Helium retention, bubble superlattice formation and surface blistering in helium-irradiated tungsten

The microstructural evolution and helium retention in polycrystalline tungsten (W) implanted with 100 keV helium ions at different temperatures to a wide range of different fluences were studied. Rutherford backscattering spectrometry (RBS) shows that helium atoms were well retained in tungsten, and no loss of helium occurred before surface blistering/flaking occurred. Within certain windows of fluence and temperature, ordered bubble superlattices form. A minimum fluence and sufficiently high irradiation temperature are necessary to establish a bubble superlattice, but continued introduction eventually destroys the order via bubble linkage and crack nucleation occurring at the depth of peak helium deposition. Scanning electron microscopy showed that typical sizes of flakes were roughly the same, independent of helium fluence. With increasing fluence, the number of flakes increased, and often linked together, forming a massive peeled off region. Using RBS-measured helium retention and TEM-measured bubble density, we obtained an interfacial energy of 2.53 +/- 0.72 J/m(2). There is no observation of bubble denuded zones at grain boundaries, suggesting that bubbles represent a stronger defect sink than grain boundaries or cracked grain boundaries. (C) 2020 Elsevier B.V. All rights reserved.