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increased oxygen diffusion path. 12 The following epitaxial orientation relationships between the growing oxide and the matrix have been proposed:

(111) [110]Aí//(110) [110]oxide on (110) A1 or (111)Aí (110)[110]AI//(110)[110] ox id e on (110)A1 (110) [100] A I//(100) [100]oxide on (001)Aí The addition of Mg is known to accelerate the oxidation rate of Al alloys above 400°C 13,14 and this results in a large wastage of material in commercial alloy production. Whereas the presence of Mg at impurity levels appears to enhance y-Al 2O 3 nucleation, 4 j 6 at concentrations higher than about 1 at pct the formation of y-AI 20 3 is entirely suppressed in favor of crystalline MgO. Electron diffraction studies have confirmed that initially the oxide films contain crystalline MgO 15916 but that on prolonged oxidation the thermodynamically favored spinel MgAl z0 4 may form. It has been proposed that the MgO forms either by the reduction of the amorphous Al,O 3 at the oxide/metal interface, l5 ' 16 or by direct growth at the oxide/oxygen interface . l ' Little information, however, exists as to the morphology, density or growth rate of the MgO crystals. In situ experiments in a transmission electron microscope are particularly useful in the study of oxidation, as the early stages of nucleation and growth can be followed and the morphology and crystallography of the oxide can be established. However, in situ alloy oxidation in a conventional 100 kV microscope may not provide meaningful results, typical of bulk material, due to insufficient solute within the thin (1000 to 3000A) foil specimen. This difficulty can be overcome by the increased penetration provided by a high voltage electron microscope (HVEM). A further limitation of conventional 100 kV microscopy is that environmental or gas reaction cells, which enable the oxygen partial pressure to be controlled around the specimen during examination, are not generally available. In situ experiments at 100 kV are thus limited to the oxidation at low pressures of materials which do not contain VOLUME 6A, NOVEMBER 1975-2055

volatile components, e.g., Al. 12 The requirement for an accurately controlled and calibrated heating stage and gas reaction cell for in situ oxidation studies is considerably easier to fulfill in a HVEM due to the increased dimensions of the specimen chamber. The advantages of using such an instrument in the study of oxidation have recently been reviewed 1 8 In this study, in situ observations of the oxidation of Al, Al-Mg, Al -Zn and an Al -Zn -Mg alloy have been made in the AEI EM? HVEM equipped with a specially designed hot stage and gas reaction cell. 19 Although low pressures are necessary in controlled studies of crystalline oxide nucleation, 12 it is known that at low pressures and high temperatures Mg evaporation is faster than the oxidation rate. s 13 For this reason the provision of a gas reaction cell was essential to the work in assessing the role of Mg evaporation during oxidation in the microscope vacuum. The ternary alloy was included in the stu