Faceted He-Filled “Pancakes” Confined within Nanoscale Metal Layers

Helium (He)-filled cavities in single-phase metals typically take on nearly equiaxed shapes, such as spherical bubbles or voids. However, in multiphase metal composites, the morphology of the constituent phases and the structure of the interfaces between them alter the shapes of such He-filled cavities. In this work, a He-implanted three-layer composite consisting of a 5-nm-thick Cu layer sandwiched between two V layers was reexamined. In addition to He-filled nanochannels, which have been reported in this material before, we discovered faceted He-filled “pancakes,” viz. high-aspect-ratio cavities spanning the thickness of the Cu layer and extending several tens of nanometers parallel to the Cu/V interfaces. We propose that such pancakes form nanochannels via a multistage coarsening process.