Formation of helium platelets in molybdenum

Abstract

Over the past two decades, helium introduced into metals either by ion implantation or by the (n, α) reaction has been studied over a wide range of concentrations and temperatures1,2. Early studies were relevant to fission reactor technology3,4 but more recent emphasis has been on the role of helium in fusion reactor environments. One area of interest concerns the interaction of helium ions from the plasma with the first-wall surface while the influence of the helium from the (n, α) reaction on void nucleation, and on grain boundary bubble nucleation, is also under investigation5. One long-established property of insoluble helium and other inert gases in metals is their tendency to precipitate as bubbles. However, the inference that inert gas clusters always assume a three-dimensional form may not be justified. We present here experimental evidence showing that, at least in molybdenum, helium can initially aggregate in an unexpected planar form. Furthermore, we have found a bubble nucleation route involving the formation of several small bubbles from a single helium platelet.