Comparison of thermal expansion path of the Fe2Mo Laves phase calculated using the DFT-based phonon and Debye-Grüneisen approaches

Abstract

Precipitation of the intermetallic Fe2Mo Laves phase at the interface between the nuclear fuel and the fuel element cladding can significantly weaken the strength characteristics of the cladding and fuel. Despite the importance of designing materials for the cladding of fuel rods, the thermodynamic properties and the trajectory of the thermal expansion of the Fe2Mo remain poorly understood. The thermodynamic properties of the Fe2Mo have been studied using the finite-temperature quantum mechanical calculations within the frame of the density functional theory under the quasiharmonic approximation. The vibrational contribution to the free energy was obtained using phonon calculations. The thermal expansion of Fe2Mo was predicted by comparing between free energies calculated in different directions. A path with the least energy was chosen as the trajectory of thermal expansion. The obtained result was compared with the direction calculated in previous theoretical work used the Debye–Grüneisen approach and accounted magnetic subsystem to calculate the vibrational and magnetic contributions to the free energy. This comparison reveals that these two approaches are in good agreement with each other. The work shows that the Fe2Mo possesses a non-isotropic thermal expansion. The heat capacity and volumetric expansion at constant pressure are modeled. The calculated results analyzed and are in satisfactory agreement with the experimental data. Obtained results can be useful for further design of fuel element cladding materials intended for generation IV reactors, the operating temperature of which should be above 873 K.