Oxidation kinetics of niobium in the temperature range of 873 to 1083 K
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under 0.133 to 0.013 bar (100 to 760 Torr). There exists also disagreement as to the type of oxide(s) formed. Kolski 5 reports the presence of monoclinic a-Nb205 below 973 K and of both a-Nb205 and 7-Nb205 between 998 and 1098 K. Hurlen, 2 on the other hand, reports the existance of only ~-NbzO 5 over the range of 673 to 1073 K.
EXPERIMENTAL Specimens of approximate size of 1.0 • 1.5 • 0.20 cm were cut from a N b sheet supplied by Fansteel, Inc., Baltimore, MD, and containing the following impurities inwtppm:C=38, O = 6 2 , N = 4 7 , H = 5, T a = 2 0 0 , Hf, W, Mo ~ 100 each, Zr, Fe, Si ~ 50 each, Ni, Ti, A1, Mn, Ca, Cu, Sn, Pb, Mg, V, Co < 20 each, Cd = 5. The samples were ground and polished using SiC emery papers from 200 through 600 grit and 7-A1203 polishing compounds from 15 to 0.5/zm. The final, mirror-like, polish was obtained by polishing with 0.25 ~ m diamond paste lubricated with a light oil. After acetone cleaning and drying, each specimen was measured for length, width and thickness at five different locations in order to obtain a good average value of the total surface area and volume. Each sample was weighed before exposure to oxygen. To prevent abnormal grain growth during subsequent oxidation and to obtain a homogeneous composition and structure, all specimens were annealed in 1.33 • 10 -7 mbar vacuum at 1123 K for 3 h. After annealing each sample was placed in a self-recording, Cahn RH, electrobalance with a sensitivity of 0.8 /.tg/cm 2 for samples having a total area of about 3.3 cm 2 and exposed to oxygen at a pressure of 1.0 bar (750 Torr) at five temperature levels: 873, 943, 993, 1043, 1083 K. The furnace temperature was controlled to _+2 K and the oxygen pressure was regulated to _ 2 m bar. The ultrahigh purity, Matheson, oxygen used contained the following impurities in wt ppm: N 2 = 6.5, Kr = 13.6, H 2 0 ~ 1.0, Ar, N20, CO 2 and C H 4 ~ 0.0, each. X-ray diffraction patterns from the oxide scale formed on N b were obtained with a Debye-Scherrer camera of 114.59 m m in diam using a Ni filtered CuK~ radiation. To prevent excessive darkening of the X-ray film, a very thin A1 foil was used as a wrap around the film. A minute a m o u n t of fine powdered KC1 was
ISSN 0360-2133 / 80 / 0 8 1 1 - 1 3 8 5 5 0 0 . 7 5 / 0 METALLURGICAL TRANSACTIONS A 9 1980 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 1IA, AUGUST 1980--1385
added to the oxide powder in order to make corrections to the interplanar spacings, d, and thereby increasing the accuracy of phase identification. Samples of the inner oxide used for X-ray identification were procurred by scraping the surface of the oxidized metal after mechanical removal of the outer oxide. For hardness measurements across the oxidized metal-substrate, a T u k o n hardness tester with a Vickers ( D P N ) indenter under a load of 50 g was used. The thickness of the surface oxide was determined by using a filar eyepiece under 200 times magnification. The metal recess, that is, the thickness of metal consumed in the formation of the o
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