Anomalous phonon stiffening associated with the (111) antiphase boundary in L1(2) Ni3Al

Antiphase boundaries (APBs) play a crucial role in the anomalous yield behavior exhibited by Ni3Al with L1(2) structure. We investigated the changes in the vibrational properties associated with the formation of (001) and (111) APBs in Ni3Al by employing first-principles calculations. The phonon density of states of Ni3Al with and without (001) and (111) APBs revealed an interesting result: the (001) APB softens the phonons in its vicinity, while the (111) stiffens them. We also calculated the finite-temperature (001) and (111) 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 (001) APB and is negative (e.g. 0.0157 mJ K-1 m(-2) at 300 K) for the (111) APB over the entire temperature range. We also find a significant change in the thermal electronic free energy due to the creation of the (001) or (111) APB. The anisotropy ratio of the APB energies, i.e. the ratio of the (111) APB free energy to the (001) APB free energy, changes from 2.9 at 300 K to 15.9 at 1000 K. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.