Investigation of effects of dilute oxygen solute on nucleation of zirconium

Classical nucleation theory is the foundation of considerations of the freezing of liquids. The presence of a dilute solute has been shown to affect nucleation, however, classical theory does not account for the rates seen experimentally. This work investigates the effects of a dilute amount of oxygen solute on the nucleation of solidification in otherwise pure zirconium.; Undercooling distributions were obtained for mid and high purity zirconium samples via electrostatic levitation experiments and the critical work of formation, ΔG*, and kinetic prefactor, log K_v, were determined using statistical analysis. Oxygen content of each sample was determined using glow discharge mass spectrometry. Both kinetic parameters were seen to increase with increasing sample purity.; In order to examine these effects of oxygen, a sharp interface model was assumed to allow separation of total free energy change accompanying nucleation into volume free energy and interfacial free energy components. It was shown that dilute amounts of oxygen (<0.2 at %) had a minimal effect on the volume free energy change for this system. Thus, the interfacial free energy term was found to be the dominant factor in the total free energy change for nucleation. Spaepen and Turnbull's temperature-dependent reduced interfacial free energy, α, was used to examine the interfacial free energy which arises as a result of the decrease in configurational entropy at the interface. Experimentally-determined values for α were significantly less than theoretical predictions of their negentropic model, which assumed an infinite planar interface between a pure liquid and pure close-packed crystal. The interface configurational entropy is greater than the theoretical prediction due to the curvature of the interface and the presence of oxygen. The addition of oxygen increases the number of available configurations, increasing the configurational entropy at the interface, decreasing the interfacial free energy and decreasing ΔG*, the critical work of formation.