Papers by Author: Jerzy Jedlinski

Abstract: The 18O2 exposure-based approach was used to investigate the failure mechanisms of the oxide scales growing on alumina-forming materials. The scale spallation mechanisms and cracking processes were studied at various oxidation stages of FeCrAl alloys and β-NiAl intermetallic compound. High spatial resolution SIMS was applied to determine the distribution of the oxygen isotopes and other elements in the scales. It was found that the scales spall away according to an adhesive mode. However, t is process usually occurs at temperatures high enough to cause the reoxidation of the exposed bare substrat which results in thin oxide film on the metal. The thickness of this film and its composition depend on the alloy and the region at the interface. Spallation on reactive-element free FeCrAl alloys occurs at relatively high temperatures and the film is fairly thick, while on Zr-containing material very thin oxide layer is formed because the scale is better resistant to spalling and this process occurs at quite low temperature. The thin oxide layer formed on smooth regions comprises essentially the alumina, while the sequence of iron, chromium and aluminium oxide appears on regions exhibiting 'oxide imprints'. The applied approach enabled to find that the through-scale cracking observed at early oxidation stages of β-NiAl occurs at high temperatures and not during cooling. Formation of such cracks affects the further growth of the scale in terms of its microstructure, morphology and generation of stresses. Oxygen inward penetration through cracked scales formed during thermal cycling of FeCrAl alloys occurred mainly via oxide grain boundaries.

Abstract: The results of the study of the scale growth mechanism on the 50 µm thick foil of Fe18Cr5Al are reported. The experiments were carried out at 1093 K using a two-stage-oxidation approach. In the first stage, the samples were exposed to the SO2 + 1 vol. % O2 mixture for 1.5 h and 24 h in order to produce the primary scale. Subsequently, the samples were cooled down to the room temperature and moved to the furnace where they were exposed to the atmosphere rich in 18O2. The duration of the second stage was 0.5 h and 4.5 h for samples previously oxidized for 1.5 h, 8 h and 72 h, for the samples oxidized previously for 24 h. The elemental distributions were determined using high resolution SIMS, while the scale morphology was observed by means of SEM. In addition the radioisotopic studies using 35SO2 were carried out in order to assess whether sulfide phases developed at the initial stage of oxidation. All the results were interpreted in terms of the transport processes in the oxide scale.

Abstract: In the modeling of the oxidation and degradation processes of the superior metallic materials in terms of the high temperature oxidation resistance, being the so-called alumina formers, the matter transport only via grain boundaries of α-Al2O3 is usually taken into account. This paper indicates that such an approach needs to be re-visited. It is pointed out that the following factors should be taken into account (i) diffusion via point defects in the unstable alumina polymorphs (γ, δ, θ) which grow during the early oxidation stages; (ii) dislocation-related processes; (iii) formation of the oxide in three-dimensional defects, being the cracks in the oxide layer; (iv) outward oxide growth resulting from the formation of the new oxide within the existing layer. The mentioned above effects as well as the other ones, related to the interfacial and stress-induced processes are illustrated and discussed. It is pointed out that alumina formers constitute a wide group of materials and several categories should be distinguished among them with respect to their composition and production route on one hand, and the oxidation and degradation mechanisms, on the other hand. It is shown that the comprehensive approach enables better and self-consistent modeling of the oxidation and degradation mechanisms of alumina formers.