Experimental studies and modelling of surface loss during segregation
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Date
2015-01
Authors
Cronje, Shaun
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Journal ISSN
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Publisher
University of the Free State
Abstract
Afrikaans: Metallurgiese produkte speel ‘n onontbeerlike rol in die alledaagse lewe. Die soeke na metale met beter eienskappe soos sterkte, slytasie- en korrosieweerstand asook metodes om vervaardigingskostes en – tye te verminder duur steeds voort. Die invloed van onsuiwerhede in metale is van besonderse belang. Tydens die vervaardigingsproses van metallurgiese produkte is dit algemeen om een of ander hittebehandelingsproses te volg. By hoë temperature is die atome van onsuiwerhede, sonder uitsondering, mobiel en kan dus na korrelgrense en ander oppervlakke diffundeer, wat weer ‘n groot invloed op die eienskappe van die materiale het. Hierdie herverdeling van atome tussen die oppervlak en die bulk van die materiaal staan bekend as segregasie.
Die segregasie van Sb uit ‘n Cu(100)-kristal, gedoteer met 0.05 at% Sb, is eksperimenteel gemeet. Tydens ‘n liniêre temperatuurveeg, is die oppervlak-konsentrasie van Sb met behulp van Augerelektronspektroskopie gemeet. Die segregasieprofiel is gepas met behulp van die Gemodifiseerde Darkenmodel en die volgende segregasieparameters is onttrek: 𝐷0 = 1.5x10-5 m2.s-1, 𝐸 = 177.0 kJ.mol-1, Δ𝐺 = -89 kJ.mol-1 en Ω = -3 kJ.mol-1. Hierdie data is ‘n waardevolle toevoeging tot vorige gemete segregasieparameters van Sb na Cu-oppervlakke met verskillende oriëntasies.
In ‘n aantal binêre-legerings is die oppervlakverdamping tydens segregasie-eksperimente gerapporteer. Die invloed van verdamping tydens segregasiestudies het egter baie min aandag gekry. In besonder het onlangse segregasiemodelle die invloed van segregant-verdamping geïgnoreer. In hierdie studie is ‘n gemodifiseerde weergawe van die Hertz-Knudsenvergelyking gebruik om die Gemodifiseerde Darkenmodel te verbeter. Hierdie rekenaarprogrammetuur stel navorsers in staat om beide die kinetika- en ewewigsegregasie, met inagneming van die oppervlakverdamping van die segregant, te voorspel. Die invloed van die verdampingsparameter wat in die Hertz-Knudsenvergelyking ingevoer is, word bespreek en toon die sensitiwiteit van die segregasieprofiel vir selfs baie klein verdampingstempos. Dit word duidelik getoon dat indien verdamping geïgnoreer word onakkurate segregasieparameters gegenereer kan word. Riglyne word gegee hoe om segregasieparameters, wat met behulp van die Gemodifiseerde Darkenmodel, sonder verdamping onttrek is, te korrigeer. Die interpretasie van die segregasieparameters, in terme van verdamping, word ook bespreek.
Veranderinge is aangebring aan ‘n Augerelektronspektroskopie-instrument om verdamping met behulp van ‘n Inficon XTC/3s deponeringsbeheerder te meet. Hierdie veranderinge en voorgestelde eksperimentele prosedures, maak voorsiening vir die verstuiwing en analise van die oppervlak wat vir die verdampingsstudies gebruik word. Deur van hierdie sisteem gebruik te maak is die verdampingstempo van suiwer Sb gemeet. Alhoewel daar kwalitatiewe ooreenkomste rakende die verdampingsgedrag verkry is, is gevind dat die oorspronklike Hertz-Knudsenvergelyking die verdampingstempo ver oorskat.
Geen verdamping van Sb kon vanaf die gedoteerde Cu(100)-oppervlak gemeet word nie, aangesien die verdampingsvloed laer as die deteksielimiet van die instrument was. Dit word toegeskryf aan die baie lae verdampingstempo van Sb, soos in hierdie studie vir suiwer Sb waargeneem is, indien vergelyk word met voorspellings deur die Hertz-Knudsenvergelyking.
Nogtans toon vergelykings tussen die eksperimentele segregasieprofiel en segregasieprofiele wat met behulp van die opgedateerde rekenaarprogram gegenereer is, dat Sb verdamping wel vanaf die Cu(100)-oppervlak tydens segregasie plaasvind.
Hierdie studie toon duidelik hoe belangrik dit is om verdamping tydens segregasiestudies in ag te neem. Die studie lê ‘n belangrike fondament vir toekomstige verdampingstudies vanaf die oppervlakke van metale tydens segregasie, deur die nodige apparatuur in ‘n Augersisteem te monteer, ‘n eksperimentele protokol daar te stel en die opgradering van ‘n rekenaarprogram van die bestaande Gemodifiseerde Darkenmodel om segregasieprofiele te simuleer.
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.
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.
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Keywords
Antimony, Auger electron spectroscopy, Copper, Evaporation, Thesis (Ph.D. (Physics))--University of the Free State, 2015