Anomalous phonon stiffening associated with the (1 1 1) antiphase boundary in L12 Ni3Al

Antiphase boundaries (APBs) play a crucial role in the anomalous yield behavior exhibited by Ni3Al with L12 structure. We investigated the changes in the vibrational properties associated with the formation of (0 0 1) and (1 1 1) APBs in Ni3Al by employing first-principles calculations. The phonon density of states of Ni3Al with and without (0 0 1) and (1 1 1) APBs revealed an interesting result: the (0 0 1) APB softens the phonons in its vicinity, while the (1 1 1) stiffens them. We also calculated the finite-temperature (0 0 1) and (1 1 1) APB Gibbs free energies from the first-principles quasi-harmonic approximation. The vibrational entropy of formation is positive (e.g. 0.053 mJ K−1 m−2 at 300 K) for the (0 0 1) APB and is negative (e.g. −0.0157 mJ K−1 m−2 at 300 K) for the (1 1 1) APB over the entire temperature range. We also find a significant change in the thermal electronic free energy due to the creation of the (0 0 1) or (1 1 1) APB. The anisotropy ratio of the APB energies, i.e. the ratio of the (1 1 1) APB free energy to the (0 0 1) APB free energy, changes from 2.9 at 300 K to 15.9 at 1000 K.