Abstract
Cobalt, nickel and iron-based alloys containing 25wt.%Cr and strengthened either by chromium carbides or by HfC were oxidized at 1000, 1100 and 1200°C to obtain external oxide scales. The spallation of these oxides during the post-oxidation cooling was studied by exploiting the cooling part of the thermogravimetry curves. The best resistant alloys against scale spallation were the iron-based alloys which did not lose oxide. The nickel-based alloys well behaved too. The worst alloys were the cobalt alloys, principally due to the complex composition of the oxide scales. Beside the base element, the thickness of the oxide was also identified as a major parameter. The presence of Hf with contents much higher than usual was also beneficial for the resistance against spallation.
Highlights
Among the most important properties which characterize the high temperature oxidation resistance of refractory metallic alloys there is the isothermal oxidation rate [1, 2]
The samples were still entirely covered by the oxide scale while other samples had lost a significant part of the scales externally developed at high temperature (Fig. 6)
Even if the resistance against oxide spallation at cooling is better specified by thermal-cycling tests the present results obtained only for one cooling are interesting. These ones, which were deduced by the exploitation of the last part of the files issued from isothermal thermogravimetry tests usually devoted to the study of isothermal oxidation kinetic, allowed revealing the important role of several parameters
Summary
Among the most important properties which characterize the high temperature oxidation resistance of refractory metallic alloys there is the isothermal oxidation rate [1, 2] Another important criterion in this field is the resistance of the formed protective oxide scale against spallation during temperature variations. The well-known parameters which influence this phenomenon are the ratio between the average thermal expansion coefficients of the oxide and of the alloy, the cooling rate and the chemical composition of the alloy (notably the base element, the presence in small quantities of active elements as hafnium or yttrium) [3, 4]. The mix of the influence of several parameters may lead to various behaviours in spallation of the oxide scales developed isothermally at high temperature during cooling
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