Dumbbell interstitial configuration

The differences in the atom coordination around the interatomic cavities manifest themselves in the distribution of the interstitial formation energies. In LRC s as well as in crystals, the formation of dumbbell interstitial configurations is possible. The expressions for equilibrium concentrations of interstitials are similar to those given above for vacancies (6.19 and 20), with the difference, however, that one accounts in them for the difference of interstitial cavities and the possibility of dumbbell configuration formation. 

The embedded-atom technique was applied to the study of interstitials in NiAl (Caro and Pedraza, 1991) and NisAl (Caro et al, 1990 Pedraza et al., 1991). In Ni3Al, the enthalpy differences between dumbbell, octahedral, and crowdion Ni interstitial configurations are small, typically 0.1 to 0.2 eV. The <100> Ni-Ni dumbbells in pure Ni planes IE = 3.63 eV) are favored Ni-Al and Al-Al dumbbells have larger formation energies (4.45 and 6.22 eV respectively, which is understandable on the basis of size arguments) and hence convert into Ni-Ni dumbbell + Al i antisite defects. This behavior is very similar to that studied experimentally in CU3AU (see Sections 4 and 7). 

Interstitialcy migration depends on the geometry of the interstitial defect. However, an a priori prediction of interstitial defect geometry is not straightforward in real materials. For an f.c.c. crystal, a variety of conceivable interstitial defect candidates are illustrated in Fig. 8.5. The lowest-energy defect will be stable and predominant. For example, in the f.c.c. metal Cu, the stable configuration is the (100) split-dumbbell configuration in Fig. 8.5d [3]. 

Figure 8.5 Geometric configurations for a self-interstitial defect atom in an f.c.c. crystal (a) octahedral site, (b) tetrahedral site, (c) (110) crowdion, (d) (100) split, dumbbell, (e) (111) split., (f) (110) split crowdion [2].

Diffusion of Self-Interstitial Imperfections by the Interstitialcy Mechanism in the F.C.C. Structure. For f.c.c. copper, self-interstitials have the (100) split-dumbbell configuration shown in Fig. 8.5d and migrate by the interstitialcy mechanism illustrated in Fig. 8.6. The jumping is uncorrelated,8 (f = 1), and a/ /2 is the nearest-neighbor distance, so... 

Figure 13. Configuration and migration of the Cu-Cu split interstitial proposed for ordered CujAu. The labelling I [010] means the dumbbell is oriented along the <010> direction, and centered on the cube face perpendicular to the X axis (From Alamo et at., 1986)...

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