A time dependent regular solution model for the thermal evaporation of an Al-Mg alloy

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Abstract
A series of Al-6.27 at. % Mg alloys were thermally evaporated in a vacuum at 1910 K. The length of time during which the alloy was molten and was evaporating was varied from very short times to a length of time sufficient for complete evaporation of the alloy. The thickness and average composition of the deposited films were determined with thin-film x-ray microanalysis in the analytical electron microscope. A solution/flux model was developed to simulate the evaporation process. The model treated the liquid Al-Mg alloy as a regular solution using experimentally determined Raoultian activity coefficients. The evaporative flux was calculated according to the expression of Langmuir. The solution and flux equations were numerically integrated with respect to time to accommodate changes in mass and liquid alloy composition as the molten alloy evaporated. The agreement between the model and the experimental data (evaporation rate, rate of composition change in the molten alloy, film thickening rate, and average film composition) was excellent. The experimental data and the results of the solution/flux model show that the Mg is evaporated from the melt very quickly. The time required for Mg depletion is approximately 3% of the time required for total evaporation of the alloy. The solution/flux model calculates an average activation energy for evaporation of 72 400±4000 cal/mol over the temperature range 1800–2400 K, which is in good agreement with the thermodynamic enthalpy change for vaporization of 72 300±2000 cal/mol at 2110 K.