Alloying elements, effect cerium

Today most of the advanced materials used at temperatures around 1000 °C rely on the effect of active element additions to the alloy. In particular, for protective coatings on gas turbine materials which were optimized for oxidation resistance, the active element effect plays the key role for longterm oxidation protection of the structures. The most common active elements in this respect are yttrium and cerium and to some extent also rhenium, hafnium, and zirconium. 

Figure 2.16 shows the charge-discharge cycle characteristics of alloys in which part of the nickel component was replaced with cobalt. Misch metal (Mm), which is a mixture of rare earth elements such as lanthanum, cerium, praseodymium, and neodymium, was used in place of lanthanum. It was found that the partial replacement of nickel with cobalt and the substitution of the lanthanum content with Mm was very useful in improving the charge-discharge cycle life. However, such alloys have insufficient capacity, as shown in Figure 2.17 [18]. From study of the effect that their compositions had on the charge-discharge capacity, it was concluded that the best alloy elements were Mm(Ni-Co-Al-Mn)This alloy led to the commercialization of sealed nickel-M H batteries. All the battery manufacturers who use a rare earth-nickel-type alloy for the negative electrode material employ similar alloys with slightly different compositions.

The various rare earths are used in the foundry industry as rare earth silicides, in which the rare earth content is about 30%. Other alloys are used in which the level of rare earths is about 10% (10% cerium, 2% other rare earths) with silicon and iron comprising the bulk of the remaining elements. In the magnesium-ferrosilicon alloys, the rare earths are present in amounts from about 0.1% to 1.0%. These alloys are used differently by the various consumers. However, the effects of the rare earth elements, introduced by whatever means, are the same. 

Rare earth elements, such as yttrium, cerium and samarium, are well known to stabilize tetragonal Zr02 at lower temperatures, although the tetragonal Zr02 is generally stable at and above 1373 K. In the present study the effect of rare earth element addition on the catalytic properties of the amorphous Ni-Zr alloy-derived catalysts has been examined in order to improve the catalytic activity of the catalysts. 

The last chapter (134) in this volume is an extensive review by Colinet and Pasturel of the thermodynamic properties of landianide and actinide metallic systems. In addition to compiling useful theiTnodynamic data, such as enthalpies, entropies, and free eneigies of formation and of mixing, the authors have made an extensive comparative analysis of the thermodynamic behavior of the rare earths and actinides when alloyed with metallic elements. They note that when alloyed with non-transition metals, the enthalpies of formation of uranium alloys are less negative than those of the rare earths while those of thorium and plutonium are about the same as the latter. For transition metal alloys the formation enthalpies of thorium and uranium are more negative than diose of the rare earths and plutonium (the latter two are about the same). The anomalous behaviors of cerium, europium and ytterbium in various compounds and alloys are also discussed along with the effect of valence state changes found in uranium and plutonium alloys. 

The addition of small amounts of reactive elements such as cerium, yttrium, hafnium, thorium, lanthanum, or their oxide dispersions greatly increases the high-temperature oxidation resistance of Fe-Cr alloys under isothermal or cyclic conditions. [11], Beneficial effects also result from ion implantation of the active element or from surface-applied coatings [11]. The ion implantation work of Bennett et al. [12] concerns the oxidation behavior of a 20Cr-25Ni-Nb stainless steel in CO2 at temperatures in the range of 900 to 1050°C. SIMS can be used to locate the position of the reactive element after oxidation. 

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