Compositional variation of the phonon dispersion curves of bcc Fe-Ga alloys

Abstract

Inelastic neutron scattering techniques have been used to measure the phonon dispersion curves of bcc ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}$ ($x=10.8$, 13.3, 16.0, 22.5) alloys as a function of Ga concentration. The phonon frequencies of every branch were found to decrease significantly with increasing Ga concentration. The softening was most pronounced for the ${T}_{2}[\ensuremath{\xi}\ensuremath{\xi}0]$ branch and, to a lesser extent, the $L[\ensuremath{\xi}\ensuremath{\xi}\ensuremath{\xi}]$ branch in the vicinity of $\ensuremath{\xi}=\frac{2}{3}$. The concentration dependence of the shear elastic constant ${C}^{\ensuremath{'}}=1∕2({C}_{11}\ensuremath{-}{C}_{12})$, calculated from the slope of the ${T}_{2}[\ensuremath{\xi}\ensuremath{\xi}0]$ branch, was found to agree with the results of sound velocity measurements. For the higher concentration sample measured, 22.5 at. % Ga, new branches appeared, an effect associated with the increase in the number of atoms per unit cell.