Grain boundary structure in B2 Fe-Al ordered alloys: an atomic-scale simulation
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Grain boundary structure in B2 Fe-Al ordered alloys: an atomic-scale simulation R. Besson, C. S. Becquart, A. Legris and J. Morillo1 Laboratoire de Métallurgie Physique et Génie des Matériaux - C.N.R.S. U.M.R. 8517 U.S.T.L. - Bât. C6 - 59655 Villeneuve d’Ascq Cedex - France [email protected], [email protected], [email protected] 1 Structure des Systèmes de Basse Dimensionnalité - C.N.R.S. - C.E.M.E.S. 29, Rue Jeanne Marvig - 31055 Toulouse Cedex 4 - France [email protected] ABSTRACT We calculated the atomic structure of the (310)[001] symmetric tilt grain boundary (GB) in B2 ordered Fe-Al, using empirical and ab initio potentials. Including a proper treatment of the influence of small departures from bulk B2 stoichiometry on chemical potentials through a thermodynamic point-defect model, we obtain low energy GB variants geometrically close to the usual ones deduced from the coincidence site lattice (CSL) theory. In Al-rich alloys, both methods predict GB Al segregation whereas in Fe-rich alloys, the empirical (resp. ab initio) approach leads to Fe (resp. Fe or no) segregation. With both methods, strong GB chemical effects triggered by the bulk composition appear, showing that in B2 Fe-Al, GB properties may be strongly influenced by small bulk composition changes. INTRODUCTION In spite of good high-temperature properties, ordered iron aluminides with B2 structure suffer from strong intergranular brittleness, the origin of which has not been elucidated yet [1]. To understand the mechanical properties of these alloys, a better knowledge of their GB properties is helpful but little information is available about them. The present paper is therefore devoted to atomic-scale simulations of GBs in B2 Fe-Al, using both empirical (Embedded Atom Method-like, hereafter noted EAM) [2] and ab initio (Density Functional Theory, DFT) potentials. We chose the Σ = 5 (310) [001] symmetric tilt GB for simulation commodity reasons (its short period), and because it is representative of a wide class of GBs. A previous paper [3] was concerned with an EAM study of this interface in perfectly stoichiometric FeAl. However, the connection between bulk and interface composition was not addressed. This question being essential for alloy designers, it was chosen as the purpose of the present article, in which we assumed T = 0 K and small bulk departures from B2 stoichiometry (50 % Al). Ab initio methods provide the most accurate state-of-the-art potentials but have a high computational cost, compared to semi-empirical (tight-binding) or empirical (EAM) models. Consequently, when using ab initio methods, only a partial investigation of the GB configurational space is tractable. Conversely, given their high computational efficiency, EAM potentials, at the cost of numerical accuracy, allow for a wider inspection of the GB degrees of freedom, including not only the rigid-body translations (RBTs) of grains, but also atomic relaxations and local composition changes. Both methods having specific advantages,
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