Microstructural characteristics of β precipitates in Zr-1Nb alloy

The β phase precipitates in a Zr-1Nb alloy annealed at 600 °C and 700 °C (below and above the monotectoid temperature, respectively) for 3 h were investigated by transmission electron microscopy. It was found that the uniformly distributed β phase obtained at 600 °C was spherical, with diameter of 30-60 nm, volume fraction of ∼2.5% and Nb content of ∼68 wt.%. However, the β phase obtained at 700 °C was stick-shaped, with ∼100 nm in diameter and ∼300 nm in length, volume fraction of ∼9.8% and Nb content ∼10 wt.%. The distinctions in volume fractions and compositions were consistent with the lever rule of the Zr-Nb phase diagram. The distinction in shapes resulted from the change in the interfacial structure, which depended on the lattice misfit. The orientation relationship between α matrix and stick-shape β precipitates satisfied Burgers' orientation relationship, i.e., (101¯ 0)α//(11¯ 2)β for the broad face, (0001)α//(110)β for the side facet and (112¯ 0)α//(1¯11)β for the edge. The outline of the stick-shaped β precipitates was composed of straight and curved interfaces, and the broad faces and side facets together formed the straight interface, and the edges formed the curved one. As the Nb content of β phase increased, the lattice misfits on broad face and side facet increased and their interfacial structures changed from coherent (obtained at 700 °C) into incoherent (obtained at 600 °C), and the misfits of edge decreased, and their interfacial structures were always incoherent for the two heat treatments. Therefore, the curved interfaces had a higher growth rate than the straight interfaces at 700 °C and stick-like β phase precipitate was formed. While the three planes have a comparable growth rate at 600 °C and spherical β phase precipitate was formed.