Defect and Diffusion Forum Vols. 289-292

Abstract: Powder metallurgical (PM) stainless steels can be used for high-temperature applications. However, their characteristic porosity dramatically affects the resistance of the stainless steels to the oxidative attack and modifies the oxidation mechanisms. In this work, it is discussed how the processing parameters of PM stainless steels can modify the diffusion process when the material is exposed at high-temperature in oxidative environments. Processing parameters affect not only the amount but also the nature of the formed oxides. For powders of a given composition, the pressing method, the sintering atmosphere (vacuum, 100% H2 or 75%H2/25%N2) and the sintering temperature can modify the amount of porosity and its shape, often promoting the formation of less-protective oxides, instead of chromia. The different oxygen partial pressures in the inner pores and on the outer surface of the material also tend to make oxides formed inside the stainless steel pores more protective than those formed on their surface. X-ray diffraction, SEM and EDS studies of surface and cross-sectional views of the oxidized materials are used to prove these differences.

Abstract: The performance of Ni-based alloys at high temperature strongly depends on the formation of a protective chromia scale which prevents from fast growth NiO scale. The evolution of the oxide scales will be shown to be strongly dependent on the chromium content especially for the shortest oxidation times. Indeed, transient stages will be reported to condition the subsequent oxidation behavior at longer times until the oxidation rate constant becomes invariant. To this end, a comparative study on the outward and inward diffusion of species upon oxidation at 800°C for short times (0.5 / 1 / 2 / 4 / 8 h) has been evaluated on commercial available Ni20Cr and Ni30Cr alloys through in-situ Raman spectroscopy and supported by post-mortem X-ray diffraction (XRD) as well as scanning electron microscopy coupled to energy dispersive spectrometry (FEG-ESEM / EDS) investigations.

Abstract: Research on the effect of ion implantation on the corrosion behaviour of metals has been carried out for years, but some difficulties arise in the comparison of the obtained results due to variations in experimental conditions (alloys, surface preparation, doses, experimental techniques...). This work tries to overcome those differences, presenting the effect of several elements (Ce+, N+, Cr+ and Cr+ N+) implanted in similar conditions on the pitting corrosion resistance of AISI 430 stainless steel. Potentiodynamic measurements in 1M NaCl demonstrate the beneficial effect of all the implanted elements, showing that Ce+ is the less efficient ion, while Cr+ N+ co-implantation gives the best results in terms of localized attack resistance. Pitting morphology is explained in terms of the XPS and GIXRD data that allow chemical and structural characterization of the implanted layer. Those results help to enlighten the protection mechanisms involved in the considered implantations.

Abstract: Oxygen uptake and oxygen diffusion in Mayenite (Ca12Al14O33) were investigated using the stable tracer 18O2. Mayenite contains one intrinsic, highly mobile oxygen anion. It was shown that for high temperatures (above 700 °C), the diffusion goes through a interstitialcy mechanism, where the interstitial oxygen anion knocks out a lattice oxygen anion. The activation enthalpy for this process is around 0.9 eV, suggesting that the ionic migration is very fast.

Abstract: Ultracentrifuge experiments were performed on the twinned Y1Ba2Cu3O7-x (Y123) single crystal at much lower temperatures than the melting point. Two layers structure with slightly different compositions was observed in the sample ultracentrifuged at 250°C(380,000 G), which might be due to the sedimentation of atoms. In the strong gravity layer, it was found that the Y123 phase disappeared, and unknown XRD peaks appeared. Decomposition occurred in the sample ultracentrifuged at 400°C.

Abstract: Oxygen diffusion coefficients were measured in polycrystalline ZnO by means of the gas-solid exchange method using the isotope 18O as the oxygen tracer. The diffusion annealings were performed at 892oC and 992oC, in an Ar+18O2 atmosphere under oxygen partial pressures from 0.1 to 1atm. After the diffusion annealings, the 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). Increasing the oxygen pressure leads to an increase of the oxygen diffusion in ZnO. The bulk diffusion coefficients depends on oxygen pressure according to , at 882oC, or , at 992oC, which indicates that the oxygen bulk diffusion mechanism should preferentially take place by means of interstitial oxygen having a null effective charge. The grain boundary diffusion coefficients show little dependence on oxygen pressure at 882oC, given by , which should correspond to a diffusion mechanism by means of interstitial oxygen, with a double negative charge, but at 992oC this dependence is corresponding to a diffusion mechanism by interstitial oxygen having a null effective charge. The results also show that the grain boundary is a fast path for the oxygen diffusion in polycrystalline ZnO.

Abstract: Diffusion of oxygen (O) in amorphous silicon dioxide (SiO2) was investigated by means of Si3N4/natSinatO2/28Si18O2/28Si isotope heterostructures grown by thermal oxidation and plasma enhanced chemical vapour deposition. Diffusion experiments with and without a silicon nitride (Si3N4) cap, which serves as diffusion barrier for the gasses in the ambient, were performed. In particular, we determined the impact of the ambient gas, of the thickness of the isotopically enriched SiO2 layer, and of the annealing time and temperature on diffusion. Our results are compared with data given in the literature on oxygen and silicon diffusion in silica and are discussed in the framework of the experimental conditions established at the sample surface and at the buried 28Si18O2/28Si interface. Taking into account a point defect model that is predicted by recent atomistic simulations all experimental results can be explained consistently.

Abstract: The oxidation mechanism of FeCrAl (+RE), RE: reactive elements: Y and Hf) thin foils was studied at temperatures ranging from 1093 K to 1173 K in SO2+1%O2 atmosphere. Materials were subjected to isothermal and thermal cycling exposures as well as to the so-called two-stage-oxidation. In the latter, an oxygen isotope 18O2 was used as a tracer. Starting materials and scales were characterized using Grazing Angle X-Ray Diffraction (GA-XRD), EDX, SEM, XPS and High Spatial Resolution Secondary Ion Mass Spectrometry (HSR-SIMS). The obtained results showed within the studied range of exposure conditions the scales on all the studied alloys grow via outward mechanism typical for transient oxides and not for the -Al2O3 which is consistent with phase composition results and scale morphology and/or microstructure. It was also found that ‘as received’ foils are not bare metals but complex oxide-on-metal systems resulting from their manufacturing procedure. The obtained results are discussed in terms of the diffusion-related transport properties of the scale and of the scale phase composition.

Abstract: One possibility to run a zero CO2-emission power plant is the oxyfuel combustion process. An efficient technology to separate oxygen from air are processes with oxygen transport membranes. It is important that these membranes have a high permeability additional to high chemical stability under power plant conditions. La0.58Sr0.4Co0.2Fe0.8O3- (LSCF) is a mixed ionic electronic conducting material (MIEC) which is considered to have good permeation properties and a high stability. In this work LSCF based membranes are investigated. Data from a TGA (Thermo-Gravimetric Analyser) have been used to determine the diffusion coefficient of oxygen vacancies in LSCF perovskite type oxides for different temperatures.

Abstract: A nanostructured surface layer with a gradient microstructure was produced on a Cu plate by means of the surface mechanical attrition treatment (SMAT). Diffusion of Ni in the nanostructured layer was investigated by the radiotracer technique at temperatures from 383 to 438 K. The measured diffusion profiles consist of two distinct sections with different slopes, the steep one corresponding to the top surface layer with the grain size of 10 to 25 nm and the shallow one corresponding to a subsurface layer with a grain size of 25 to 100 nm. The effective diffusivities derived from both sections are more than 2 orders of magnitudes higher than the grain boundary diffusivities in coarse-grained Cu. The significantly accelerated diffusion rates are expected to be associated with the “non-equilibrium” states of interfaces in the nanostructured surface layer induced by SMAT. The difference between the diffusivities in the top and sub- surface layer might result from the fact that most interfaces developed from twin boundaries in the former while produced by dislocation activities in the latter.