GAS-BUBBLE SUPERLATTICE FORMATION IN BCC METALS

The spatial ordering of small helium bubbles (diameter 1.5-2.0 nm) pro duced in the bcc metals V, W, Mo, Cr, Fe and Ta by ion implantation wi th 30-50 keV He+ is studied using transmission electron microscopy (TE M) and earlier studies are reappraised. Implantations are at temperatu res in the range 300-773 K. Gas-bubble ordering onto {110} planes is o bserved in all the metals studied. For V, W and Mo, the bubbles order fully in three dimensions to form a bcc gas-bubble superlattice orient ed parallel with the metal lattice. A similar conclusion is reached al so for Cr, Fe and Ta although the study of these metals is less detail ed. The bubble lattice parameters are found from bubble spacings deter mined from electron diffraction patterns. The results, confirmed from bright-field micrographs, are as follows: V, approximately 3.9 nm at 3 00 K; W, approximately 6.2 nm at 775 K; Mo, approximately 6.2 nm at 67 5 K; Cr, approximately 5.1 nm at 300 K; Fe, approximately 6.0 nm at 30 0 K; and Ta, approximately 6.4 nm at 575 K. The results suggest that o rdered bcc bubble superlattices are a consistent feature in bcc metals following high-dose implantation with helium at temperatures approxim ately-equal-to 0.2T(m), where T(m) is the melting temperature of the m etal.