Grain fragmentation and twinning in deformed Zircaloy 2: Response to positron lifetime measurements

Microstructural developments in deformed Zircaloy 2 were studied using techniques ranging from positron annihilation spectroscopy (PAS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscattered diffraction (EBSD). Grain fragmentation during plastic deformation, which is expected to involve dislocation realignment and recovery, showed a clear correlation with drop in mean PAS lifetime (τm). An increased τm, on the other hand, was associated with annihilation of deformation twins. The deformation twins, {101-bar 2}<1-bar 011> type tensile twins, were identified from axis-angle relationship: 94.8 deg. <1-bar 21-bar 0>. A deviation of more than 5 deg. from the exact twin orientation relationship was considered as annihilation. Such annihilation, caused by developments of in-grain misorientations, is expected to involve accommodation through interfacial dislocations. Presence of such dislocations, and associated point defects, offers an explanation for the observed increase in τm.