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During some hypothetical loss-of-coolant accidents (LOCA), Zircaloy fuel sheathing is expected to reach temperatures in excess of 1273 K and to oxidize rapidly. The objective of the current work was to determine the effect of oxygen on the mechanical behavior of Zircaloy-2 at temperatures above 1273 K.
Zircaloy-2 containing 1 200, 4 000, 8 000, and 12 000 ppm oxygen was deformed in compression at 1275, 1375, and 1475 K at constant true strain rates of 10-4 to 10-1 s-1. The flow stress of Zircaloy-2 increases by a factor of 3 to 4 as the oxygen concentration is increased from 1200 to 12 000 ppm. Approximately half of this increase is due to solid solution strengthening by oxygen and approximately half to the effect of oxygen on the volume fraction of the much stronger α-zirconium. The stress exponent of the strain rate of both phases appears to be unaffected by oxygen, being 3.5 to 4.0 for β-zirconium and 5 to 6 for α-zirconium. In the (α + β)-phase field, the stress sensitivity at low strain rates is low, suggesting that some form of boundary sliding mechanism contributes to flow.
The stress-strain curves are either “normal” flow curves, with work hardening and saturation of the flow stress, or “abnormal” flow curves, in which a maximum or peak stress is reached at low strains, followed by flow softening. The flow stress peaks are attributed to an aging or clustering effect that occurs when the α phase is enriched in iron and chromium, and the flow softening is attributed to strain-enhanced declustering or coarsening of the precipitates.
deformation, Zircaloy, oxygen, high temperature
Combustion Engineering Superheater Ltd., Sherbrooke, Quebec
Atomic Energy of Canada Limited, Chalk River, Ontario
Professor of mechanical metallurgy, McGill University, Montreal, Quebec