Effects of chemical composition and B2 order on phonons in bcc Fe-Co alloys

The phonon density of states (DOS) gives insight into interatomic forces and provides the vibrational entropy, making it a key thermodynamic function for understanding alloy phase transformations. Nuclear resonant inelastic x-ray scattering and inelastic neutron scattering were used to measure the chemical dependence of the DOS of bcc Fe-Co alloys. For the equiatomic alloy, the A2 -> B2 (chemically disordered -> chemically ordered) phase transformation caused measurable changes in the phonon spectrum. The measured change in vibrational entropy upon ordering was -0.02 +/- 0.02 k(B)/atom, suggesting that vibrational entropy results in a reduction in the order-disorder transition temperature by 60 +/- 60 K. The Connolly-Williams cluster inversion method was used to obtain interaction DOS (IDOS) curves that show how point and pair variables altered the phonon DOS of disordered bcc Fe-Co alloys. These IDOS curves accurately captured the change in the phonon DOS and vibrational entropy of the B2 ordering transition. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3456500]