Observations of a fcc helium gas-bubble superlattice in copper, nickel, and stainless steel

Abstract

Transmission electron microscopy is used to investigate the spatial arrangement of the small gas bubbles produced in several fcc metals by 30 keV helium ion irradiation to high dose at 300 K. In what is a new result for this important class of metals it is found that the helium gas bubbles lie on a superlattice having an fcc structure with principal axes aligned with those of the metal matrix. The bubble lattice constant a_l , is measured for a helium fluence just below the critical dose for radiation blistering of the metal surface (∼4 ± 10¹⁷ He/cm²). Implantation rates are typically ∼ 10¹⁴ He ions cm⁻² sec⁻¹. The values of a_l obtained for copper, nickel and stainless steel are (7.6 ± 0.3) nm, (6.6 ± 0.5) nm and (6.4 ± 0.5) nm respectively. Above the critical dose the bubble lattice is seen to survive in some blister caps as well as in the region between blisters. Bubble alignment is also observed in the case of hydrogen bubbles produced in copper by low energy proton irradiation to high fluence at 300 K. The presentation of this data was accompanied by a cine film illustrating the behaviour of the gas bubble lattice in copper during post-irradiation annealing in the electron microscope. A summary of the film is given in the appendix.