Site Occupation and Structural Phase Transformation of the (010) Antiphase Boundary in Boron-Modified L1₂ Ni₃Al

The site occupancy of boron (B) in L1₂ γ′-Ni₃Al and its (010) antiphase boundary (APB) are studied by first-principles calculations in the present work. Based on the electronic structures of the (010) APB, 12 initial tetrahedral sites for B are identified and reduced to 6 distinct configurations due to symmetry, which are transformed into 4 octahedral sites presented by the atomic trajectories during relaxations in first-principles calculations. It is revealed that B atoms prefer to occupy a site far away from the fault layers within the (010) APB of γ′-Ni₃Al, agreeing well with previous experimental observations. Bonding charge density is utilized to provide a novel insight into the local L1₂ → D0₂₂ structural phase transformation of the APB and the corresponding tetrahedral-octahedral transition. The energetic site occupation of B is dominated by the lower electron density of the octahedral sites than that of tetrahedral sites.