Phonon Modes in FeCrAl Alloys

The ferritic Iron-Chromium-Aluminum (FeCrAl) alloy is being considered for use as an accident tolerant fuel/clad material in the current light water nuclear reactors due to its high-temperature oxidation and corrosion resistance, and its structural stability under irradiation. It has a body-centered cubic lattice with the Cr and Al atoms randomly distributed throughout the lattice. Due to the compositional disorder in the FeCrAl structure, there does not exist a discernable unit cell that is appropriate for the traditional phonon mode calculations; further complications arise when FeCrAl undergoes structural changes due to radiation damage that leads to significant loss of translational symmetry. This work furthers our recent research efforts on using atomic current correlations [1] to draw out the phonon modes of disordered multicomponent systems. We begin by showing that the Fourier modes of the atomic current, without relying on a repeating unit cell, are theoretically and numerically consistent with phonon modes derived from the dynamical matrix approach with certain constraints on the resolvable phonon modes. Using atomistic simulations, we apply our method to obtain the phonon dispersion of unirradiated FeCrAl (18 at.% Cr and 5 at.% Al). Next, we use non-equilibrium displacement cascade simulations to probe the effects of radiation damage on the phonon modes. The simulation domain consists of a cubic box containing 65,000 atoms, approximately. A primary knock-on atom imparts excess kinetic energy to simulate a neutron interaction; several atomic layers are provided at the boundaries to absorb the excess heat and to damp the pressure waves emanating from the radiation knock. Relatively stable defects are formed after 10 ps with most of them concentrated near the knocked region. Using the current correlations, we note a significant broadening of the phonon linewidths arising from the glass-like irradiation defects. Of particular interest is the emergence of nearly discontinuous bands in the wave-vector space. We conclude by delineating the key differences in the phonon dispersion between the irradiated FeCrAl and its native state.

[1] A. Fullmer and J. Eapen, Theory of phonon mode interactions from current correlations and applications in disordered solids and liquids, March APS Meeting, Las Vegas (2023).