Abstract
• Helium implantation resulted in significant hardness increase of beryllium. • Dislocation loops and helium bubbles are responsible for hardening. • Low purity grade has higher irradiation induced hardening after 200°C implantation. • Crystallography leads to significant scatter in indentation hardness. • Softer grains exhibited higher irradiation induced hardening. The effect of ion irradiation on evolution of microstructure and hardening of beryllium with different impurity levels was investigated using TEM and nanoindentation. High purity S-65 grade and less-pure S-200-F grade were implanted by helium ions at temperatures of 50°C and 200°C. 11 different energies were used, so as to create a quasi-homogeneous 3 µm irradiated layer with average radiation damage of 0.1 dpa and average He content of 2000 appm. Nanoindentation experiments demonstrated that before irradiation, the S-200-F and S-65 grades have an average hardness of 3.7±0.8 GPa and 3.4±0.8 GPa correspondently. After implantation the hardness of both grades increased by about 60% for the 200°C irradiation and 100% for the 50°C irradiation. The crystallographic analysis of indented grains demonstrated that in the as-received materials the hardness is about 2.5 times higher when the indentation direction is close to the [0001] c-axis of beryllium compared to indentation perpendicular to [0001]. Hardness anisotropy significantly decreased after irradiation: the “soft orientation” was most sensitive to irradiation-induced hardening, with hardness increasing by about 140% after irradiation at 50°C and 100% after irradiation at 200°C, compared to about 15 - 20% for the “hard” orientation at both irradiation temperatures. The higher purity grade had smaller increase of the “soft orientation” hardness: 2.5±0.3 GPa for the S-65 and 2.9±0.2 GPa for the S-200-F. At both temperatures in both grades, under TEM investigation the radiation damage appears as “black dots” which are likely to be small dislocation loops with the number density of ~ 10 22 m −3 . No bubbles were observed by TEM inside grains and at grain boundaries. Analysis of the possible hardening contribution demonstrated that the observed “black dots” could be responsible for up to half of the measured hardening, while the rest of the hardening should originate from helium bubbles with the size below the TEM resolution (at or below 1.5 nm).
Highlights
Due to a unique combination of mechanical and physical properties, beryllium is extensively used in a wide variety of nuclear facilities
In bulk materials it is generally observed that a high yield stress to elastic modulus (σ y/E) ratio and a high strain-hardening rate both promote a spread of deformation deeper into the specimen and the surface around the indenter sinks-in; while the converse (low (σ y/E) values and low strain hardening) favour localisation of deformation around the indents and the creation of pile-ups [50,59]
The observed crystallographic dependence of pile-up/sink-ins in non-irradiated beryllium is in good agreement with crystal plasticity finite element modelling (CPFEM) results [35], which demonstrated that the stress field and geometrically necessary dislocations (GND) penetrate deeper into the material in the “hard” orientation relative to the “soft” orientation, consistent with a higher (σ y/E) ratio, the plastic strain under the “hard” indent is higher consistent with high GND hardening
Summary
Due to a unique combination of mechanical and physical properties, beryllium is extensively used in a wide variety of nuclear facilities. Nanoindentation is widely used for evaluation of local mechanical behaviour of materials at micro-scale and is useful for ion irradiated materials where only a shallow damaged layer is available It has been successfully used for screening of irradiation induced hardening in different nuclear materials, including: ferritic-martensitic [27,28,29], austenitic [30] and RPV [31] steels, ODS alloys [32] and tungsten [33,34]. The load - indentation depth curves from different beryllium states (material grade, irradiation condition) are presented and the popin events (horizontal plateaux observed on the load-displacement at some at some critical loads) are characterised to highlight differences between different states of the materials This is followed by a general discussion of the origins of the observed irradiation induced hardening and the applicability and significance of the obtained data for engineering design considerations for nuclear facilities with beryllium elements under irradiation
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