Binary Alloy Surfaces. Surface Segregation and the Effects of Ion Bombardment on the Surface Composition of Cr-Mo Alloys
The 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.
The Cr-Mo binary systems 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 and of the composition range to the other.
Quantitative surface analysis was performed using Auger Electron Speotroscopy. A major limitation to the accuracy attained was identified with differences in the 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 deteoted Auger current was quantified and procedures established for its minimization.
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 ration K=1.73±0.04 in good agreement with theoretical prediction and the ratio of the sputter yields of the pure metals.
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.
Layer calculations using a monolayer segregation model, have been used to determine the outer 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.