Experimental studies and modelling of surface loss during segregation

English: Metallurgical products play an essential role in everyday life. The search for metals with better material properties such as strength, wear and corrosion resistance and for ways to reduce production costs and time continues to this day. The effect of impurities in metals is of particular interest. It is common to apply some sort of heat treatment during manufacturing of metallurgical products. At elevated temperatures, impurity atoms are invariably mobile and can diffuse to grain boundaries and other surfaces which can have a major influence on material properties. This redistribution of solute atoms between the surface and the bulk of the material is known as segregation. Experimentally, the segregation of Sb from the bulk of a Cu(100) crystal doped with 0.05 at% Sb was measured. The Sb surface concentration was monitored using Auger Electron Spectroscopy during a linear temperature ramp. The segregation profile obtained was fitted with the Modified Darken model and the segregation parameters were determined to be 𝐷0 = 1.5x10-5 m2.s-1, 𝐸 = 177.0 kJ.mol-1, Δ𝐺= -89 kJ.mol-1 and Ω = -3 kJ.mol-1. This data is a valuable addition to previous measurements of segregation parameters made for Sb to Cu surfaces of different orientations. In a number of binary alloy systems surface evaporation during segregation experiments has been reported. The effect of evaporation has however received very little attention in previous experimental studies. In particular, contemporary segregation models omit the influence of segregant evaporation. In this study a modified version of the Hertz-Knudsen equation was used to update the Modified Darken model, producing software that enables researchers to predict both kinetic and equilibrium segregation while including the effects of surface evaporation of the segregant. The effect of the proposed evaporation parameter introduced into the Hertz-Knudsen equation is discussed and shows how sensitive the segregation profile is to even very small segregant evaporation rates. It is clearly demonstrated that omitting evaporation from the simulations can generate inaccurate segregation parameters. Guidelines are given on correcting segregation parameters extracted from Modified Darken model fits made with software before updating for the influence of evaporation. Interpreting segregation parameters in terms of evaporation is also discussed. Modifications were made to an Auger Electron Spectroscopy system to measure evaporation by using an Inficon XTC/3s deposition controller. This modification and proposed procedures allows the surface of a sample used for evaporation studies, to be sputter-cleaned and analysed. Using this modified system the evaporation rate of pure Sb was measured. While there was qualitative agreement regarding the evaporation behaviour, it was found that the original Hertz-Knudsen equation highly over estimates the evaporation rate. No evaporation of Sb from the surface of the Cu(100) sample could be measured as the evaporation flux was below the detection level of the equipment used. This is attributed to the very low evaporation rate of Sb, as observed in this study for pure Sb, compared to predictions from the Hertz-Knudsen equation. Nevertheless, comparison of the experimental segregation profile and simulations generated with the updated software suggest that evaporation of Sb does in fact take place from the Cu(100) surface during segregation. This study has demonstrated the importance of taking evaporation into account during segregation studies and laid an important foundation for future studies of evaporation from the surfaces of metals during segregation by installing the necessary hardware into an Auger system, establishing experimental protocols and updating the existing Modified Darken software for simulating segregation profiles.