APB energy

The interatomic interaction is described by an EAM potential specifically developed for NiAl in the B2 structure [12]. Compared to the older potential [16], which was used in most of the previous atomistic studies, our new potential gives considerably higher antiphase boundary (APB) energies = 0.82 J/m, yj pg = 1.06 J/m in good agreement with the APB... 

In spite of the very high APB energy, most of the dislocations split into two 1/2(111) super-partials as shown in Figure 2. This is true for all dislocation characters studied, except for the pure screw dislocation which is displayed in Figure 3. 

BWG and anti-phase boundary energies. The bond-breaking method first described by Flinn (I960) allows an estimate to be made for the structural component of anti-phase boundary (APB) energies. As good agreement can be obtained... 

It has been found experimentally that alloying NiAl with Cr, which reduces the APB energy, may indeed increase the propensity to <111> slip (Field etal.. 

The results for the ANNI model (thick curves in figure 10) show that the presence of even small not-nearest-neighbor interactions leads to the drastic change in the low-temperature behaviour of the APB energy at T — 0, the a T, c) value becomes finite and concentration-dependent. However, with elevating temperature the influence of such small interactions rapidly weakens, and at T 0..3 — 0.4, the APB energies for the NNl and ANNI models do virtually coincide. 

Figure 11 (as well as figure 9) shows the temperature and concentration dependences of APB energies calculated for a more realistic, Ni-Al-type model with the Llo order. The nearest-neighbor interaction for this model much exceeds the other interactions... 

Tables 1-6 contain a list of energies of shear APBs expressed in terms of the ordering energies of the nearest neighbors (V) and the second nearest neighbors (IV) (the formulas are taken from Yamaguchi and Umakoshi (1990)). Flinn (1960), who Hrst derived analytical solutions for LI2 and B2 structures, argued that the APB energies should be associated principally with nearest-neighbor interactions since second-neighbor interactions had been estimated to be only one-tenth as...

These departures are usually taken as indications for much stronger interactions among high-order neighbors. Structural relaxations have also been shown to contribute to reducing the degree of APB energy anisotropy in LI2 alloys (Beauchamp et al., 1987). 

The APB energy has also been proposed to correlate with the stability of the ordered structure vs. the disordered structure, being reflected, in the Ising model, in the relationship between the APB energy and the critical temperature T,. of the disorder-order transition. Models for the disorder-order transformation based on the nearest-neighbor interactions have yielded in some structures a proportional relationship between the ordering energy V and the transition temperature T, i.e. 

Figure 6. Variation of APB energies with temperature observed in some Ll compounds. For Ni,AI the measurements were made for (001) APB. Curve 1 was obtained using weak-beam TEM (Veyssi re et at., 1989) and curve 2 was a measurement using lattice-resolution TEM (Crimp, 1989). For other compounds the APB is on 11111. The APB energies are in fractions of the value at room temperature (70)...

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