Effect of Extreme Disorder on the Lattice Dynamics and Phonon Scattering in Concentrated Solid Solution Alloys

Composition complexity has a profound influence on lattice dynamics and phonon scattering of solid solution alloys. Here we combine inelastic neutron/X-ray scattering measurements and theoretical calculations based on the itinerant coherent potential approximation (ICPA) and the phonon unfolding method (PUM) to study phonon spectrum of a series of equiatomic solid–solution alloys: NiCo, NiFe, NiFeCo, NiCoCr, NiFeCoCr. Because phonons in these alloys are dominated by force constant disorder they present a considerable challenge to theory. We find that the phonon dispersion relationships are similar across all alloys, however, the disorder induced linewidths are very q-dependent and vary considerably between different alloys. We find excellent overall agreement between experiment and the PUM for dispersions and linewidths and that the broadening reflects strong fluctuations in the pairwise interactions. Furthermore, an underestimation by ICPA of linewidths in NiFe results from neglect of fluctuations in the species-dependent pair-wise interactions.