Effect of Heat Treatment on the Carbon Distribution in Zirconium and the Alloy Zr + 2.5%Nb

x-Ray diffraction and Auger spectroscopy are used to establish that vacuum annealing of zirconium at 900–1200°C is accompanied by the formation of zirconium oxycarbide on the metal surface. The tagged-atom method is used to show that the oxycarbide is formed as a result of the diffusion of impurity carbon, which is present in zirconium and in the alloy, from the bulk to the surface of the samples. It is shown experimentally that the segregation of carbon on the grain boundaries and subgrains is a consequence of oxygen concentrating on these boundaries and the formation of a high-temperature α phase whose carbon solubility is much higher than that of the β phase.