Growth and annihilation kinetics of intermetallic compound ally layers in Fe-Al system

Abstract

The growth kinetics of intermetallic compound alloy phases between Fe-Al system were studied employing hot-dipping processes and annealing treatment of diffusion couples of various species of materials of the system. The growth of intermetallic phases was analyzed by an optical microscope and identified by X-ray diffractometer and electron probe microanalyzer. The major phase developed, in hot-dipping process, was $Fe_2Al_5(\eta)$. The growth of other thermodynamically stable phases -$FeAl_2(\zeta)$ and $FeAl_3(\theta)$- were also observed in diffusion annealing experiments and hot-dipping treatment respectively. However, due to very slow growth rate of $FeAl_2(\zeta)$, it was not detected in hot-dip aluminizing processes. It was also found that $FeAl_3(\theta)$ dissolved in liquid aluminum in hot-dip aluminizing processes, not forming a continuous layer at the interface. The major intermediate alloy layer observed consists of parallel grains of $Fe_2Al_5(\eta)$ which was grown from the aluminum into the iron side. The serrated interface was found between the alloy phase and the iron basis. This phenomena was appeared due to the nucleation of each $Fe_2Al_5(\eta)$ grain in different time at the interface. The addition of carbon and silicon in the iron or the silicon in the aluminum bath leads to the concurrent nucleation of $Fe_2Al_5(\eta)$ grain, resulting the planar interface rather than the serrated one. The growth of the $Fe_2Al_5(\eta)$ obeyed a parabolic rate law irrespective of alloying elements in iron and aluminum. As the growth of the $Fe_2Al_5(\eta)$ was controlled by interdiffusion of iron and aluminum in the phase, chemical diffusion coefficients in $Fe_2Al_5(\eta)$ were obtained through the consideration of the flux and compound stoichiometry and the layer growth rate. The activation energy for the growth of $Fe_2Al_5(\eta)$ was found to be 31.2 Kcal/mol. The addition of carbon and silicon in the iron increased the activation energy, while silicon...