Antiphase boundary defects

Using the constructed potentials the y-surface for the (111) plane was calculated. (For more details see Girshick and Vitek 1995). T e lowest energy minimum on this surface corresponds to the ideal Llo structure. However, there are three different metastable stacking fault type defects on (111) the antiphase boundary (APB), the complex stacking fault (CSF) and the superlattice intrinsic stacking fault (SISF). The displacements... 

Figure 1.1 Defects in crystalline solids (a) point defects (interstitials) (b) a linear defect (edge dislocation) (c) a planar defect (antiphase boundary) (d) a volume defect (precipitate) (e) unit cell (filled) of a structure containing point defects (vacancies) and (/) unit cell (filled) of a defect-free structure containing ordered vacancies. ...

In a crystal containing twin defects, the crystal lattices continue across the twin boundaries without a break. Another similar defect, the antiphase defect, is formed by a shift of the crystal by half a unit cell along the antiphase boundary. This defect can also contribute to strong image contrast as shown in Figure 10.3b. 

Virtually all minerals contain defects. In addition to point defects (e.g., vacancies that exist in a thermodynamically determined equilibrium number, impurities etc ), macroscopic minerals contain line defects (dislocations), and planar defects such as stacking foults, antiphase boundaries and twins. Intergrown layers of different structure or composition, and polytypic disorder also may be present. 

Besides the mechanical alloying of elemental powders, ball-milling of an intermetallic compound can also lead to amorphization, as demonstrated for several alloys [3.18, 19, 130, 131] (for more details see Chap. 2). This cannot be explained by the above statements, since in this case no composition-induced destabilization provides the driving force for an interdiffusion reaction. Amorphization by milling starting from powders of crystalline intermetallics is attributed instead to the accumulation of lattice defects - mainly the creation of antiphase boundaries - which raise the free enthalpy of the faulted intermetallic above that of the amorphous alloy. Therefore, there exists some similarity with irradiation-induced amorphization [3.20]. 

Despite continuing progress in the crystal growth, 3C-SiC films still contain many lattice defects. In particular, twins, stacking faults and antiphase boundaries (APBs) have been reported [64,65]. APBs occur as a common defect when a polar film, SiC in this case, is heteroepitaxially grown on a non-planar substrate. To eliminate this particular defect in 3C-SiC films, Si substrates misoriented from the (100) plane have been used, as stated above [39,53],... 

As for the translation domains of B, they occur because the unit cell of the primitive lattice of B is three times larger than for A, so that each R atom of A can give an Rj or R2 or R3 atom of B, as seen previously. This gives three translation domains, separated by planar defects of translation vectors g[132] and -g[132] which are in this case antiphase boundaries. Each kind of epitaxy of B on A gives 6 x 3 = 18 equivalent domains. When we deal with B regions made of two twinned regions the number of domains is doubled. We can see that the study of the transformation domains is rather complicated for the B structure. It is probably... 

Such interconnections are sometimes observed only in one slice of the crystals. An example is shown in the HREM micrograph of Fig. 33a. The contrast at the edge of crystd, characterized by double rows of white dots corresponds to a triple slab "PbO-Cu-PbO" at the level of the defect such a double row disappears and is replaced by a triple row of small spots. This defect is explained by the fact that a "45°" antiphase boundary is interrupted at the level of the "SrO" planes as a consequence, this limited defects can be interpreted as the replacement of triple "PbO-Cu-PbO" layer by a "Cu02-Y-Cu02" layer (Fig. 33b). 

Simulation methods based on first principles are becoming available, but the number of atoms that is tractable at present is significantly fewer than what dislocation studies require (De Fontaine, 1992 Pettifor, 1992), implying approximations at various levels. As with most techniques, simulations are not in general fully reliable (see antiphase-boundary (APB) energy calculations in NijAl and in NiAl (Chapter 21 by Sun in this volume), and the situation does not improve when dealing with more complicated defects such as dislocation cores. Simulations may nevertheless be helpful in solving problems that would otherwise require intuition. 

It is well-known that the antiphase boundaries (APBs) in intermetallic compounds affect many material properties, such as the dislocation structures, the associated plastic deformation, and microstructures of many intermetallic compounds. APBs are two-dimensional defects, i.e. intercrystalline interfaces similar to grain boundaries. Therefore, the phenomenology of APBs has many parallels with that of grain boundaries. The structure and chemistry of APBs are discussed in Chapter 21 by Sun in this volume. 

Larger etch pit densities of VCo.88 than of VCo,s3 form the subgrain boundaries characterized by the presence of substructure such as antiphase boundaries due to the formation of an ordered compound (150). The hardness of NbC decreases with carbon content and the hardness anisotropy of NbCo.8 is less pronounced than that of NbCo.9 (Fig. 11), which would be due to (a) deviation from stoichiometry of the crystal and (b) ordering of carbon vacancies. A high-resolution electron microscopy (HRFM) study gives very detailed information about defect order... 

Twins and some stacking faults produce bond distortions but no broken bonds and are usually of limited consequence. Stacking faults producing antiphase boundaries in ordered materials are high energy structures and result in significant numbers of charged defects. 

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