A temperature threshold for gas bubble ordering in molybdenum

Abstract

Molybdenum foils were implanted to the critical dose for radiation blistering and 0.75 of the critical dose with 160keV helium ions in a Van de Graaff accelerator. Disc samples were then punched from these foils, electrochemically thinned, and examined in a transmission electron microscope. At implantation temperatures of 200°C and above, the presence of satellite reflections in electron diffraction patterns showed that the gas bubbles ordered onto planes parallel to the {110} planes of the host lattice. For an implantation temperature of 300°C and critical dose, the presence of normally disallowed reflections in electron diffraction patterns indicated that bubbles were ordered onto planes other than the close-packed {110} planes. This was taken to show that the ordering of the bubbles was genuinely three-dimensional. At implantation temperatures between -50°C and 120°C disordered bubble arrays were observed. A sample implanted to critical dose at 120°C was depth profiled using ion milling, but even for depths near the mean projected range for the incident helium ions, the bubble arrays were disordered. It is concluded that there is a temperature threshold which must be exceeded if ordered bubble structures are to be formed. Bubble size and spacing in the disordered bubble distributions were found to be comparable to those in ordered lattices, suggesting that the bubble array was at a stage where it could order if conditions were favourable. In many of the samples containing random bubbles, a bright ring was present around the transmitted beam in diffraction mode, indicating a preferred inter-bubble spacing even though the bubbles were randomly oriented. Calculation of thermal vacancy mobility in molybdenum and copper suggested that the temperature at which bubble ordering commenced was correlated with the temperature at which the mean free path length of a vacancy became comparable to the inter-bubble spacing. Results are presented showing that a random bubble array produced under implantation can evolve to an ordered state simply under annealing, and without further ion bombardment.