The Effect of Lanthanum on the Scales Developed on Thin Foils of Fe-20Cr-5Al at Very High Temperatures

A study has been undertaken of the effects oflanthanum on the oxidation of thin foils of Fe-20Cr-5Alin air at 1150°C. The addition of lanthanum causesthe time to breakaway to increase from about 24 hr for Fe-20Cr-5Al to over 400 hr. Oxidationof the lanthanum-containing alloy occurs in threestages. During the first stage, an-Al₂O₃ layer is establishedand thickens with time until the aluminum in the foil is depleted sufficiently for alayer of Cr₂O₃ to become stableand develop at the scale-alloy interface. This continuesto thicken at a relatively slow rate until breakawayoccurs. The main emphasis in the present paper has been anexamination and analysis of the scale established on thelanthanum-containing alloy in cross section in theanalytical transmission electron microscope (TEM), after an exposure period that coincides with thesecond stage of oxidation, prior to breakaway. The scaleat that time consists of three layers. The outer layeris composed of equiaxed Al₂O₃grains. The intermediate and inner layers consist of columnarAl₂O₃ grains and equiaxedCr₂O₃ grains, respectively.Numerous voids are observed in the oxide grainboundaries and at the intermediate-inner layerinterface. Lanthanum segregates in the oxide grain boundaries andits concentration increases toward the outermost surfaceof the scales. These results are consistent with thedynamic segregation model to account for the effects of reactive elements on thegrowth of Al₂O₃ scales.     

Effect of lanthanum rare earth addition on low temperature plasma nitriding

Abstract

The effect of lanthanum additions on the plasma nitriding of nanocrystalline steel 3J33 is reported. Nitriding was carried out in 20N₂ + 80H₂ at a pressure of 400 Pa at 350 and 410°C. Glow discharge spectrometry shows that although its atomic radius is larger than that of Fe, La can diffuse into the surface layer to a significant depth; the La addition increases the compound layer thickness. Based on the microhardness profile, the diffusion layer thickness also increases due to La addition. XRD analyses indicate that the presence of La did not change the phase structure of the surface, i.e. a compound layer of γ′-Fe₄N type. The incorporation of La not only enhances the microhardness but also improves the toughness of the nitrided layer. The atomic and electron structures of phases γ′-Fe₄N and γ′-(Fe,La)₄N were calculated using the plane wave pseudopotential method. The (Fe,La)₄N phase is more stable than Fe₄N and the shear/bulk modulus ratio G/K of (Fe,RE)₄N is much smaller. This finding provides a possible mechanism for the increased microhardness and improved toughness of nitrided layers containing La.     

Influence of rare earths on shrinkage porosity in thin walled ductile cast iron

Abstract

Ductile cast iron has been cast in test bars with thickness from 2 to 10 mm. The rare earth elements La and Ce have been added to some of the castings to evaluate their influence on microstructure and shrinkage tendency. Both La and Ce increased the graphite nodule count, especially for thickness of 6 mm and below. La gave the best nodule size distribution with many small nodules. La had less shrinkage tendency than Ce in the 10 mm test bars. This tendency was less pronounced for the 6 and 4 mm test bars and other factors may have a large influence at these thicknesses. Increasing the temperature T ₁, which is controlled by the growth of off-eutectic austenite dendrites, increased the shrinkage tendency.     

Development of intermediate temperature SOFC using LaGaO3 based oxide film

Abstract

The effect of the NiO–Fe₃O₄ (NiFeO)/Ce_0·8Sm_0·2O_2-δ (SDC) ratio on the microstructure and electrochemical activity of cermet substrates was systematically investigated. Dense NiFeO–SDC substrates, which are suitable for the deposition of thin oxide films for example, La_0·9Sr_0·1Ga_0·8Mg_0·2O_3-δ (LSGM) tend to be porous and applicable as anodes in intermediate temperature solid oxide fuel cells (IT SOFC). The porosity and electrical conductivity of the reduced substrates were evaluated with increasing NiFeO ratios. The lowest anode overpotential was measured in a substrate with a NiFeO/SDC ratio of 6:4, especially at a high current density. In a reduced substrate with this composition, continuous frames of SDC and NiFe alloy can be formed simultaneously, thus leading a longer triple phase boundary.     

Combustion synthesis and characterisation of lanthanum hexa-aluminate

Abstract

Lanthanum hexa-aluminate (LHA) is considered a promising material for thermal barrier coatings for gas turbine applications as well as oxidation resistant coatings for fibres in oxide based ceramic composites. Combustion synthesis has attractive advantages such as simple experimental setup, low cost, short reaction time, energy savings due to exothermic reaction, better control of stoichiometry and fine powder size. The present study involves combustion synthesis of LHA using lanthanum and aluminium nitrates as oxidising precursors, and urea as fuel. The precursor salts were dissolved in a minimum amount of deionised water and the solution was heated in a mantle heater to temperatures of around 300–350°C, at which combustion takes place with rapid evolution of gas. The LHA powder was also prepared by a precipitation route for comparative study. The as synthesised powders were calcined at up to 1450°C and subjected to X-ray diffraction for phase evolution studies. Product powders were then characterised. The influence of the lanthanum/aluminium molar ratio in the starting composition and the effect of temperature on the formation sequences of lanthanum monoaluminate (LMA) and LHA and on the conversion of LMA to LHA are discussed.