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

Citation

Swan-Wood, Tabitha Liana (2006) Vibrational Entropy Contributions to the Phase Stability of Iron- and Aluminum-Based Binary Alloys. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3PTA-J395. https://resolver.caltech.edu/CaltechETD:etd-09012005-143247

Abstract

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 Fe_0.50Cr_0.50 prepared as a solid solution, and in increasingly un-mixed states induced by annealing the solid solution at 773 K. Mossbauer spectrometry was used to characterize the extent of decomposition after annealing. A neutron-weight correction was performed, using results from the Mossbauer 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 Fe_0.50Cr_0.50.

The vibrational entropy of formation of vacancies in FeAl is studied in detail. Born von Karman 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 k_B/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.

The anharmonic entropy of Al_0.40Ag_0.60 have been measured to be extremely large between 20 C and 520 C. The origins of this anharmonicity are unclear. The origins of this anharmonic entropy of Al_0.93Ag_0.07 between 20°C and 520°C was found to be fully described by lattice expansion. A large Ag resonance peak was measured in Al_0.93Ag_0.07 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.

Thesis Files