AL DEPOSITION ON FE - FORMATION OF AN IRON ALUMINIDE SURFACE ALLOY

An investigation has been made of the formation of a surface iron-alum inum alloy through aluminum adsorption (and subsequent reaction) on Fe (100) single-crystal and polycrystalline Fe surfaces. On the Fe(100) s urface, Auger electron spectroscopy and low-energy ion scattering spec troscopy (LEISS) studies indicate that at low Al coverages (<1 ML), an d a very low Al deposition rate (<0.03 ML/min), a surface alloy grows uniformly at 25 degrees C to yield an aluminide with an approximate av erage stoichiometry of FeAl3. At higher Al exposures the surface becom es more aluminum rich. This Al enrichment is due to a kinetic limitati on in the formation of the surface alloy. Heating the surface (300 deg rees C during or after Al deposition) partially overcomes this kinetic limitation, and the topmost surface layer changes to a stable stoichi ometry with some reduction in the relative aluminum concentration. Low -energy electron diffraction observations made during the Al depositio n (or subsequent heating experiments) gave no indication of long-range order, suggesting the formation of a disordered surface alloy. LEISS experiments show that at 25 degrees C, the final stoichiometric config uration of the surface is highly dependent on the average arrival rate of Al at the surface. A higher deposition rate ultimately leads to a more aluminum-rich surface than for an equivalent dose at a lower rate . This may be rationalized by noting that at higher deposition rates t he Al atoms are more likely to encounter other Al atoms at the surface and may proceed to form two-dimensional microclusters of Al instead o f reacting to form a surface or subsurface aluminide alloy. The thermo dynamic implications of these observations in the context of the forma tion of intermetallic alloy phases seen here will be discussed.