Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Materials Science and Engineering

First Advisor

O. Norman Carlson


Three investigations associated with the fast transport behavior of cobalt in thorium have been conducted. These include (1) a study of a metastable ThCo[subscript] x phase and the solid solubility of cobalt in thorium in dilute thorium-cobalt alloys, (2) a study of the mechanism of fast diffusion of cobalt in thorium using diffusion and internal friction experiments and (3) a study of the thermotransport behavior of cobalt in thorium;A metastable plate phase, having a stoichiometry of about ThCo[subscript]0.08, forms in alloys containing greater than 0.004 at.% Co that are quenched from the single-phase field. Upon aging between 773 and 1073 K the plate phase transforms to rod-like precipitates of the equilibrium Th[subscript]7Co[subscript]3 phase. The solid solubility of cobalt in thorium increases from 0.05 at.% at 1120 K to 0.4 at.% at 1350 K according to the relation, c[subscript] s (at.%) = 7044 exp(-110.5 kJ mol[superscript]-1/RT). The terminal solid solubility at the eutectic temperature of 1373 K is 0.45 at.%;The activation energy for diffusion of cobalt in [alpha] thorium is 83.7 kJ/mol whereas that associated with the anelastic relaxation process for cobalt in thorium is 78.0 kJ/mol. The diffusion and internal friction results can be interpreted as supporting either the host-solute diplon mechanism or the interstitial mechanism of fast diffusion. The observed cobalt internal friction peak may be due to a host-solute diplon or a substitutional-interstitial pair;The heat of transport, Q*, for cobalt in thorium was determined by the single-phase steady-state technique and a new technique referred to as the two-phase nonsteady-state technique. Q* is temperature dependent, decreasing from about 20 kJ/mol at 1125 K to about -61 kJ/mol at 1458 K and is described by the relation, Q* = 274 - 0.24 T kJ/mol. The observed temperature dependence of Q* is consistent with a current model for the electronic contribution to Q*.



Digital Repository @ Iowa State University,

Copyright Owner

Steven C. Axtell



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135 pages

Included in

Metallurgy Commons