Vacancies high-temperature alloys

At T=OK, the criterion of minimum energy dictates that only one defect, the constitutional defect, is present. In NiAl for example this is the Ni antisite (Cab) in Ni-rich alloys, and the Ni vacancy (Cva) in Al-rich alloys [6]. The constitutional defects will be the dominant ones at high temperatures. The criterion that the A antisite should be the constitutional defect in A-rich alloys is ... 

Two types of transformations can be very broadly distinguished. The first is the formation of a solid solution, in which solute atoms are inserted into vacancies (lattice sites or interstitial sites) or substitute for a solvent atom on a particular sublattice. Many types of synthetic processes can result in this type of transformation, including ion-exchange reactions, intercalation reactions, alloy solidification processes, and the high-temperature ceramic method. Of these, ion exchange, intercalation, and other so-called soft chemical (chimie douce) reactions produce no stmctural changes except, perhaps, an expansion or contraction of the lattice to accommodate the new species. They are said to be under topotactic, or topochemical, control. 

Volume diffusion mechanism On the basis of diffusivities extrapolated from high-temperature data, volume diffusion is considered inoperable in higher-melting alloys at room temperature. However, if the formation of excess surface vacancies (monovacancies or vacancy aggregates such as divacancies) by the dissolution of less noble atoms from terrace sites is taken into account, a new situation arises Provided that the annihilation of these vacancies at sinks is outperformed by their production, the excess surface vacancies may diffuse into the interior of the alloy. The interdiffusion coefficient of the alloy components in the vicinity of the surface, D, may then approximately be expressed as... 

For most of the intermetallic compounds, deviation from the stoichiometric composition is accommodated by antisite atoms CuZn, CusAu, NiTi, NisAl. Paired antisite defects are also formed thermally in high concentration for those alloys which present an order-disorder transition in the solid state (CU3AU, CuZn) their concentration is directly related to the LRO parameter value. The absence of structural vacancies has been checked by positron annihilation in the case of CuZn (Kim and Buyers, 1980), CujAu (Doyama et fl/.,1985a,b), FejAl (Schaefer et al., 1990), and TiAl (Shirai and Yamaguchi, 1992). In all cases, thermal vacancies form at high temperature. 

Excess vacancies may be obtained, at lower temperatures by quenching the solid after heating it to a high temperature. Such excess vacancies affect many different properties, such as diffusion, where they allow solid-state reactions to take place at low temperatures. The important age hardening reaction in aluminum-base alloys requires quenched-in vacancies. An Al-2 at. % Cu alloy becomes much stronger... 

Thus, the growth of an external oxide scale by the reaction between a pure metal or a metallic alloy and a gaseous or liquid oxidant phase at high temperature is also a combination of diffusion processes and interfacial reactions, and Fig. 2.1 also applies to such corrosion processes that are formally similar to solid-state reactions in poly-phase and multi-constituent systems. Such a similarity will be considered to extend the treatment of the Kirkendall effect for two-phase diffusion couples to the growth of an oxide scale on a pure metal or on an alloy. The roles of interfaces will be analysed more particularly in relation to some specific topics related to oxide scaling processes such as interface displacement, growth stresses and injection of point defects (vacancy or interstitial). 

We work with the pair approximation of a binary alloy system. The two species are written as A and B (i=l and 2), and we do not include vacancies. The nearest-neighbour (NN) interactions are taken into account, and the numerical examples are given for a 2-D square lattice as well as for an fee lattice. Although we consider thermal vibrations, we assume the temperature is high enough so that the classical treatment is applicable and the equipartion law of the kinetic energy is justified. Therefore, we work only on the potential energy part of the... 

Iron oxide scale Oxidation of iron or low-alloy steels at temperatures >570 C (wustite stable) leads to a scale composed of an inner thick layer of wustite, Fei j,0, and two outer thinner layers of magnetite Fe3 04 and hematite Fe203. The disorder of Fei yO has been described in Sect. 6.2.2.1.3, its very high-iron vacancy concentration being the reason for fast outward cation diffusion and rapid growth. 

Fewer EXAFS works have been devoted to the study of catalytic systems under reaction conditions due, as already said, to inherent limitations of the technique at high, working temperatures characteristic of catalytic reactions. Among these, a majority include studies of Cu/ZnO (Cu/Si02) in methanol synthesis.The solid state physics of the active copper phase in methanol synthesis is a rather intriguing problem which has not achieved consensus concerning the oxidation state and the hosting of the Cu phase characteristics. The EXAFS works mentioned above elucidated mainly the importance of the metallic state in the reaction. Similarly, the metallic state has been shown to be of importance in the water gas shift reaction (WGS) in Cu, Au, and bimetallic Pd-Cu systems supported on ceria. The importance of ceria vacancies on the activation of water and of the metal (and more precisely, of the metal at support boundaries) for CO activation appear as key elements for this reaction. The bimetallic Pd-Cu work analyses the modulation of Pd behaviour by effect of the alloy with the base... 

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