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. http://resolver.caltech.edu/CaltechETD:etd-09012005-143247
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. 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 kB/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 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 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.
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.
|Item Type:||Thesis (Dissertation (Ph.D.))|
|Subject Keywords:||Al-Ag; Fe-Al; Fe-Cr; neutron scattering; phase stability; phonons; vibrational entropy|
|Degree Grantor:||California Institute of Technology|
|Division:||Engineering and Applied Science|
|Major Option:||Materials Science|
|Thesis Availability:||Public (worldwide access)|
|Defense Date:||12 August 2005|
|Default Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Imported from ETD-db|
|Deposited On:||06 Sep 2005|
|Last Modified:||26 Dec 2012 02:58|
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