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Point defect phonons in intermetallics: the case of NiAl3 by atomic-scale simulation

Rémy Besson 1David Tingaud 2

1. Laboratoire de Métallurgie Physique et Génie des Matériaux (LMPGM), Université de Lille 1, Villeneuve d'Ascq 59655, France
2. Laboratoire Science des Procédés Céramiques et de Traitements de Surface - UMR 6638 CNRS - Université de Limoges, 123 avenue Albert Thomas, Limoges 87060, France

Abstract

Although little is known about the influence of phonons on the point defect properties of ordered compounds, experimental measurements (positron annihilation) have pointed out the occurrence of unexpectedly strong effects, especially in iron aluminides. As a complementary approach, the present study, devoted to an atomic-scale simulation study of Al-rich Ni-Al intermetallic phases by means of embedded-atom potentials, aims at providing further insight into the role of this factor, presumably non negligible since it may drastically influence the diffusion of matter, and thus the microstructural evolution during the elaboration and ageing phases of the materials. The present work concerns NiAl3 of cementite-type D011 structure, with a preliminary investigation of its low-temperature point defect structure, based on an independent-point defect approximation. The calculation of the T = 0 K grand canonical (GC) energies E0GC(d) of the point defects shows the mixed antisite-vacancy character of this compound at low temperature. The defect-induced phonon properties are then tackled by means of a harmonic analysis around the T = 0 K relaxed configuration of each type of point defect: although point defects entail only minor changes in the overall shape of the phonon density of states with respect to that pertaining to the undefected reference crystal, they significantly modify the defect parameters, the relevant quantities becoming the (phonon-corrected) GC free energies FGC(d) = E0GC(d) + FvibGC(d), with FvibGC= EvibGC -TSvibGC. Our analysis demonstrates that, in the latter expression, none of the energy and entropy terms can be neglected in general. Moreover, the FvibGC term being a decreasing (resp. increasing) function of T for antisites (resp. vacancies), phonon-induced inversions of defect populations are expected at moderate temperatures.

 

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Presentation: Oral at E-MRS Fall Meeting 2008, Symposium J, by Rémy Besson
See On-line Journal of E-MRS Fall Meeting 2008

Submitted: 2008-04-25 11:18
Revised:   2009-06-07 00:48