Helium Implantation of Ultrafine Grained Tungsten within a TEM
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Helium Implantation of Ultrafine Grained Tungsten within a TEM K. Hattar1, O. El-Atwani2,3, M. Efe2, T.J. Novakowski2, A. Suslova2, and J.P. Allain4, Sandia National Laboratories, Albuquerque, NM 87185, U.S.A. 2 Purdue University, West Lafayette, IN 47906, U.S.A. 3 Birck Nanotechnology Center, West Lafayette, IN 47906, U.S.A. 4 University of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A. 1
ABSTRACT Many theoretical predictions have suggested that the confined length scales and increased interface density of various nanostructured materials may result in desired thermal, mechanical, and radiation properties. An important aspect of this for next generation nuclear reactors is understanding the change in swelling resulting from helium evolution in tungsten alloys, as a function of grain size and grain boundary type. This study investigated this using a new ion irradiation transmission electron microscope (TEM) facility that has been developed at Sandia National Laboratories and is capable of ion implanting helium at energies up to 20 keV. It was demonstrated in this feasibility study that helium could be implanted into an ultrafine grained tungsten TEM sample produced by severe plastic deformation. The size and density of the helium bubbles formed during the experiment appear nearly constant; while the larger voids formed appear to be dependent on the local microstructure. Future work is underway to both optimize the facility, as well as better understand the evolution of ultrafine grained tungsten resulting from both helium implantation and displacement damage. INTRODUCTION A number of high-temperature refractory materials candidates have been investigated for use in plasma facing components in fusion reactors such as ITER. Currently, tungsten is considered to be one of the best candidates due to its high thermal conductivity, low tritium retention, low sputtering yield, and low erosion rate [1]. However, studies involving helium irradiation on tungsten surfaces indicate that sufficient fluences and temperatures give rise to certain microstructural and morphological changes, which can have negative effects on the mechanical integrity. These are manifested in the form of voids, bubbles, and fuzz through a dynamic microstructural evolution [1]. The observed changes during helium irradiation are largely considered to be due to helium implantation and radiation-induced defect formation. It is also known that the grain boundaries serve, as good sinks for both helium and interstitial atoms. The interstitials can more freely move through these grain boundaries, and even recombine with nearby vacancies, helping to suppress surface evolution [2]. Accordingly, it is speculated that increasing grain boundary density, tailoring the grain boundary types to maximize sink strength, or combinations thereof can enhance the ability of tungsten to resist radiation damage under plasma-burning conditions [3]. As a result, ultrafine and nanocrystalline grained tungsten are being investigated to create new materials to handle high-
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