Calculation of thermal expansion of iron-aluminides with transition metal additives
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Calculation of thermal expansion of iron-aluminides with transition metal additives Tatiana Seletskaia, Leonid Muratov, Bernard R.Cooper, West Virginia University, Physics Departement, Hodges Hall, PO Box 6315, Morgantown, WV 26506-6315 Abstract The addition of transition metal elements can significantly modify physical properties of intermetalic compounds. We studied the influence of Molybdenum and Vanadium additives on thermal expansion coefficient (CTE) of Fe3Al and FeAl over the wide range of temperatures. The site preference of both transition metals was determined by full-potential LMTO method within the grandcanonical formalism. At low temperatures CTEs were found directly from the FP-LMTO calculations by incorporating them into the Debye model of a solid. The obtained thermal expansion for pure Fe3Al and FeAl is within 10% of its experimentally measured values. At high temperatures we performed Molecular Dynamics (MD) simulations based on our manybody atomistic potentials. The parameters were fitted to reproduce the total energy of a crystal under various types of deformations obtained by FP-LMTO method and were tested with respect to different structures and vacancy formation energies. Our calculations show that addition of V decreases the CTEs of both iron-aluminides, while the addition of Mo makes Fe3Al DO3 structure unstable. Introduction Recent experiments have shown that addition of Mo and V leads to a remarkable stabilization of Fe3Al DO3 structure and therefore improves its strength [1]. Thus, there is a natural interest in studying the effect of these additives on the other physical properties of ironaluminides. Fe3Al in DO3 structure has three non-equivalent lattice sites: FeI site surrounded by eight iron atoms, FeII site with four iron and four aluminum nearest neighbors and Al site. Since experimental data on site-selection of Mo and V in Fe3Al is still incomplete [1], we solved this problem using FP-LMTO method. One of the most important characteristics of iron-aluminides is the strong covalent bonding between Fe and Al atoms implying high activation energy of the optical phonons. Thus, thermal expansion of these compounds can be calculated directly by FP-LMTO method incorporated into the Debye model of a solid. This model was succesfully applied by C.L.Fu et al to calculate anisotropic CTEs of molybdenum-silicides [2]. We tested the validity of the Debye model for pure iron-aluminides and in the presence of Mo and V additives by comparing the calculated thermal expansion with the one obtained from MD simulations. Further, we studied the effect of Mo and V addition on thermal expansion of FeAl and Fe3Al. Methodology FP-LMTO method A complete description of FP-LMTO method can be found in [3]. It is a highly accurate technique for solving bulk density functional problem within Local Density Approximation (LDA). Our FP-LMTO method has a full potential both in the muffin-tin and interstitial regions. In addition, the muffin-tin orbitals are not constrained to have zero kinetic energy in t
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