Vibrational Entropy Contributions to the Phase Stability of Iron- and Aluminum-Based Binary Alloys

This work considers phonon entropy effects on phase stability of three binary alloys: Fe-Cr, FeAl, and Al-Ag. In all cases the vibrational entropy plays an interesting role. The phonon density of states was measured on body-centered cubic Fe0.50Cr0.50 prepared as a solid solution, and in increasingly un-mixed states induced by annealing the solid solution at 773 K. Mössbauer spectrometry was used to characterize the extent of decomposition after annealing. A neutron-weight correction was performed, using results from the Mössbauer spectra and recent data on inelastic nuclear resonant scattering from 57Fe-Cr. The vibrational entropy of decomposition was found to be –0.17 ± 0.01 k B/atom, nearly equal to the change in configurational entropy after spinodal decomposition. Vibrational entropy has a large effect on the critical temperature for spinodal decomposition in equi-atomic Fe0.50Cr0.50. The vibrational entropy of formation of vacancies in FeAl is studied in detail. Born von Kármán calculations show that the point defects due to vacancy formation have a strong stiffening effect on one of the transverse acoustic branches in the (1 1 0) direction. The vibrational entropy of vacancy formation is measured to be –0.75 kB/vacancy. The anharmonic vibrational entropy of FeAl is measured in the temperature range of 10 K to 1323 K. It is shown that there is an abnormally large softening between 10 K and 300 K, which is attributed to a local magnetic moment corresponding to Fe anti-site defects at 10 K. Also measured is an anomalously small anharmonic entropy between 300 K and 1323 K. This could be caused by thermal vacancies and point defects. vi The anharmonic entropy of Al0.40Ag0.60 have been measured to be extremely large between 20 ◦C and 520 ◦C. The origins of this anharmonicity are unclear. The anharmonic entropy of Al0.93Ag0.07 between 20 ◦C and 520 ◦C was found to be fully described by lattice expansion. A large Ag resonance peak was measured in Al0.93Ag0.07 at 20 ◦C. The Mannheim method was used to show that this peak could make a large contribution to the increased solubility of Ag in Al at high temperatures.