The Surface Segregation and Initial Stages of Oxidation on Ni-Co Alloys

Abstract

<p> This thesis consists of studies carried out on the Nickel-Cobalt alloy system. The surface segregation behaviour has been studied over the range of composition from 14 to 90 atomic percent Nickel using Auger Electron Spectroscopy (AES). An analysis procedure has been developed that is based on computer simulation of observed spectra using pure metal standard spectra and has resulted in excellent matches. Nickel has been observed to segregate over the entire range of compositions studied and the driving force for segregation has been investigated by monitoring the equilibrium surface composition over a range of temperatures from 813 to 1100 K. The enthalpy and entropy of segregation has been determined on single grains of orientations (111) and (210) in a sample of composition 56.0 atomic percent Nickel as well as on the (111), (100) and (110) faces of a 50.0 atomic percent Nickel single crystal.</p> <p> The initial stage of oxidation has also been studied here using Scanning Auger Mapping and digital image processing. The initial stage of oxidation carried out in-situ has been shown to proceed via an island nucleation and growth mechanism which has been confirmed by imaging islands based on the oxygen Auger signal. A preliminary study of the effects of temperature and pressure on the island growth stage of oxidation has been completed and results are reported.</p>