Atomic and electronic basis for solutes strengthened (010) anti-phase boundary of L1₂ Co₃(Al, TM): A comprehensive first-principles study

The crystallographic and electronic structures of (010) APB of L1₂ Co₃Al_0.75TM_0.25 are studied by high-resolution transmission electron microscopy and first-principles calculations. Effects of solute atoms (TM = Cr, Hf, Mo, Ni, Re, Ru, Ta, Ti, W and Y) on the formation energy, lattice parameters/distortion, magnetism, and bonding strength of the (010) APB in Co₃Al_0.75TM_0.25 are obtained from first-principles calculations. Comparing to the equilibrium volume of Co₃Al, it is found that the volume change of the Co₃Al_0.75TM_0.25 with and without the presence of APB increases linearly with the volume of the corresponding FCC elements, indicating the contribution of the solute atoms on lattice distortion of bulk and (010) APB. Particularly, the strong dependence of the APB energy on the composition is comprehensively discussed together with the available experimental and theoretical data in the literature. The negative (010) APB energy indicates that the formation of (010) APB could stabilize the ordered L1₂ (or the FCC-lattice) Co₃Al, and the local L1₂ → D0₂₂ phase transformation can occur. The physical natures of lattice distortions caused by the fault layers of APB and the solute atoms are characterized by bonding charge density. It is found that the solute atoms, occupying Al site of L1₂ phase and its (010) APB, increase the local bonding strength along (010) through the electron redistribution during forming the chemical bonds with Co, revealing an intrinsic solid-solution strengthening mechanism. This work provides an insight into the atomic and electronic basis for solid-solution strengthening mechanism of L1₂ Co₃Al_0.75TM_0.25.