Oxidation of Nb-19at%Al Alloy in Oxygen During a Constant Rate of Heating up to 1000°C
Mieko Okamoto, Isao Tomizuka, Akimitsu Miyazaki
Nb-19at%Al alloy was heated in oxygen up to 1000°C at a rate of 2°C/min. Variation during the heating was observed by a thermomicroscope and by a thermobalance, respectively. Process of the oxidation was discussed based on the data obtained by SEM, X-ray diffractometry and EPMA techniques. Obtained pieces of information were: 1) While weight change was observed only above 800°C, surface appeared to be obviously changing already at 350°C under the thermomicroscope. 2) A lamellar structure, being discerned faintly already at room temperature, was increasingly better defined with increase in temperature and was preferentially oxidized above 800°C. 3) Scale over the lamellar structure was thick and composed of a unique textile fabric pattern across its cross-section. 4) The lamellar structure was considered to appear because that part of the alloy in as-cast condition comprised a cell structure of A15-phase enmeshed in a network of A2-phase, or because that part of the alloy transformed to such a cell structure by dissolution of oxygen into the alloy, followed by internal oxidation of aluminium which resulted in supersaturation of niobium and, finally, precipitation of a network of A2-phase from A15-phase. The unique structure of the scale was considered to be caused by oxidation of a cell-structure containing internally oxidized alumina, together with gradual expansion of the scale due to a series of stepwise oxidation of various niobium oxides. 5) Scale was composed of alumina and niobium oxides.