THE INFLUENCE OF LI SURFACE SEGREGATION ON THE INITIAL OXIDATION OF AN AL-LI ALLOY AT ROOM-TEMPERATURE

The initial oxidation (0-25 L O2) of nonsegregated and segregated surf aces of an Al-6.5at%Li alloy has been studied at room temperature by A uger electron spectroscopy (AES), synchrotron radiation-photoelectron spectroscopy (SR-PES), static secondary ion mass spectroscopy (SSIMS), work function (DELTAphi) and nuclear reaction analysis (NRA). The ini tial sticking coefficient on the nonsegregated surface was found to be 0.048. The reaction of this surface with oxygen leads to oxide format ion at exposures < 10 L by a two-step process: oxygen atoms react firs t with the lithium atoms located at the surface for exposures of 0-1.5 L and then also react with the aluminum atoms at higher exposures. By contrast, oxygen reacts simultaneously with Al and Li atoms on the se gregated surface, leading to the formation of a mixed oxide at low exp osure (0.4 L). This oxide is possibly converted to another mixed oxide at higher oxygen exposures. The oxidation rate is promoted (by a fact or of 20.8) by the formation of a (square-root 3 X square-root 3 )R30- degrees Li surface-segregation superstructure that exhibits a sticking coefficient of unity for oxygen.