Dislocation structures in zirconium and zircaloy-4 fatigued at different temperatures

The development of characteristic dislocation structures in pure zirconium and zircaloy-4 fatigued under pull-push strain control as the testing temperature and the cyclic strain range varied was examined using a thin-foil transmission electron microscopy (TEM) technique. The slip planes and the twinning planes were determined by a standard stereographic trace analysis technique. The first-order prismatic slip {1010} is the primary deformation mode in zirconium and zircaloy-4 fatigued from room temperature to 873K. The pyramidal slip {121D1} is activated at 673K and at high cyclic strain ranges, whereas the basal slip {0001} only appears in those specimens fatigued at 873K. The {1012}, {1121}, and {1122} types of twins were detected in specimens fatigued at room temperature. Twinning becomes less frequent as the testing temperature increases. The schematic map of the cyclic deformation modes as a function of the plastic strain range and the test temperature is described. The dislocation configurations in fatigued pure Zr specimens evolve from a planar arrangement to a cell structure as the test temperature and the strain range increase. For zircaloy-4, the fatigued dislocation structure is parallel dislocation lines at room temperature, cells at 673K, and two sets of approximate mutually perpendicular dislocation bands at 873K, respectively. Finally, the fatigued dislocation-structure evolution map with the cyclic strain range and the test temperature are qualitatively established for Zr and zircaloy-4, respectively. The effect factors on the fatigue mechanism and the thermodynamic and dynamic criteria of the dislocation-pattern evolution are discussed.