Binary Alloy Surfaces: Surface Segregation and the Effects of Ion Bombardment on the Surface Composition of Cr-Mo Alloys

Abstract

<p>This thesis reports on a comprehensive investigation of the relationship between binary alloy surface composition and the externally controllable parameters of irradiating conditions, temperature and bulk composition.</p> <p>The Cr-Mo binary system was used as the model system as it provides appreciable differences in the driving forces considered to play a significant role in surface segregation: atomic interaction energy and size. The Unified Segregation Model, adopted for theoretical analysis, predicted a cross-over in the segregating element from one end of the composition range to the other.</p> <p>Quantitative surface analysis was performed using Auger Electron Spectroscopy. A major limitation to the accuracy attained was identified with differences in surface roughness between the alloy and elemental standards. An investigation was carried out to establish a correlation between sputtering, crystal orientation and surface roughness. The effect of surface roughness on the detected Auger current was quantified and procedures established for its minimization.</p> <p>Surface compositional modifications of thirteen Cr-Mo alloys, subject to 2 KeV argon ion bombardment, have been established. A preferential sputtering of Cr has been observed across the entire composition range. The results have been interpreted in terms of a phenomenological preferential sputtering model yielding a sputter rate constant ratio K = 1.73 ± 0.04 in good agreement with theoretical prediction and the ratio of the sputter yields of the pure metals.</p> <p>The surface segregation phenomenon has been investigated across the entire composition range. In the temperature range of 875-1075 K, the results reveal non-equilibrium effects which can occur when using sputter-cleaned alloys for segregation studies. A kinetic analysis of the segregation has yielded an insight into the activated processes that occur upon annealing a sputter-modified surface and the corresponding activation energies for diffusion.</p> <p>Layer calculations, using a monolayer segregation model, have been used to determine the outer monolayer composition from that averaged over the finite escape depth of the Auger electrons. The calculated compositions have provided a qualitative and a quantitative test of the theoretical segregation model. The results have also yielded an enthalpy of segregation and an experimental estimation of the inelastic mean-free paths of the Auger electrons used in the analysis.</p>