Mechanistic insights into twinning and detwinning during fatigue short crack growth in zirconium alloys

This study investigates fatigue short crack growth in Zircaloy-4 alloy, revealing phenomena of twinning and detwinning of {$10\overline{1}2$}<$10\overline{1}1$> tensile type at the vicinity of the crack. Twinning primarily occurs at the crack tip, crack deflection sites, and adjacent heterogeneities such as grain boundaries and triple junctions. Both twinning and slip contribute simultaneously to crack tip plasticity. The occurrence of twinning significantly increases the average stress and stored energy density at the crack tip, thereby accelerating short crack propagation. Twinning forms earlier in Z-type samples (with the texture pole aligned with the Z-direction) compared to Y-type samples, resulting in an earlier and more pronounced increase in crack growth rate. Detwinning is observed exclusively in Z-type samples, induced by substantial changes in stress distribution at the crack tip, likely due to dislocation pile-up at the twin boundary during crack growth.