Thermophysical properties of zirconium alloy E110 after oxidation in air atmosphere

Full Text: The subsecond resistive pulse heating technique has been used for the study of thermophysical properties of the E110 zirconium alloy (Zr-1 % mass. Nb) after oxidation in air. The tubular samples were oxidized in the course of multiple cycles of pulse heating up to desired temperature - subsequent free cooling to 700 K approximately. The quantity of these cycles was the same for the each sample and equal to 50, but the maximum temperatures at heating were different: 1200, 1300, 1400, 1500 and 1600 K accordingly. After this preparation the structure of the test samples at room temperature was the oxygen dissolved α- phase with the oxide layer on the surface. Two identical samples were prepared for each value of the maximum oxidation temperature: one - for the pulse measurement of thermophysical properties, and other - for the analysis of the structure. The experiments were performed in argon, the chamber was filled up to small overpressure after vacuum pumping-out. The temperature dependencies of the specific heat capacity cP, spectral emissivity ελ and electrical resistivity ρ of the test samples of E110 alloy, oxidized at different conditions, were measured in the temperature range from 1000 K to melting. For comparison, the same measurements with the sample of initial composition have been done. It was obtained that the thermophysical properties depend on the content of oxygen in the alloy and the thickness of the oxide layer, dependent in our case, on the difference of the maximum temperature of oxidation. The observed broadening of α-β phase transition temperature range at the increase of the maximum oxidation temperature, that is associated with the increase of the oxygen concentration in the alloy, corresponds to the available phase diagram of Zr-O system. The increase of the electrical resistivity of the oxidized samples in comparison to the sample of initial composition was observed in the whole measurement temperature range. This increase was the greatest in the case of the samples with the maximum oxidation temperature 1600 K. When the maximum temperatures of sample oxidation are equal to 1500 or 1600 K, the peaks at temperature about 1450 K on the dependence cP (T) appear. These peaks correspond to the phase transition in the oxide film. The first peak on the dependence cP (T), corresponding to the α-β phase transition in the alloy, decreases when the maximum oxidation temperature increases. The heat effect due to the endothermic reaction of dissolution of the oxide film in the alloy was observed for the samples of all oxidation conditions as a steeper increase of cP at temperatures above 1830 K. The dark underlayer of the non-stoichiometric zirconium oxide explains the high values of the emissivity of the oxidized alloy in comparison with the initial one. The thin white stoichiometric surface film of the zirconium oxide does not influence noticeably on the ελ. (author)