Abstract
A brief literature survey of Rapid Solidification (RS) techniques, and in particular of High Pressure Gas Atomization (HPGA) is presented together with a more detailed study of the literature of the analysis of the surfaces of materials by electron spectroscopy. Surface oxidation is reviewed with particular emphasis on the Cabrera-Mott theory of very thin oxide film growth and earlier experimental work on the surfaces of RS materials is discussed. A protocol is established for the surface analysis of model RS materials and of HPGA aluminium alloy powder by X-Ray Photoelectron Spectroscopy (XPS). Some preliminary results on the degassing of RS powders are also presented. Useful experimental techniques, including specimen preparation and curve fitting are introduced and developed for the measurement of oxide thickness and surface segregation by XPS. A theoretical approach is presented for oxide thickness calculation for powders. The segregation of certain alloying elements on the surface of HPGA powder is quantified with the introduction of the enrichment factor. For HPGA aluminium alloy powders whose alloying elements exclude magnesium and lithium, the oxide is thin, in most case less than 2nm and appears to be relatively inert to ambient atmospheres over short (up to 12 hours) exposures. Aluminium alloys that contain magnesium or lithium have a thicker oxide which is prone to oxide growth during powder handling in ambient atmospheres. This problem is exaggerated by the surface segregation of these elements, which also affect the degassing characteristics of the powder. The relatively thin oxide on the surface of the HPGA Powder is discussed in terms of a two stage oxidation mechanism. At least a monolayer of oxide is formed at the molten stage, (high temperature oxidation), and the majority of the oxide growth occurs in the collection box during powder handling (low temperature oxidation). The prediction of the surface segregation of alloying elements is complex. The results indicate that lithium, magnesium and zinc will always segregate in the surfaces of RS materials, other elements (e.g. nickel, copper and iron) will segregate depending on their concentration in the alloy, while some elements (e.g. chromium, zirconium and hafnium) do not segregate at all.