Point-defect and threshold displacement energies in Ni3Al II. Events at the displacement threshold

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Abstract
Molecular dynamics simulations with the many-body interatomic potentials used in part I have been employed to investigate the displacement threshold energy E d at 0 K and the defect structure created at the displacement threshold in the ordered alloy Ni3Al. The crystallography of the L12 structure requires four unit stereographic triangles to be considered: one for Al primary recoils and three for Ni. E d for Al is generally higher and has a much stronger orientation dependence than in pure Al. For Ni recoils, on the other hand, E d is similar to that in pure Ni in directions where Ni-Al interactions are not strong; exceptions occur along closely packed directions, for which replacement collision sequences (RCSs) are comparatively short in the ordered alloy. There are significant differences from pure Ni for directions where the Ni primary recoil makes close contact with an Al neighbour. The stable defect arrangement in Ni3Al is usually much more complex than for pure metals because of the requirements of stoichiometry and the fact that six basic defect types can occur in the alloy. Generally, however, defects on the Ni sublattices are the majority species. The principal exception to this occurs for ⟨110⟩ recoils along mixed Ni-Al-Ni-Al-… rows, for which RCSs generate chains of antisite defects. These break, however, to create two Frenkel pairs for each primary recoil atom. All these results are discussed in terms of the structure and atomic interactions in Ni3Al and are illustrated by computer-generated defect plots. The same potentials have also been used to compute E d in pure Ni and Al, and the values are in reasonable agreement with the sparse experimental data in the literature. They show that, as in the ordered alloy, E d is strongly influenced by the number of atoms displaced by the recoil atom and the extent to which RCSs are a feature of the defect structure.