Fast transport of cobalt in thorium
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*.