FREE-ENERGY AND VIBRATIONAL ENTROPY DIFFERENCE BETWEEN ORDERED AND DISORDERED NI3AL

We have calculated free energy and vibrational entropy differences in Ni3Al between its equilibrium ordered structure and a disordered fee s olid solution, The free energy and entropy differences were calculated using the method of adiabatic switching in a molecular-dynamics forma lism. The path chosen for the free-energy calculations directly connec ts the disordered with the ordered state. The atomic interactions are described by embedded-atom-method potentials. We find that the vibrati onal entropy difference increases with temperature from 0.14k(B)/atom at 300 K to 0.22k(B)/atom at 1200 K. We have calculated the density of states (DOS) of the disordered phase from the Fourier transform of th e velocity-velocity autocorrelation function, The disordered DOS looks more like a broadened version of the ordered DOS. Analysis of the par tial density of states shows that the Al atoms vibrations are most aff ected by the compositional disorder, The phonon partial spectral inten sities along the [100] direction show that the vibrational spectrum of the disordered phase contains intensities at optical mode frequencies of the ordered alloy. We find that the volume difference between the ordered and disordered phases plays the most crucial role in the magni tude of the vibrational entropy difference. If the lattice constant of the two phases is set to the same value, the vibrational entropy diff erence decreases to zero. [S0163-1829(98)06202-X].