Effect of antiphase boundaries on the electronic structure and bonding character of intermetallic systems: NiAl

The possible origin of the high degree of brittleness, i.e., low ductility, of the NiAl-based alloys in the B2 structure is investigated by all-electron self-consistent total-energy linear-muffin-tin-orbital calculations. Using a supercell approach (with up to 20 atoms/unit cell), the energetics of the two simplest antiphase boundaries (APB's) for NiAl in the B2 structure--namely the 1/2〈111〉 on {110} and 1/2〈111〉 on {112}--are calculated assuming no relaxation at the interface. We find APB energies of the order of 800 mJ/m² for both cases. Since the calculated APB energies are very high, 〈111〉 slip is unlikely to occur--as suggested experimentally and by some phenomenological theories. By substituting Ni or Al with V, Cr, or Mn at the APB interfacial planes, remarkably decreased APB energies are obtained. These theoretical results on simplified model systems may suggest a way to decrease the APB energy in order to eliminate one of the possible reasons for the brittleness at ambient temperatures.