A comparative study on the in situ helium irradiation behavior of tungsten: Coarse grain vs. nanocrystalline grain

Z Chen and LL Niu and ZL Wang and LF Tian and L Kecskes and KG Zhu and QM Wei, ACTA MATERIALIA, 147, 100-112 (2018).

DOI: 10.1016/j.actamat.2018.01.015

Increasing the density of sinks such as grain boundaries and interfaces for irradiation-induced defects and implanted ions has been demonstrated to be an effective way to improve the irradiation resistance of materials. To understand the effects of grain boundaries on the degradation mechanism of nano structured materials, nanocrystalline tungsten was fabricated by high pressure torsion (HPT-W). Morphological changes of HPT-W and coarse grain tungsten (CG-W) during helium ion irradiation were evaluated in situ in a helium ion microscopy. It has been shown that the degradation mechanisms of CG-W and HPT-W are remarkably different. Blister occurs on the surface of CG-W when the irradiation dose increases up to 5.0 x 10(21) m(-2), and orientation dependence of blistering has been observed. However, no blister is formed on the surface of HPT-W even when the irradiation dose increases up to 1.0 x 10(23) m(-2). Instead, crack formation along grain boundaries is the major degradation mechanism during helium irradiation of HPT-W, supporting a different irradiation degradation mechanism. This explains the unprecedented irradiation tolerance of HPT-W in terms of blistering. Molecular dynamics results also show that grain boundaries and helium clusters play an important role during the propagation of a crack. The zigzag crack planes are attributed to the coalescence and growth of helium blister/bubble-induced crack. The results document that grain boundaries play decisive roles in the irradiation resistance of nano structured materials, and provide a new perspective to the design of plasma facing materials with excellent irradiation resistance. It is thus suggested that excellent irradiation resistance can be achieved by a meticulous design of grain boundaries based on "interface engineering". (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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