CYCLIC BEHAVIOR AND MICROSTRUCTURE DEVELOPED IN ZIRCALOY-4 BETWEEN 300 AND 873K

ALVAREZ ARMAS, IRIS; ARMAS, A. F.

Nuclear Materials Research Development

Nova Science Publishers, Inc.

Lugar: New York; Año: 2007; p. 341 - 367

A detailed description of the low-cyclic fatigue behavior and microstructure development on Zircaloy-4 in the temperatures range from 300 to 873K is given. Based on the cyclic response, three temperature regimes could be defined. In the `Low Temperature Regime’, 300-573K, the cyclic behavior characterizes by a softening stage, which diminishes as the temperature rises. An analysis of the cyclic stress components, the back and the friction stress, has shown that the unlocking of dislocations from oxygen atoms is responsible for this softening behavior. In the `Intermediate Temperature Regime’, 623 – 823K, the cyclic curves exhibit basically a pronounced cyclic hardening rate due to the activation of an atomic process, which intensifies with temperature within the range. Because of the lack of symmetry of the slip systems in hexagonal crystals, the dislocation structure form an unusual non-equiaxial or squared cell structure of dislocations. Moreover, for the temperature where the maximum effect of Dynamic Strain Aging is observed, a high dislocation density characterizes the microstructure. A model of dislocation dynamics has been proposed for the hexagonal metal. Finally, in the `High Temperature Regime’, above 823K, the cyclic response characterizes by a state of saturation practically from the beginning to the end of the fatigue life. The associated dislocations structure corresponds to band of dislocations, which has been identified as the recovered state of the intermediate temperature dislocation structure, where the edge dislocation components were mostly annihilated by climb and the screw components have made cross slip to pyramidal planes.