Growth Mechanism of Fe₂Al₅ Phase during Aluminizing Process Using a Marker Experiment

Abstract

In order to investigate the formation mechanism of η phase (Fe₂Al₅ phase) during aluminizing process, a so-called marker experiment has been developed in this study. Angular and small MgO particles have been placed firmly on the surface of iron plate. Using iron-saturated aluminum melt bath, which was kept at 700 ºC, an iron specimen was dipped for 11 minutes. Then the cross section was metallographically analyzed. MgO particles were found on the boundary between the Al-Fe alloy region and η phase. This fact indicated that Al diffuses into the α-iron but Fe does not diffuse out into the aluminum melt. It also indicated that the formation site of η phase is on the η/α-iron interface. Since the thickness of η phase is proportional to the square root of dipping time, this process is controlled by the volume diffusion of Al atoms in the η phase. It also found that the thickness of iron plate increased with increasing dipping time. This can be explained semi-quantitatively taking into account of the change in the phase and volume between α-iron and η phase.