The high temperature oxidation of Al-4.2 Wt Pct Mg alloy
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INTRODUCTION
MAGNESIUM additions to aluminum form a technologically important alloy system having good mechanical and physical properties in conjunction with excellent aqueous corrosion resistance. However, these alloys oxidize rapidly both when molten and during high temperature heat treatment of solid product forms, giving rise to thick surface films of magnesium oxide which may be accompanied by severe metal blistering in moist environments. Before further processing can take place, the surface films must be removed which results in an expensive loss of metal, and can make processes such as scrap recycling commercially uneconomical. Methods of controlling the rapid oxidation of A1-Mg alloys have been established, such as by tracealloying additions, with the most effective additive being toxic Be. 1 The oxidation kinetics of A1-Mg alloys have been measured thermogravimetrically by several investigators 2'3'4 but when comparing weight gains the surface preparation of specimens prior to oxidation appears to be an important factor, affecting overall weight gains by orders of magnitude, and this factor has not always been closely controlled. From the literature evidence it is clear that weight gain measurements alone have shed relatively little light on the oxidation mechanism(s) operating during the high temperature oxidation of A1-Mg alloys. In recent years the application of modem TEM techniques to high temperature oxidation 5 has become more commonplace, and microstructural studies in conjunction with thermogravimetry offer the opportunity of gathering structural information which can then be related to the oxidation weight gain. This approach was adopted in this investigation.
D. J. FIELD, Research Scientist, and G. M. SCAMANS, Principal Scientist, are with Alcan International Limited, Banbury Laboratories, Banbury, Oxon OX16 7SP, England. E. P. BUTLER, formerly at Imperial College, London, is Principal Scientist with Alcan International Limited, Banbury. Manuscript submitted August 8, 1985. METALLURGICALTRANSACTIONSA
Aluminum and Aluminum Alloy Oxidation
During the initial stages of oxidation over a wide range of conditions, aluminum and aluminum alloys rapidly develop a thin tenacious film of amorphous T-A1203.5 This oxide layer provides a barrier between the metal substrate and the environment, and it is this film which has been shown to control the early stages of crystalline oxide formation at high temperatures. 6-9 At temperatures above about 425 to 450 ~ the amorphous T-A1203 overlayer undergoes a discontinuous change in structure. 6 Rapid migration of oxygen to the oxide/metal interface is readily detectable at these temperatures and is evidenced by the nucleation and growth above about 450 ~ of crystalline "y-A1203 as a new phase below the amorphous layer. Growth of amorphous "y-A1203 continues by cation diffusion to the oxide-oxygen interface in a fashion of mutual independence with respect to any underlying crystallites of T-A1203 which form at the oxide/metal interface and penetrate down into
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