Kinked basal dislocation loops for anomalous annealing hardening in irradiated zirconium

Annealing is a traditional pathway to mitigate irradiation hardening in metals, while an anomalous annealing hardening is widely observed in neutron-irradiated zirconium (Zr), which is counterintuitive and intriguing, and affects the performance of Zr components in nuclear reactors. Here, we report that the anomalous annealing hardening in irradiated Zr originates from thermally activated formation of three-dimensional kinked <c> dislocation loops. Through concurrent in-situ heating experiments inside a transmission electron microscope, we demonstrate that irradiation-induced planar <c> dislocation loops progressively merge into zigzag-shape kinked configurations between 400°C -500°C. Atomistic simulations reveal that partial dislocations generated by the dissociation of 1/6<2$\overline{2}$$\overline{2}$03> loops glide on pyramidal planes driven by the inter-loop attraction forces, ultimately forming kinked steps. These three-dimensional kinked <c> loops act as strong obstacles for prismatic <a> dislocations, leading to a pronounced hardening. This discovery provides a universal framework for understanding the annealing-induced hardening in hexagonal close-packed metals.