits corrosion of the iron by cathodic protection. However, there are discrepancies in the literature concerning both the nature of the intermetallic phases that can form in iron-zinc (Fe-Zn) alloys and the details of the Fe-Zn phase diagram. Many of the problems that are associated with this system can be attributed to the difficulty of preparing homogeneous alloys of known compositions because of the high vapor pressure of zinc relative to iron. For many of the same reasons, it has proved difficult to produce thin foils for examination in the transmission electron microscope (TEM) from bimetallic Fe-Zn materials. Transmission electron microscopy of the bimetallic Fe-Zn couple in cross section is desirable because microstructural information by imaging techniques, phase identification by diffraction techniques, and chemical analysis by energy dispersive spectroscopy can be achieved with one instrument.
In the present investigation, a sample preparation technique for the production of cross-section TEM foils of Fe-Zn couples is described in detail. In an attempt to obviate the problem associated with the thinness of commercial Fe-Zn couples, pure Fe-Zn couples have been produced that are in excess of 3 mm in thickness. These samples can be cut directly to the required dimensions for thin foil preparation. Electron transparency has been achieved by liquid nitrogen ion milling with specimen dimpling. Preliminary results from the zinc side of the Fe-Zn couple are presented. The zeta (ζ) phase has been identified, and it is believed that these are the first TEM micrographs and electron diffraction patterns of this phase observed from cross-sectioned Fe-Zn couples. The microstructural evolution of the η (Zn) → η + ζ→ζ phase transitions within the diffusion zone is discussed.