SEGREGATION OF MG TO THE SURFACE OF AN AL-MG SINGLE-CRYSTAL ALLOY AND ITS INFLUENCE ON THE INITIAL OXIDATION AT ROOM-TEMPERATURE

The segregation of Mg to the surface of a (110) Al-1.45at%Mg single crystal alloy and its influence on the initial stages of oxidation at 300 K was investigated using Auger electron spectroscopy (AES), work function measurements (Delta phi), low electron energy diffraction (LEED), nuclear reaction analysis (NRA), and static secondary ion mass spectrometry (SSIMS). The diffusion coefficient of Mg in the alloy is obtained from the kinetics of segregation and is found to be about 100 X smaller than previous determinations. A rapid increase in the Mg concentration at the (110) surface is observed at 500 K. A surface concentration (Mg/(Al + Mg)) of 0.47 is obtained at equilibrium. This rapid increase in Mg surface concentration is associated with a fast decrease in Delta phi. The formation of a c(2 x 2) structure is observed at the equilibrium concentration. The initial sticking coefficient for oxygen on the nonsegregated surface is found to be 0.12. The reaction of this surface with oxygen leads to oxide formation at exposures > 40 L. In contrast, the oxygen sticking coefficient for the segregated surface is found to be 1. The reaction mechanism of oxygen with the segregated surface is similar to that found with the non-segregated surface but the oxidation rate is much larger. Saturation of the segregated surface occurs after a 30 L exposure, thereby yielding an oxide film consisting of a combination oxide of Al2O3 and MgO.