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

Al-Ag;Fe-Al;Fe-Cr;neutron scattering;phase stability;phonons;vibrational entropy

Swan-Wood, Tabitha Liana ; Fultz, Brent T.

University:California Institute of Technology

Department:Engineering and Applied Science

关键词: Al-Ag;    Fe-Al;    Fe-Cr;    neutron scattering;    phase stability;    phonons;    vibrational entropy;   

Others  :  https://thesis.library.caltech.edu/3306/1/Thesis_FinalVersion.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

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.