Papers by Keyword: Scale Morphology

Abstract: Isothermal oxidation of two g-TiAl-based intermetallic alloys: Ti48Al and Ti46Al8Nb alloys was studied in synthetic air at 1073-1223 K for up to 240 hrs. Mass change per unit area for the oxidized samples followed approximately the parabolic rate law. The k_p values for the studied temperature interval were in the range from 7.2×10^-13 to 1.8×10^-11 g²cm^-4s^-1. The activation energy for oxidation of Ti48Al alloy in air at 1073-1223 K was E_a = 165±12 kJ/mol. Niobium addition to Ti48Al alloy in the amount of 8% increased its oxidation resistance. Structure and chemical composition of the oxidation products, and morphology of the oxidized samples were investigated using XRD, SEM-EDS, and TEM. The oxide scales formed on Ti48Al and Ti46Al8Nb alloys were well adherent to the metallic substrates and exhibited a multilayer structure. Depending on the oxidation temperature and the alloy composition, the scale consisted of variable amounts of TiO₂ and Al₂O₃. Additionally TiN, and niobium rich particles were also identified in the appropriate oxide scale. To understand the growth mechanism of oxide scale formed on Ti46Al8Nb alloy, two stage oxidation experiment was performed using ¹⁶O₂/¹⁸O₂, followed by SNMS and TEM-EDS. Particular attention was paid to the use of TEM in order to precisely characterize the reaction products on the Ti46Al8Nb alloy.

Abstract: The oxidation behaviour of a Ferritic 10%Cr steel in Ar-H2O mixtures was investigated at 650°C. The studies aimed at elucidating the effect of water vapour content as well as the gas flow rate on the mechanisms of oxide scale formation. An important observation of the present investigation is, that H2 produced by the reaction of water vapour with the steel, can play a significant role in the oxidation process. It affects the possibility to form an external haematite layer and may alter the oxide scale growth rate. The extend by which the H2 affects the oxidation behaviour depends on the gas flow conditions, the water vapour content and the exposure time. To confirm these observations a number of specimens were oxidized in Ar-H2-H2O mixtures. This atmosphere guarantees a very low equilibrium oxygen partial pressure, in which H2 formed by reaction of the gas with the metal, does not substantially alter the thermodynamic properties of the gas.