STUDY OF POINT-DEFECT MOBILITIES IN ZIRCONIUM DURING ELECTRON-IRRADIATION IN A HIGH-VOLTAGE ELECTRON-MICROSCOPE

A high-voltage electron microscope (HVEM) was used to investigate the nature of intrinsic point defects in alpha-Zr by direct observation of dislocation climb and cavity growth or shrinkage. The material used was Marz grade Zr that had been pre-irradiated with neutrons at about 740 K in the Doureay Fast Reactor. Dislocation loops of vacancy character that had been produced during the neutron irradiation were studied by further irradiation with electrons in the HVEM. Growth of the loops was observed at temperatures as low as 230 K indicating that, under the conditions of the experiment, some vacancy-type defects were mobile in the temperature regime 230-300 K. The nature of these defects is unknown. One possibility is that these defects are not intrinsic in nature, but may be vacancy-Fe complexes. In addition to the climb of dislocation loops, c-component network dislocations and cavities were also studied. Basal-plane climb of the network dislocations was observed at 573 K, but was not readily apparent at 320 K. This suggests that preferred climb planes (and possibly loop habit planes) are sensitive to temperature. Cavities that were already in the foil after neutron irradiation, or were induced by electron irradiation, grew along the c-axis and shrank along a-directions during electron irradiation. This radiation-induced shape change of the cavities strongly suggests the existence of a diffusional anisotropy difference between interstitials and vacancies in alpha-Zr.