ROLE OF MICROSEGREGATION IN FRACTURE OF COLD-WORKED ZR-2.5NB PRESSURETUBES

Results from Zr-2.5Nb pressure tubes removed periodically from CANDU r eactors for surveillance tests have revealed significant tube-to-tube variability in fracture toughness. If the microstructural factors resp onsible for such variations were known, the toughness of future pressu re tubes could be improved and their service life extended. Evidence i s presented which demonstrates that the variation in toughness for spe cimens fractured on the radial-axial plane is related to decohesion or fissuring on the transverse-axial plane as a result of microsegregati on of Cl and C (as carbide). In the absence of such species and decohe sion it is shown that very high toughness can be achieved.