Cyclic softening mechanisms of Zircaloy-4

Cyclic tests with different total strain amplitudes were conducted on Zircaloy-4 at room temperature. The results show that cyclic softening behaviour is the main mechanical feature observed in this material. A detailed analysis of the hysteresis loops was conducted in order to determine the friction and back stresses. The friction stress corresponds to the resistance which the dislocations have to overcome to keep moving in the lattice. The back stress depends on the density of long-range impenetrable obstacles that are created by the dislocations movement such as pile-ups. The cycling softening that Zircaloy-4 exhibits is related with the decrease of the friction stress. Moreover, the independence of the friction stress behaviour on the total strain amplitude as well as the influence of thermal treatment on interrupted cyclic tests permit the conclusion that the unlocking of dislocations from oxygen atoms is responsible for the observed softening.