Abstract
The lattice dynamical and thermodynamic properties of Fe–Ni invar alloys with three stoichiometric compositions, FeNi3, FeNi and Fe3Ni, have been investigated from first principles density functional theory. Phonon dispersions of L12 FeNi3 and L10 FeNi predict dynamically stable phase along the high symmetry directions throughout the first Brillouin zone. Phonon dispersion of L12 Fe3Ni unveils a softening of transverse acoustic mode along X - M - Γ - R directions. In the phonon dispersion of D03 and Z1 phases of Fe3Ni, the significant softening of transverse acoustic mode is disappeared. Our results predict that Z1 Fe3Ni can be used as a phonon filter. The finite temperature/pressure thermodynamic properties; the isothermal bulk modulus, room temperature thermal equation of state, coefficient of thermal expansion, heat capacity, Debye temperature and Grüneisen parameter are also presented. In the D03 Fe3Ni, negative thermal expansion is observed for the temperature above 500K.
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