Previous experiments showed that the thickness of a thick prior-oxide layer formed on Zircaloy-4 fuel cladding can decrease during the first seconds at very high-temperature, before re-growing. We confirmed these results with oxidations performed at 1200 • C on prior-oxidized Zircaloy-4. The initial reduction of the prior-oxide was explained by the balance of the oxygen fluxes at the metal/oxide interface and successfully reproduced by numerical simulations using a diffusion-reaction model. Different hypotheses were considered for the diffusion coefficients of oxygen in the different layers. This allowed discussing the effect of the prior-oxidation on the kinetics of oxygen embrittlement of the metallic substrate. Abbreviations: ␣ red , ␣Zr(O) phase formed by reduction of the low temperature oxide layer; Exp, experimental, i.e. data link with experimental results; EKINOX-Zr, Estimation KINetics OXidation numerical model for Zr alloys; HCP, hexagonal close-packed; HT, high temperature; HTox/HT ZrO2, oxide layer formed at high temperature (T ≥ 1000 • C); LOCA, loss-of-coolant-accident; LT, Low temperature, i.e. the in-service temperature of the PWR (T≈320 • C); LTox/LT ZrO2, oxide layer formed at low temperature; Num., numerical, i.e. data link to numerical results; PBR, Pilling-Bedworth ratio; Prior-Zr, phase formed from the cooling of Zr phase stable at high temperature; PWR, pressurized-light-water-reactor.
The numerical tool EKINOX-Zr has been upgraded to simulate oxygen tracer experiments during the high-temperature oxidation of a metal with a high oxygen solubility limit. The penetration of 18 O tracer is calculated during the dynamic evolution of the ZrO 2-x /a Zr(O) /b zr system. The numerical approach allows to explicitly take into account the variation of the tracer diffusion coefficient through the oxide scale as a function of the vacancy concentration. A classical twostages 16 O 2 / 18 O 2 tracer experiment has been simulated. It is shown that a classical fitting procedure on the 18 O concentration profile obtained for short-time experiments leads to the identification of the oxygen chemical diffusion coefficient. The second type of tracer experiment is proposed using a three-stages 16 O 2 / 18 O 2 / 16 O 2
Due to high solubility of oxygen and nitrogen in titanium alloys, the influence of the diffusion zone on the macroscopic tensile properties of pre-oxidized annealed Ti-6Al-4V tensile specimens was examined at room temperature. Thin microtensile specimens were prepared with different thicknesses ranging from 100 µm to 500 µm and then exposed at 750°C for durations between 5 and 200h. A dedicated gripping technique was developed in the present study to investigate the brittleness of such pre-oxidized and ultrathin specimens at room temperature. Tensile testing was paired with digital image correlation techniques to assess both macroscopic deformation and full-field strain maps. High temperature pre-oxidation treatments significantly decreased the ductility of the specimen and the tensile strength of the materials (yield strength and ultimate tensile strength). Fractographic examinations revealed typical brittle fracture features in the oxygen/nitrogen-affected diffusion zone in the periphery of the cross-section while the fracture remained ductile in the core of the specimen for most of the specimens. Some specimens fully failed in a brittle manner for “(pre-ox. duration)1/2/thickness” configurations with ratio equal or higher than 0.45 h1/2.µm-1.
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