Crack tip plasticity in single crystal UO2: Atomistic simulations

The fracture behavior of single crystal uranium dioxide under mode-I loading is studied using molecular dynamics simulations at room temperature. The initial cracks are introduced as elliptical notches on either {1 1 1} or {1 1 0} planes. Two crack tip shielding mechanisms, dislocation emission and metastable phase transformation are identified. Crack extension is observed for cracks residing on {1 1 1} plane only. The dislocations have a Burgers vector of 〈1 1 0〉/2 and glide on {1 0 0} planes. Two metastable phases, Rutile and Scrutinyite, are identified during the phase transformation, and their relative stability is confirmed by separate density-functional-theory calculations. Examination of stress field near the crack tips reveals that dislocation emission is not as an effective shielding mechanism as the phase transformation. The formation of new phases may effectively shield the crack if all phase interfaces formed near the crack tips are coherent, as in the case of cracks residing on {1 1 0} planes.