GAS-BUBBLE SUPERLATTICE FORMATION IN BCC METALS

The spatial ordering of small helium bubbles (diameter 1.5-2.0 nm) produced in the bcc metals V, W, Mo, Cr, Fe and Ta by ion implantation with 30-50 keV He+ is studied using transmission electron microscopy (TEM) and earlier studies are reappraised. Implantations are at temperatures in the range 300-773 K. Gas-bubble ordering onto {110} planes is observed in all the metals studied. For V, W and Mo, the bubbles order fully in three dimensions to form a bcc gas-bubble superlattice oriented parallel with the metal lattice. A similar conclusion is reached also for Cr, Fe and Ta although the study of these metals is less detailed. The bubble lattice parameters are found from bubble spacings determined from electron diffraction patterns. The results, confirmed from bright-field micrographs, are as follows: V, approximately 3.9 nm at 300 K; W, approximately 6.2 nm at 775 K; Mo, approximately 6.2 nm at 675 K; Cr, approximately 5.1 nm at 300 K; Fe, approximately 6.0 nm at 300 K; and Ta, approximately 6.4 nm at 575 K. The results suggest that ordered bcc bubble superlattices are a consistent feature in bcc metals following high-dose implantation with helium at temperatures approximately-equal-to 0.2T(m), where T(m) is the melting temperature of the metal.