XPS characterization of heat-treated aluminum alloys
Abstract
XPS provides information on the thickness and chemical composition of oxides formed the surfaces of Aluminum alloys under different process operations, such as cutting, extrusion, rolling, forging and heating. This knowledge of the dependence of surface characteristics on process conditions and alloy composition is of special interest since it can be used to assist construction of emissivity compensation algorithms, which provide accurate and reliable non-contact (radiometric) temperature measurements during manufacturing. Angle Resolved XPS (ARXPS) was used for surface analysis of freshly-processed Al-alloys. For full quantitative ARXPS data analysis we have improved an existing model in the literature to account for the changes of the emission angle of photoelectrons (take-off angle) across the surface associated with roughness geometry. This model, based on an assumption of a sinusoidal surface roughness predicts with accuracy the oxide layer thickness and gives a very good fit to the angular dependent XPS intensity ratios. Oxide layer thicknesses of 40-70 A are typical of freshly processed surfaces of a variety of alloys. A combination of Ar$\sp+$ sputtering with XPS was used to measure the concentration depth profiles of the thick oxide layers formed after long heating times. At heating temperatures higher than 670 K, XPS results showed a non-uniformly thick, top Mg-oxide layer overlying a mixed-phase Mg-Al-oxide layer, which is very rich in MgO and contains Al inclusions. The 6061 forged alloy, however, does not form a thick top Mg-oxide layer while its oxidation is strongly influenced by the presence of the graphite lubricant and the forging process itself. Relatively thin, mixed-phase Mg-Al-oxide layers which are free from Al inclusions are formed even after heating the alloys for long times at temperatures below 670 K. The top MgO layer is not thicker than two monolayers, even on the 5052 alloy. Emissivity measurements of the heated alloys showed that the oxide layer thickness and the carbon and graphite surface residue for the 6061 alloy, may control the emissivity behavior.
Degree
Ph.D.
Advisors
Delgass, Purdue University.
Subject Area
Chemical engineering|Metallurgy
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