Ordered alloys domain structures

Regardless of whether the non-imaging of a species is due to preferential field evaporation or to preferential field ionization, the distinguisha-bility of alloy components in ordered alloys makes much easier the identification of lattice defects and of all types of domains, such as orientational and translational domains, and the discernment of order-disorder phase boundaries in ordered alloys, as well as facilitating the study of clustering and order-disorder phase transformation, etc.88 In most cases, image interpretations become self-obvious. For example in PtCo, which has the LI 0 structure, a Co layer can be distinguished from a... 

Figure 18.7 Interfaces resulting from two types of continuous transformation, (a) Initial structure consisting of randomly mixed alloy, (b) After spinodal decomposition. Regions of B-rich and B-lean phases separated by diffuse interfaces formed as a result of long-range diffusion, (c) After an ordering transformation. Equivalent ordering variants (domains) separated by two antiphase boundaries (APBs). The APBs result from A and B atomic rearrangement onto different sublattices in each domain.

Mechanical properties of disordered alloys are also different to those of ordered alloys. This can have a bearing on the techniques used to prepare the alloys. Ordered structures are usually harder than disordered ones. In the former, dislocations have higher energy the Burgers vector is larger because it is defined on the basis of the superlattice. Ako, dislocation movement is hindered by antiphase domain boundaries which may be present in the ordered state. 

Smith et al have investigated the Pd Ce alloy-H system and have shown that there is a greater solubility in the disordered form. The solubilities in both forms is rather low. Ordered Pdr Ce has an anti-phase domain structure with 0.25 interstices having only palladium atom nearest neighbors. If the disordered structure is completely random, there are 0,Vl8 interstices with only palladium atoms as nearest neighbors. 

The microstnictural evolution process is calculated by substituting Eq.(22) into TDGL Eq.(21) and the results are presented in Fig.ll. The microstructure is visualized by ffay levels representing different values of /= ), +fjj +tj,. fn this calculation, the system is annealed at T = 1.6. One sees that a triple junction of APB s is formed in the later period. This is one of characteristic features of the anti-phase domain structure in Llo ordered alloy and is in marked difference with those shown in Figs.9 and 10. 

It was Sato and Toth who showed that when low-energy imperfections such as antiphase domain boundaries were introduced into an ordered structure without changing the near-neighbor coordination, the positions of (some of) the Brillouin zone boundaries were altered so that they followed an expanding Fermi surface and maintained structural stability despite the increase in electron concentration due to alloying. Consider, for example, the ordered AuCu I structure. The reduced Brillouin zone is made up of 100 planes and the second extended zone is made up of (002) and (110 -type planes. Mapped back in the reduced zone, the second zone has a square cross section normal to the axis. With two electrons per primitive cell, the Fermi surface overlaps the 001 planes of the first zone and touches the 110 planes of the second zone. 

Alloys about AujCd have the M = 2 antiphase domain structure (AujZr type). With increasing Cd content (increasing electron concentration) modulated structures (polytypes of the M = 2 structure) are obtained in the order 1R , 4H , 6H , and 3R (12R, 4H, 6H, and 36R in Ramsdell s notation), in agreement with estimates from the electron diffraction patterns of the order of increasing volumes included within their Brillouin zones. ... 

C.2.1. PdlAl20s Preparation and Structural Properties. To prepare a thin well-ordered AI2O3 model support, a NiAl(l 10) alloy single crystal was oxidized in 10 mbar of O2 at 523 K (290). The structure of the alumina film was examined by a variety of techniques (see Reference (101) and references cited therein), and recently it was even possible to image its atomic structure by STM at 4K (Fig. 19) (215). The alumina film was only approximately 0.5 nm thick and hydroxyl-free, and one should also keep in mind that its exact structure may deviate from those of bulk aluminas (101,215,292,293). Its properties are certainly influenced by the observed line defects (antiphase domain boundaries and reflection domain boundaries). 

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