Cylindrical Damage Shells in Irradiated Vanadium

Transmission‐electron‐microscopy studies of neutron‐irradiated vanadium single crystals have revealed the presence of cylindrical regions of high damage concentration surrounding elongated fibrous precipitates. These precipitates have been identified as tetragonal V₃B₂ and are oriented along $\text{〈100〉}$ directions in the cubic vanadium lattice. The cylindrical damage shells are duplex in nature, consisting of a highly damaged inner region of about 1.3‐μ radius and a less highly damaged outer region of about 2.6‐μ radius. These two damage shells consist of small defect clusters produced by recoiling lithium ions and alpha particles, respectively, generated by ¹⁰B(n, α)⁷Li transmutation reactions. Post‐irradiation annealing at temperatures up to 1400°C eliminates the defect clusters and produces dislocation tangles and blocky precipitates in the inner shell and spherical cavities or bubbles in the outer shell. The precipitates in the inner shell are believed to be LiV₂O₅, while the bubbles in the outer shell are evidently formed by the agglomeration of helium atoms. Although spherical damage shells surrounding boron‐containing particles have been observed in several other irradiated metals and alloys, this represents the first known observation of nonspherical shells.