Quenching, from high temperature

NixAlj x (homogeneous between 42 and 69 at.% Ni) with good mechanical and oxidation resistance properties. By quenching from high temperatures the formation of an ordered martensite is obtained which can be considered for its shape memory behaviour. For a discussion on substitutional additions to CsCl-type alloys (site preference for dilute additions to NiAl, FeAl, CoAl, etc.) see Kao et al. (1994). 

Density measurements clearly establish that the system contains a close-packed anion array with cation vacancies. Moreover, neutron-diffraction data on samples quenched from high temperature have identified the presence of tetrahedral-site iron interstitials surrounded by four octahedral-site vacancies, but the nature of the elementary cluster associated with such an interstitial has been more difficult to estab-lish " . Some of the clusters considered are shown in Fig. 16. From powder neutron diffraction at 985 and 1075 °C, Gavarri et al. have found a ratio of octahedral-site vacancies to interstitial iron to be... 

Samples quenched from high temperature (1100°C for Nd2 xCexCu04, 890°C for Nd2Cu04 xFx) contain little or no superlattice. On annealing these samples at 400-700°C regions of superlattice form within minutes this process can be observed directly in the electron microscope by means of a hot stage. 

The defect structure of Fei O with the NaCl-type structure had been estimated to be a random distribution of iron vacancies. In 1960, Roth confirmed, by powder X-ray diffraction, that the defect structure of wiistite quenched from high temperatures consists of iron vacancies (Vp ) and interstitial iron (Fcj) (there are about half as many FCj as Vpe). This was a remarkable discovery in the sense that it showed that different types of crystal defects with comparable concentrations are able to exist simultaneously in a substance, Roth also proposed a structure model, named a Roth cluster, shown in Fig. 1.84. Later this model (defect complex = vacancy -F interstitial) was verified by X-ray diffraction on a single crystal and also by in-situ neutron diffraction experiments. Moreover, it has been shown that the defect complex arranges regularly and results in a kind of super-structure, the model structure of which (called a Koch-Cohen model) is shown in Fig. 1.85 together with the basic structures (a) and (b). 

Cinibulk, M.K., Kleebe, H.-J., Schneider, G.A. and Ruhle, M., (1993b), Amorphous intergranular films in silicon nitride ceramics quenched from high temperatures , J. Am. Ceram. Soc., 76 (11), 2801-2808. 

Figure 3 Comparison of typical Mossbauer spectra obtained from Eastern ashes quenched from high temperatures in reducing (top) and oxidizing (bottom) atmospheres.

This is not the only reactive form of oxygen, as La20j also is effect in generating CHj radicals, but there is no evidence for 0" ions on this material [Ref. 16], Rather, the epr spectrum of a surface O2 has been detected after the oxide was quenched from high temperatures [Ref. 17]. The superoxide ion is relatively unreactive at low temperatures however, it may become active at the high temperatures where the oxidative dimerization occurs. 

We find that the properties of the high-pressure, low-temperature phase of nitrogen obtained by "cold" compression are different fi-om those for the phase quenched from high temperature. This suggests that the C phase is metastable and/or transitions to it are sensitive to nonhydrostatic effects. Raman and infrared spectra of E-N2 above 40 GPa and are not compatible with R 3 c and R3c symmetries proposed in Ref [23] because the number of vibron bands is larger than predicted for the standard structures based on these space groups. This increase in the number of bands is probably related to the additional lowering of the symmetry and multiplication of the size of the unit cell. 

Fig. 9. Representative Raman spectra of 0 and i phases measured at 95 and 70 GPa and 297 K after quenching from high temperature. The spectra of the phase used as a starting material are shown for comparison at the same temperature and at 69 GPa.

Point defects such as vacancies are thermodynamically stable entities 18) meaning, in effect, that their concentration increases with increasing temperature because their free energy of formation is negative. A crystal that is quenched from high temperatures will, at lower temperatures, contain a supersaturation of point defects. But the excess... 

The Young s modulus of polycrystalline NiAl with a stoichiometric composition is about 235 GPa at room temperature (Harmouche and Wolfenden, 1987). The elastic moduli are functions of the composition, and the Young s modulus reaches a maximum of slightly more than 235 GPa, not at the stoichiometric composition as is expected, but at about 48 at.% Al which may be related to the difference in the defect character on both sides of stoichiometry. The effect of excess vacancies, which are produced by quenching from high temperatures, on the elastic behavior was studied... 

The dependence of the density of UO2+X upon the OAJ ratio has been obtained by Anderson (33), Grdnvold (7), Shaner (24), and Young et at. (25) all these results can be explained satisfactorily with the oxygen interstitial model and the results by Young et al. (25) are shown in Fig. 8-(a) as an example. They measured the densities of UO2+ ud U4O9 quenched from high temperature by helium-substitution method. 

Figure 9.15 Some ceramics do not show a sudden strength loss with quenching from high temperatures. In these materials, the thermal stresses often lead to a gradual increase in the degree of damage.

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