Volatilization in water vapor–containing atmospheres is an important and often unexpected mechanism of degradation of high-temperature materials during processing and in service. Thermodynamic properties data sets for key (oxy)hydroxide vapor product species that are responsible for material transport and damage are often uncertain or unavailable. Estimation, quantum chemistry calculation, and measurement methods for thermodynamic properties of these species are reviewed, and data judged to be reliable are tabulated and referenced. Applications of water vapor–mediated volatilization include component and coating recession in turbine engines, oxidation/volatilization of ferritic steels in steam boilers, chromium poisoning in solid-oxide fuel cells, vanadium transport in hot corrosion and degradation of hydrocracking catalysts, Na loss from Na β″-Al2O3 tubes, and environmental release of radioactive isotopes in a nuclear reactor accident or waste incineration. The significance of water vapor–mediated volatilization in these applications is described.

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Intermetallic-matrix composites are attractive alternatives to carbon / carbon and ceramic / ceramic composites for applications up to 1,600°C. Recent work on the intermetallic compounds MoSi 2 and Ti Si 3 has included determination of t~eir mechanical properties and deformation behavior, selection of thermodynamically compatible high-strength and ductile reinforcements, and strengthening and toughening mechanisms in silicide-matrix composites for high-temperature service. Figure 1. Melting temperature as a function of density for high-temperature intermetallic compounds. The indicated T m is the melting temperature which gives 0.8 T m = 1 ,600°C.

The structural energy differences between cubic LI2 and tetragonal DO22 crystal structures are calculated for MAl3 compounds, where M is a group III, IV, or V transition metal. The stability of the DO22 structure relative to L12 increases rapidly as the transition-metal d-electron count increases. Typical values of E(DO22) – E(L12) are 0.1–0.15 eV/atom (9600–14500 J/g-atom) for group III,  0.05 eV/atom ( 4800 J/g-atom) for group IV, and ∼ –0.2 eV/atom (∼ –19000 J/g-atom) for group V trialuminides. Similar trends are calculated for the DO23/L12 energy difference. The calculated electronic densities of states (DOS) show that each structure has a minimum in the DOS distribution at a characteristic d-electron count. The preferred crystal structure for a given compound is the one in which the Fermi level lies in the minimum.

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