Microstructural characterization of Zr-2.5%Nb extruded tubes using electronic microscopy

The CANDU nuclear plant's pressure tubes are obtained by means of the plastic deformation through the forging and extrusion of a Zr-2.5% Nb ingot at high temperatures, followed by cold deformation and stress release heat treatment. The microstructural parameters that are modified during each stage of the manufacturing process of the pressure tubes must be monitored. The domestic production of Zr 2.5%Nb pressure tubes is being developed by the CNEA's FAE (PPFAE) pilot plant using different extruded tubes from the United States as raw material, supplied by the firm Wah Chang. This work focuses on reinforcing the Project with the results of the evaluation of the test extruded tubes using scanning and transmission electronic microscopy (SEM) and (TEM) to study the morphology and size of the microstructure and the density of dislocations. The SEM micrographs show a two-phase microstructure, consistent with the α-Zr (HCP- ZrNb) and the metastable β (BCC) phase and the TEM micrographs reveal a network of (ba=1/3<1120>) and (bc=1/3<0001>) component dislocations. The width of the α phases was estimated with SEM micrographs of the tube's axial section. Values of 0.2 to 0.5 μ were obtained. The width of the β-Zr phase was measured with the TEM micrographs, revealing a value close to 0.1 μ. Using the Bailey and Hirsch method, the dislocation density was determined with thin foils whose standard coincides with the axial direction. The local thickness of the thin foil was estimated using convergent beam diffraction (CBD). The samples were observed under a two beam setting using the criteria of invisibility, in order to differentiate the and dislocations. The resulting values and morphological aspects would appear to indicate that the consignment material for manufacturing the pressure tubes will allow these tubes to be obtained in accordance with the designer's requirements for the CANDU's power reactors