A transformation-induced reduction in the oxidation rate of ordered Zr 3 Al
- PDF / 2,285,282 Bytes
- 6 Pages / 594 x 774 pts Page_size
- 57 Downloads / 188 Views
IN an earlier paper ~it was shown that the oxidation in steam of the peritectoid L12 phase, Zr3A1, is different at temperatures of 875 K and above from that at 825 K and below. For instance, the oxidation rate at the higher temperatures decreases with increasing time, at least for periods up to 3 days, and the oxide film is adherent. At the lower temperatures, on the other hand, the rate first decreases and then, at a time which increases with decreasing temperature (from < 0.3 d (days) at 825 K to > 80 d at 675 K), breaks away to a higher rate which remains constant. Concurrently, the oxide films flake off. In this paper, it is shown that a similar thermal effect occurs in air, and it is established, as previously suggested, ~ that the effect is directly related to an environmentally-induced decomposition of Zr3A1 into a lamellar Zr2A1/Zr mixture. Moreover, it is shown that the greater adherence of the oxide formed on the lamellar product is caused, at least in part, by the preferential oxidation of c~Zr, which leads to oxide platelets held between Zr2A1 lametlae. The interest in the oxidation response of Zr3A1 arises from its potential as a nuclear engineering material. 2
3. R E S U L T S A N D O B S E R V A T I O N S 3.1 Kinetics Figure l(a) shows the oxidation curves for Zr3A1. Two types of behavior are evident. At the lower temperatures, 775 K and 875 K, the oxidation rate is quite temperature sensitive, and the weight gain obeys the rate law Aw = kt", where* k = 1.9 • 10z m g / d m z * Throughout this paper d denotes days; dm, decimeters. 9 d ~ at 775 K and 1.2 x 103 m g / d m 2 9 d ~ at 875 K and where n = 0.6. At 775 K at ~ 1.1 d, a transition occurs to a higher oxidation rate after which the weight gain obeys linear kinetics (evident from separate linear plots of Aw vs t). At the higher temperatures, 975 K to 1145 K, the oxidation rate is much less temperature sensitive, at least during the early stages, and the rate law again obeys the expression Aw = kt". The exponent, however, has a much lower value; viz 0.2 to 0.25. A transition to a higher, constant rate occurs after a
AIR 50C
2. E X P E R I M E N T A L Material for this study consisted of a Zr 8.6 wt pct A1 alloy, fabricated and prepared for corrosion testing as described previously.~ Coupons, 20 • 20 • 2 mm, were oxidized in air at temperatures from 775 to 1145 K and intermittently weighed. Others were oxidized for various times, sectioned, metallurgically prepared and then examined by optical microscopy. Parts of the sectioned specimens were dissolved in a mixture of 45 ml nitric acid, 45 ml water and 5 ml hydrofluoric acid and then examined by scanning electron microscopy (SEM), eqmpped with an energy dispersive spectrometer (EDS). The oxide was identified by X-ray diffraction, using powder patterns. For comparison, Zircaloy-2 coupons (Bh grade) were oxidized also. E. M. SCHULSON, formerly of Chalk River Nuclear Labs., Atomic Energy of Canada, Ltd., Chalk River, Ontario, is now with Thayer School of Engineering, Dartmouth College, Hanover, NH.
Data Loading...
