Blistering in helium-ion-implanted (111) niobium monocrystals

Abstract

The formation of helium blisters on the (111) surface planes of monocrystalline niobium by implantation of 0.5–1.5‐MeV helium ions has been investigated for total doses ranging from 0.1 to 1.0 C/cm² for different target temperatures, angles of incidence of the projectiles, and channeling conditions. The results indicate that the blister shape is strongly dependent on target temperature. At 900 °C and for channeled projectiles, almost all the blisters have a threefold symmetry resembling a ``crow‐foot'' shape. The alignment of the crow‐foot blisters with respect to each other exhibits an asymmetry in that their prongs are aligned along only one set of 〈112〉 directions of the host niobium lattice such as [12̄1], and [112̄], and [2̄11] directions and not along the other equivalent set such as the [1̄21], [1̄1̄2], and [21̄1̄] directions. As the target temperature is lowered, the tendency to form dome‐shaped blisters increases; at room temperature only dome‐shaped blisters form. The average blister size appears to increase with decreasing target temperature, with increasing degree of channeling, and with increasing projectile energy; at room temperature the last of these variables is the one with the most pronounced effect on the size. The blister density is more than two orders of magnitude less for channeled projectiles than for unchanneled ones for irradiation at a target temperature of 900 °C. For the unchanneled projectiles, the angle of incidence of the projectiles does not appear to change the morphology of the blisters significantly.