Microsegregation of oxygen in Zr-2.5Nb alloy materials

Oxygen partitioning between the primary-α and β phases during heating at an (α+β) temperature was investigated in a Zr-2.5Nb alloy containing about 1100 ppm (wt) oxygen. Standard pressure tubes used in CANDU reactors have been produced from this material by hot extrusion of billets after preheating in the (α+β) temperature range. Secondary ion mass spectroscopy (SIMS) was found most suitable for oxygen mapping and for determining quantitatively the extent of preferential oxygen enrichment of primary-α grains in pressure tube samples previously subjected to experimental preheat treatments at an (α+β) temperature of 870 °C for different soak times up to 24 hours. For the (α+β)-treated materials, the oxygen concentration in the primary-α grains increased with soak time, reaching up to about 8 times the concentration in the primary-β matrix region. Samples from two standard pressure tubes with different fracture toughness properties were also examined. Significant differences between these tubes were found in their primary-α grain size and in the levels of oxygen enrichment of the α grains, which could provide an explanation for the difference in the toughness of these tubes.