Die segregasiegedrag van Ag en Sb in 'n Cu-enkelkristal

Abstract

English: A phase miscibility gap has as jet, not been observed experimentally for ternary alloys. To prepare a ternary alloy in such a way that a phase miscibility gap can be observed, the segregation parameters of the two segregating species should be known. These parameters are the activation energies, the diffusion constants, the segregation energies and the interactions. In this investigation a theoretical model was developed which can calculate the segregation energies of the segregating species in a binary system. The model makes use of the sublimation energies for the calculations and takes the orientation of the surface into account. The model clearly shows that the true driving force for segregation is not the difference in surface tension (between the surface tension of the pure element and the surface tension of the element in the alloy) but the difference in surface energies. The segregation energies as calculated by the model can be used as initial values for simulations. The validity of the calculations was confirmed experimentally for the three low index orientations of copper. The segregation parameters for a ternary system can be obtained by changing the temperature of the system linearly with time. A ternary alloy was prepared by evaporating silver on to an existing binary copper antimony alloy, and annealing the system at 920 °C for 30 days. Segregation profiles were measured by the surface sensitive technique Auger electron spectroscopy. From the simulations of the profiles the following segregation parameters were obtained successfully: For antimony in copper the pre exponential diffusion coefficient was (1.4 ±1.0)x10 -5 m2/s, the activation energy: was 197±7 kJ/mol, the segregation energy was 80±3 kJ/mol and the interaction between antimony and copper was 12±2 kJ/mol. For silver in copper the pre exponential diffusion coefficient was 8 (1.3±0.4) x10 m2/s, the activation energy was 215±22 kJ/mol, the segregation energy was 30±3 kJ/mol, the interaction between antimony and copper was 18±2 kJ/mol and the interaction between silver and antimony, experimentally measured for the first time, was 39±2 kJ/mol. The measured profiles show clearly a second migration process of atoms on the surface. This flux of atoms contributes to the enrichment of the species on the surface especially at low temperatures. The investigation also shows that, where there is a large difference between the segregation energies of the species, it plays an important role in the sequence of preparing a ternary system.

Afrikaans: Die verskynsel van ‘n fasemengbaarheidsgaping is nog nie eksperimenteel in ternêre legerings waargeneem nie. Om ‘n ternêre legering voor te berei sodanig dat ‘n fasemengbaarheidsgaping eksperimenteel waargeneem kan word, moet die segregasieparameters van die twee segregerende spesies bekend wees. Hierdie parameters is die aktiveringsenergieë, diffusiekonstantes, segregasie-energieë asook die interaksies. In hierdie ondersoek is ‘n teoretiese model ontwikkel wat die segregasie-energie van die segregerende spesie in ‘n binêre sisteem kan bereken. Die model maak van die sublimasie-energie gebruik vir die berekening en neem die orientasie van die oppervlak in ag. Die model toon duidelik dat die werklike dryfkrag vir segregasie nie ‘n verskil in oppervlakspannings (tussen die oppervlakspanning van die suiwer element en die oppervlakspanning van die element in die legering) is nie, maar die verskil in oppervlakenergieë. Die segregasie-energieë soos bereken deur die model kan as aanvangswaardes vir simulasies gebruik word. Die geldigheid van die berekeninge is vir die drie lae indeks oriëntasies van koper eksperimenteel bevestig. Die segregasieparameters van ‘n ternêre sisteem kan eksperimenteel verkry word deur die temperatuur van die sisteem lineêr met tyd te verander. ‘n Ternêre legering is voorberei deur silwer op ‘n bestaande binêre koper-antimoon legering te damp en die sisteem by ‘n temperatuur van 920 °C vir 30 dae uit te gloei. Segregasieprofiele is met behulp van die oppervlaksensitiewe tegniek Augerelektronspektroskopie gemeet.Uit simulasies van hierdie profiele is die volgende segregasieparameters suksesvol verkry: Vir antimoon in koper is die pre-eksponensiële diffusiekoëffisiënt (1.4±1.0) x10 -5 m2/s; die aktiveringsenergie 197±7 kJ/mol; die segregasie-energie 80±3 kJ/mol; en die interaksie tussen antimoon en koper 12±2 kJ/mol. Vir silwer in koper is die pre-eksponensiële diffusiekoëffisiënt 8 (1.3±0.4) x10 m2/s; die aktiveringsenergie 215±22 kJ/mol; die segregasie-energie 30±3 kJ/mol; die interaksie tussen silwer en koper 18±2 kJ/mol en die interaksie tussen silwer en antimoon vir die eerste keer eksperimenteel gemeet as 39±2 kJ/mol. Die gemete profiele toon ‘n duidelike tweede migrasieproses van atome op die oppervlak. Hierdie vloed van atome lewer ‘n bydrae tot die verryking van die spesies op die oppervlak veral by laer temperature. Uit die ondersoek blyk dit ook duidelik dat waar daar ‘n groot verskil in segregasie-energieë van die spesies is dit ‘n belangrike rol in die volgorde van voorbereiding van die ternêre sisteem speel.