A novel displacement cascade driven irradiation creep mechanism in α-zirconium: a molecular dynamics study

Zirconium alloys used in nuclear reactors undergo irradiation creep, which consists of a visco-plastic deformation activated by irradiation occurring under constant load. However, the fundamental underlying mechanisms have not been unraveled yet. A new high-stress irradiation creep mechanism for recrystallized Zircaloy-4 has recently been proposed based on in situ ion irradiation deformation experiments. A displacement cascade is assumed to induce a direct unpinning of a dislocation from an irradiation defect if the cascade occurs within an effective volume around the pinning point. In the present work, a systematic molecular dynamics study was performed to investigate the effect of a 20 keV cascade occurring near <a>-screw dislocation pinned on an interstitial <a>-loop. The direct release of dislocations by displacement cascades is predicted by the simulations. This release is more likely around the pinning points and with increasing stress, in agreement with experimental observations. The effective volume is roughly estimated for three stress levels equal to 0.89τ_c, 0.93τ_c and 0.97τ_c, with τ_c being the critical stress for unpinning (without irradiation). A mechanism is proposed suggesting that unpinning occurs when the displacement cascade encompasses, at its peak of damage, the two pinning points of the dislocation. A methodology requiring acceptable computation time has been implemented to estimate the effective volume for various cascade energies and loop characteristics, faithfully reproducing the MD results. The mean pinning lifetime calculated using the model provides values of the same order of magnitudes as in the in situ deformation experiments under ion irradiation at high stress levels.