J. Jedliński

Comments on the effect of yttrium on the early stages of oxidation of alumina formers

A new approach to the effect of yttrium on the early oxidation stages of alumina formers is proposed. It relies on the elucidation of processes occurring in unstable aluminas. Different mechanisms by which yttrium can accelerate or retard the phase transformation of unstable aluminas into α-Al2O3 are discussed. Using a model of the defect structure of unstable aluminas it is concluded that doped yttrium should retard the transformation as long as its amount does not exceed a critical value. It is shown that the proposed approach enables fairly consistent description of effect of yttrium and to explain various existing controversies.

Transport properties of alumina scales on the β-NiAl intermetallic

Abstract The effect of implanted yttrium on the growth mechanism of alumina scales on β-NiAl intermetallic was studied as a function of temperature (1273–1473 K) using a two-stage oxidation method with 18 O oxygen isotope as a tracer. The distribution of the tracer across the scale was determined by secondary ion mass spectrometry (SIMS) and the scale morphology by scanning electron microscopy (SEM). It has been shown that the presence of yttrium in the surface layer of the substrate strongly influences the transport processes in the growing scale by suppressing the inward diffusion of oxygen.

Comments on the use of the two-stage-oxidation method and surface-analytical techniques in studying growth processes of oxide scales

Application of the two-stage-oxidation methods and surface-analytical techniques, SIMS and SNMS, for determining transport processes occurring in thin oxide scales is discussed briefly. The most fundamental rules relevant to the interpretation of results are formulated, and the practical implications are inferred. The effects related to the morphological and microstructural features are considered.

The effect of implanted lanthanum on chemical diffusion in Cr2O3-scales on Co-40Cr alloy

Abstract A self-diffusion coefficient of chromium in Cr 2 O 3 scale preformed on “pure” and lanthanum implanted Co-40cr alloy has been estimated as a function of temperature (1473–1573 K) using diffusion-evaporation method developed by Kofstad. It has been found that the rate of chromium self diffusion in the scale on lanthanum-doped material is more than one order of magnitude lower than in the scale on unmodified substrate. It implies, that reactive element atoms do not block completely the outward diffusion of cations.

Depth profiling studies of the surface directed phase decomposition in thin polymer films

Abstract External surfaces can significantly alter the phase decomposition (PD) of polymer mixtures in thin films. The surface mode of PD orders two coexisting phases in organised structures with laminated domain morphology. Such structures are employed in new polymer-based technologies of photoelectronic devices or microelectronic circuits. We studied the surface mode of PD in thin films composed of various binary mixtures of polystyrene with its deuterated- and partially brominated- counterpart. We have examined how PD is influenced by: (a) surface active diblock copolymers admixed to decomposing blends, (b) substrate surface modification, and (c) finite film thickness. Thin films were studied by nuclear reaction analysis (NRA) and secondary ion mass spectroscopy (SIMS). The results provided composition profiles as a function of depth in the film with a nanometer precision, comparable with the polymer chain dimensions. Lateral morphology was investigated by means of the atomic force microscope and optical microscope. Various laminated structures composed of 2-, 3-, or 4- layers or column-like structures self-stratified from initially homogenous films were observed.

Mechanistic aspects of Pt-modified β-NiAl alloy oxidation

Abstract The effects of Pt addition (5, 10 and 15 at.%) on the high temperature oxidation behaviour of the intermetallic compound β-NiAl were studied at 1150°C under both isothermal and thermal cycling conditions. The scale growth mechanism was assessed using two-stage oxidation exposures with 18O2 as a tracer in conjunction with high-resolution secondary ion mass spectrometer analysis of isotopic distributions. The scale morphologies were further characterized using secondary electron microscopy, Pt was found to reduce slightly the scale growth rate and improve considerably its resistance to spallation but it did not affect the sequence of scale development. Instead, it slowed the rate at which the scale developed. The α-Al2O3 scales that were eventually established were duplex in structure, consisting of a compact inner layer and a thinner outer layer. This outer layer consisted of ridges and grew by the coarsening of these and/or by a dislocation climb mechanism.

On the application of SIMS to study the oxidation mechanisms of alumina formers

Abstract Secondary ion mass spectrometry (SIMS) is widely used to investigate the oxidation mechanism of alumina formers, being various MeCrAl (Me=Fe, Ni, Co) alloys and β-NiAl intermetallic compound. Frequently, it is combined with dedicated oxidation exposures, mostly in atmospheres containing different amounts of oxygen tracer (18O2) in order to investigate the scale growth mechanism. However, there are several practical aspects which have to be taken into account during planning the oxidation experiments as well as during interpretation of their results in order to avoid misleading conclusions. In particular, the effects related to the formation of non-uniform scales as well as to the relationship between the exposure conditions and the scale microstructure and morphology at various stages of its growth should be dealt with. This paper discusses the implications of sputtering process with respect to elemental in-depth distribution profiling and the results of the oxidation mechanism investigation of various alumina formers by means of SIMS. It is shown that the obtained shapes of the SIMS profiles and images strongly depend on the choice of the exposure conditions and on the analysis parameters, including the spatial resolution. The exposure conditions are related to the scale microstructure and morphology, the evolution of which occurs during the oxidation process. Formation of duplex alumina scale is reported which comprises an inner more compact sub-layer and an outer ridged and less-compact sub-layer.

Growth Mechanism vs Matter Transport in Thermally Growing Oxides on High Temperature Materials: A Brief Survey Based on the Case Study of Alumina Formers

This paper reviews briefly the relationship between the growth mechanism and matter transport using as an example the best currently applied metallic materials being alumina formers. The attention is paid to the experimental approach as well as to the interpretation procedure of experimental results. The scale structure, microstructure, morphology and phase composition are indicated as factors strongly affecting its growth mechanism. The attempt is made to elucidate the possible relationships between the obtained experimental results and actual scale growth mechanisms operating during oxidation exposures.

The effect of alloyed and/or implanted yttrium on the mechanism of the scale development on β-NiAl at 1100°C

Abstract This paper reports on the comparison of scale development on unmodified β-NiAl which contains volume and surface additions of yttrium. This reactive element was incorporated using conventional alloying (0.05 or 0.1 wt%) and ion implantation. The materials containing alloyed yttrium were also implanted with yttrium to study the combined influence of volume and surface additions. A two-stage oxidation approach was applied with the use of 18O2 as a tracer. The reaction temperature was 1100°C and the samples were oxidised for up to 64 h. The scale growth mechanism was followed with the aid of in-depth distributions of the oxygen isotopes across the scale using secondary ion mass spectrometry, the surface morphology of the scales was observed using scanning electron microscopy, while their phase composition was determined using photoluminescence spectroscopy. Similar stages of the scale evolution were observed for the growing scales. Initially the oxide layer consisted of transient oxides, and at its surface more or less developed blade-like surface grains were observed. Subsequently, phase-transformation-related cracks and round patches appeared, and finally ridges which successively covered the outer surface of the scale and constituted the outer layer of a duplex scale developed. The mechanism of scale growth was a mixed outward and inward, and the relative contribution of both mechanisms varied depending on the analysed scale region and on the oxidation stage. The cracks and patches were regions where the transient aluminas were preferentially transformed into α-Al2O3. The ridges formed in cracks essentially consisted of α-Al2O3. For the initial stages of oxidation, the transient aluminas and α-Al2O3 co-existed in the scales, while subsequently the latter prevailed and finally, became the only phase found in the oxide layers. The evolution rate of the scale was affected by alloyed and implanted yttrium additions: the former exhibited minor and content-dependent effect while the latter significantly retarded it.

High Temperature Oxidation and Oxidation-Induced Degradation of Alumina Formers

This paper focuses on a systematic description of the alumina formers oxidation behaviour by referring to a variety of observables, features and/or processes and their evolution during the exposure. An attempt is also made to identify the key effects relevant to degradation process. An output of this approach consists of the ‘Oxidation Routes’ and the ‘Degradation Paths’.