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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. http://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 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)
Research Advisor(s):
  • Fultz, Brent T.
Thesis Committee:
  • Fultz, Brent T. (chair)
  • Bhattacharya, Kaushik
  • Ortiz, Michael
  • Johnson, William Lewis
Defense Date:12 August 2005
Record Number:CaltechETD:etd-09012005-143247
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-09012005-143247
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3306
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:06 Sep 2005
Last Modified:26 Dec 2012 02:58

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