Effects of tube fabrication variables on the oxidation of experimental Zr-2.5Nb tubes

Experimental Zr-2.5Nb tubes, manufactured with various fabrication routes, were oxidized in air at 400 and 500 °C, and their oxide-metal interfaces are characterized by scanning electron microscopy. Various tube microstructures are formed depending to a large extent on the extrusion temperatures in the range of 650-975 °C. The second phase distribution of the tube has a great impact on the oxidation resistance. The tubes that have fine and discrete second phase structures show a superior oxidation resistance to those of coarse and continuous second phase structures. The oxide interface structure evolves as the oxide grows by mirroring the metal microstructure at the beginning of the oxidation, and then develops either a rough structure at higher oxidation rate or a flat structure at lower oxidation rate. The oxide granules in the flat interfaces grow with increases both in oxide thickness and oxidation temperature, but the granule size is held constant in the rough interfaces. Therefore, the oxidation resistance of Zr-2.5Nb tube may be improved when the tube has fine distribution of second phases with the formation of a flat structure at the metal-oxide interface.