Segregation/order factor

Fig.6. The B2(110) surface average (solid lines) and sub-lattice (dotted lines) concentrations of the segregant in AB model alloy as a function of reduced temperature calculated in the FCEM approximation for different segregation/order factors r (indicated near the plots). The difference in sub-lattice concentrations corresponds to the surface LRO parameter that vanishes at the surface transition temperature Tg that coincides with the bulk transition temperature T independently of r.

Fig.7. The B2(110) alloy surface average concentration as a function of temperature calculated in the FCEM approximation for model AcB c alloys with stoichiometric (c=0.50) and near-stoichimetric (c=0.49,0.51) bulk concentrations (segregation/order factor r =8.9).

Segregation/order factor Surface induced order/disorder or neither... 

In addition to the segregation/order factor, and depending on its magnitude, the crystal structure and surface orientation can strongly affect the surface composition in ordered alloys. For example, unlike the case of the equiatomic bulk truncated composition of Ll2(100), LRO tends to maintain the Ll2(lll) surface with nominal bulk concentration (0.25). Therefore, the two ordered surfaces are expected to exhibit quite different segregation characteristics for the same r value (Fig. 10). Moreover, SRO causes pronounced changes of surface sublattice and average compositions associated with a considerable reduction of the order-disorder transition temperature (especially in fee alloys). 

These energetic parameters were used in FCEM calculations assuming segregation at the three outmost layers only. As shown in Fig. 15, the segregation tendency prevails only in the B32 ordered alloys and the surface concentration decreases with temperature (entropy-driven monotonous desegregation). This behavior is associated with the distinctly high segregation/order factor (sec. 3.1). On the other hand, ordered bulk truncation with surface concentration very... 

Compared to SRO effects on surface segregation in solid solutions, the role of LRO should be naturally more prominent and common. Its elucidation requires calculations that take into account various factors contributing to the net segregation characteristics in ordered alloys including the temperature dependence the crystal bulk structure and surface orientation, effective bulk and surface interatomic interactions (NN, non-NN) in relation to segregation driving forces, deviation from exact stoichiometry, possible surface relaxation and reconstruction, atomic vibrations, etc. This section attempts to quantify some of these factors and present several possible scenarios of segregation/order interplay. 

Effect of mixing factors for nonfirst-order reactions. Segregation plays no role in plug flow however, it increasingly affects the reactor performance as the RTD shifts from plug to mixed flow. 

The performance of a cast nickel-based alloy is generally based on the microstructural quality, such as the amount of interdendritic segregation, secondary carbides, and intermetallic phases. With the same overall chemical composition, the corrosion rate of the same alloy can vary by several orders of magnitude, depending on its particular microstructure. The most important metallurgical factors that need to be considered are second-phase precipitation by thermal instability and the presence of cold work. The latter is especially important in cases where SCC may be expected. (Rebak)5... 

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