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This paper examines the oxidation behavior of thin specimens of cast NiCoCrAlY alloys at 1150 ∘ C through successive stages, from Al 2 O 3 growth to complete alloy conversion to oxide. Five alloy compositions were used, with varying fractions and compositions of γ and β . The time evolution of the alloy composition during Al 2 O 3 growth was simulated using the DICTRA module of Thermo-Calc and calculated analytically in the approximation of flat profiles. Simulated and experimental profiles were found to be in good agreement, indicating that the phase equilibrium and mass balance were correctly reproduced in the simulations. Local variations of alloy composition were observed in thinner specimens and found to be comparable with the variations expected from the uncertainty on the initial specimen thickness. The variations observed in the time-to- Al 2 O 3 failure were greater than expected on this basis, suggesting that additional sources of variability were in effect. Alumina failure was followed by the growth of a Cr 2 O 3 layer at the alloy–scale interface. Similarly, Cr consumption eventually led to Cr 2 O 3 failure, and Ni- and Co-containing spinel oxide formed, converting the Cr 2 O 3 at the alloy–scale interface and the Al 2 O 3 at the scale–gas interface. The remaining NiCo alloy was then converted to (Ni,Co)O. This sequence occurred without abrupt increase in the mass gain, due to the continued presence of the remnant Al 2 O 3 layer, and to the small amount of metal left to oxidize when the (Ni,Co)O eventually broke through the scale. The evolution of the scale composition throughout the oxidation stages is discussed based on an analysis of the thermodynamic conditions at the alloy–scale interface. [ABSTRACT FROM AUTHOR]

The oxidation behavior of a nickel-based superalloy was investigated from 650 to 1,000 °C for up to 1,000 h in air. Samples with thicknesses ranging from 20 to 500 µm were used to document the evolutions of microstructure and oxide scale. Oxidation products and subsurface evolution of the metal microstructure were characterized by XRD and EDS analyses. Local breakaway was observed after 600 h at 800 °C and after 100 h at 900 °C due to the full consumption of Cr from the alloy. Room temperature tensile tests were performed on aged and pre-oxidized specimens with thicknesses ranging from 20 µm to 500 µm, at 800 °C. The results were compared to tensile tests performed on the as-received metallurgical state. Both size effects due to sample thickness reduction and to sample thickness/pre-oxidation width ratio were examined onto the mechanical behavior. Interestingly, the formation of TCP phases, the oxide layer and the subsequent Cr-depleted subsurface region from either aging or oxidation treatments impairs the mechanical integrity. Bulk and subsurface regions are impacted, especially for tens-of-micrometer thin samples. [ABSTRACT FROM AUTHOR]

The steam oxidation behavior of Zr-based Zircaloy-4 fuel cladding was studied at 1273 K with two different surface roughness levels. Steam was introduced either at room temperature (RT) or at 1273 K. Weight gain kinetics were evaluated by post-test weight measurement, and the reaction products and alloy microstructure were evaluated using optical microscopy. Hydrogen pick-up was measured by the gas extraction technique. Specimen surface roughness did not affect the oxidation kinetics or the hydrogen absorption. The time to breakaway oxidation was suppressed when steam was introduced at RT, and the oxide was more adherent, suggesting superior mechanical properties. When steam was introduced at 1273 K, an undulated oxide–metal interface formed earlier and a higher amount of hydrogen was absorbed by cladding before the kinetic transition. The alloy grain grew into larger size in the condition when steam was injected at 1273 K compared to the condition when steam was injected at RT, which may affect the observed behavior. After the oxide breakaway, the rate of hydrogen absorption accelerated substantially independent of the temperature of steam injection. [ABSTRACT FROM AUTHOR]

An external ultrathin α-Al2O3 scale grown on the Ni-base alloy 602 CA during air oxidation at 800 °C was characterized by means of high-resolution TEM/EDX and electron diffraction. Alloy samples pre-oxidized at 800 °C were subsequently exposed at 1100, 1150 and 1200 °C for up to 100 h. Whereas the external alumina remained stable at 1100 °C, with the increasing exposure temperature, the pre-grown alumina scale tended to break down resulting in an external chromia scale accompanied by internal alumina precipitation. The transition from external to internal Al oxidation was investigated using SEM/EDX/EBSD. The critical Al depletion at the scale-alloy interface during the post-exposure at 1100-1200 °C was modeled using the CALPHAD-based thermodynamic-kinetic approach. [ABSTRACT FROM AUTHOR]

The paper focuses on the phenomenon of breakaway occurring in high-temperature oxidation of zirconium. The oxidation kinetics were determined by thermogravimetric analysis of samples oxidized at 500 °C under synthetic air. The time associated with breakaway was identified and compared to results of an oxidation test coupled with an acoustic emission monitoring. Breakaway corresponded to the appearance of the first cracks in the zirconia scale and which generated an acoustic activity. A significant evolution of bursts energy and an increase in the number of hits allowed to class signals into three populations corresponding to the different steps of the oxidation kinetics. Postmortem observations for qualification of crack networks confirmed crack initiation at the oxide/metal interface at breakaway. A time offset was measured between first cracks occurring and their global effect on oxidation kinetics. [ABSTRACT FROM AUTHOR]

A study has been undertaken of the effects oflanthanum on the oxidation of thin foils of Fe-20Cr-5Alin air at 1150°C. The addition of lanthanum causesthe time to breakaway to increase from about 24 hr for Fe-20Cr-5Al to over 400 hr. Oxidationof the lanthanum-containing alloy occurs in threestages. During the first stage, anα-Al2O3 layer is establishedand thickens with time until the aluminum in the foil is depleted sufficiently for alayer of Cr2O3 to become stableand develop at the scale-alloy interface. This continuesto thicken at a relatively slow rate until breakawayoccurs. The main emphasis in the present paper has been anexamination and analysis of the scale established on thelanthanum-containing alloy in cross section in theanalytical transmission electron microscope (TEM), after an exposure period that coincides with thesecond stage of oxidation, prior to breakaway. The scaleat that time consists of three layers. The outer layeris composed of equiaxed Al2O3grains. The intermediate and inner layers consist of columnarAl2O3 grains and equiaxedCr2O3 grains, respectively.Numerous voids are observed in the oxide grainboundaries and at the intermediate-inner layerinterface. Lanthanum segregates in the oxide grain boundaries andits concentration increases toward the outermost surfaceof the scales. These results are consistent with the“dynamic segregation model” to account for the effects of reactive elements on thegrowth of Al2O3 scales. [ABSTRACT FROM AUTHOR]