Structural Defects Formed in Alloy Layers of Fe-Al System by Diffusion in Air

The aluminizing process is performed in air at 1173K after the aluminum has been bonded to a carbon steel S45C surface by hot dipping or pressure bonding, Al-rich alloy layers are first formed by diffusion of Al into the steel, but as the diffusion progresses, these layers become Fe-rich. However, each of the alloy layers is found to contain structural defects. In this work, the nature of these defects has been studied by EPMA and XRD analysis. It was found that void-like defects formed during the early stages of diffusion were mainly a combination of Al2O3 with voids and Al2O3, and that the band-like defects formed at later stages in the boundary region between FeAl and αFe were mainly composed of Al2O3. These Al2O3 formations are most likely the result of oxygen penetrating into Kirkendall voids and subsequently reacting with Al.