Hardening of self ion implanted tungsten and tungsten 5-wt% rhenium
Graphical abstract
Highlights
► W+ ion implantation was used to simulate neutron damage in W and W–5 wt%Re. ► A Hardness increase in pure tungsten was seen to saturate by 0.4 dpa. ► TEM of pure W shows little change in damage levels between 0.4 and 33 dpa. ► W–5 wt%Re alloy shows a hardness saturation between 0.07 dpa and 1.2 dpa. ► Above 13 dpa rhenium clusters are seen by APT and hardness rapidly increases.
Introduction
Tungsten is one of the most attractive materials for high temperature components in any future nuclear fusion tokomak [1]. It is of particular interest for use in plasma facing components which will be subjected to the highest temperatures [2], such as the divertor [3], operating at temperatures as high as ≈2000 K [2] and as high as 3500 K under extreme events such as edge localised modes (ELMs) [4]. The use of tungsten is motivated by several attractive physical properties: high melting point (3695 K), low sputtering rates and good thermal conductivity. However these come at the expense of poor formability and low fracture toughness. Yet current divertor designs also require the use of tungsten in structural components [5]. Fracture properties and the brittle to ductile transition temperature have also been characterised for a range of tungsten materials produced by different routes [6], [7], [8], [9], [10] and a few experimental studies have focused on increasing the formability of tungsten [2].
For predicting materials properties in service, there are additional complexities from the high neutron doses. In a tokamak, it is anticipated that the divertor will see damage levels of up 20–30 displacements per atom (dpa)/year [4] along with chemical changes due to tungsten transmutation into predominantly rhenium and osmium [11] and the production of hydrogen and helium at the levels of 40–50 appm/dpa and 10–15 appm/dpa respectively.
Gilbert and Sublet [11] have calculated that pure tungsten would transmute into an alloy of composition W–3 at%Re–1.4 at%Os after 5 years in an operational reactor. The tungsten–rhenium phase diagram is complex, showing several intermetallic phases [12]. Of particular interest are the σ-phase, present above ∼30%Re and ∼5% Os on equilibrium phase diagrams, and the χ-phase, centred around ∼72%Re [12], [13]. While these phases are thermodynamically expected for relatively high Re content, they have been observed in neutron-irradiated under-saturated W–5%Re and W–25%Re alloys using transmission electron microscopy (TEM) [14] and field ion microscopy [15], [16]. More recently, Marquis et al. [17] have shown the formation of Re-Rich precipitates in W–25 wt%Re after only 2.8 dpa at 500 °C, using tungsten ion implantation. The σ and χ phases are known to be brittle [18] and deleterious to the mechanical properties of tungsten alloys [19]. Thus it is of critical importance that the mechanical properties of irradiated tungsten–rhenium alloys be understood. Additionally it is well known that additions of rhenium while in solid solution improves the ductility of the material [12], [20]; however due to the high cost and scarcity of rhenium [20] the use of W–Re alloys is not seen as a viable route for the construction of a divertor structure.
Previous studies have focused on the effect of neutron irradiation on mechanical properties of W–Re alloys. Of particular interest are the changes in hardness due to irradiation as a function of rhenium concentration, for alloys containing between 0 and 26 wt%Re [14], [21], [22], [23], [24], [25], [26]. In these studies the maximum damage level attained was only 1.6 dpa, which is significantly smaller than the potential levels of damage expected in a commercial nuclear fusion reactor, of up to 30 dpa per year [3], [4]. Hardness increases from 57 Hv in W–10%Re at 800 °C [24] and 0.62 dpa to 920 Hv in W–25%Re at 500 °C [26] were reported, with no clear trends observed. This is unsurprising due to a number of variables that are not always reported, even though they can influence the observations. These variables include dose rate, material processing and impurity levels. Fig. 1 shows increase in hardness as a function of dpa for W–5 wt%Re (a) and pure tungsten (b). There is a broad trend of increasing change in hardness as a function of dpa, but there is a wide degree of scatter in the data at low damage levels and a lack of data above 1 dpa. Several studies on the microstructural development of neutron-irradiated tungsten and tungsten–rhenium alloys have also been performed in conjunction with basic hardness measurements, but they are limited to low damage levels well below the level expected in a fully functional fusion reactor.
Neutron irradiation campaigns can take several years to plan and execute and cost large sums of money, and even then only moderate damage levels may be attained. For these reasons, there has been much recent work on using self ion irradiation in various materials and particularly Fe-based alloys to mimic the damage caused by fast neutrons [27], [28], [29]. This has the advantage of producing samples which are not radioactive and high levels of damage can be built up in relatively short time periods. However this comes at the cost of only being able to produce small depths of damage (typically several hundred nm to 10 μm depending on the energies used). Typically TEM has been used to study the dislocation structures produced by irradiation [29], [30], while atom probe tomography (APT) has been used to study the changes in local chemistry [17], [31], [32]. As the damaged layers are shallow, traditional bulk mechanical tests cannot be used to study the mechanical properties; however nanoindentation has recently been used to test the hardness of these layers, as compared to bulk materials [27], [28], [33], [34], [35].
It is clear that a better understanding of the influence of the neutron damage on the properties of W–Re alloys is necessary, particularly at higher damage levels. In particular there is a critical need to understand how radiation damage changes the basic mechanical properties of tungsten and tungsten rhenium alloys and how structural and chemical changes correlate with these properties. The present work investigates the self-ion irradiation response of W and W–5 wt%Re from the low dose into the high dose regime. The effect of self ion implantation on the hardness of the materials is related to the changes in microstructure and is investigated using transmission electron microscopy and atom probe tomography.
Section snippets
Materials
Two tungsten alloys were studied: pure tungsten and W–5 wt%Re. The pure tungsten was chosen as a reference material, and as the most likely starting material for any plasma-facing divertor components in a future DEMO reactor [3] and the W–5 wt%Re was chosen to simulate the effect of transmutation on mechanical behaviour.
The pure tungsten (W–0.006 at%O–0.005 wt%C) was produced by Plansee Inc. (Reutte, Austria), using a powder metallurgy route, followed by hot forging to produce a fully consolidated
Experimental
Ion implantation was carried out at the National Ion Beam Centre, University of Surrey, UK, using a 2 MV tandem accelerator (HVEE, Netherlands), at an energy of 2 MeV. The damage profile, as predicted using SRIM [36] (stopping range of ions in matter) is shown in Fig. 3, the displacement energy used is 68 eV [37], [38]. The damaged layer is ≈300 nm thick with peak damage at ≈100 nm. Implantations were carried out at sample temperatures of ≈300 °C, measured from a thermocouple mounted alongside the
Results
Typical load displacement curves for implanted and unimplanted pure tungsten are shown in Fig. 5a. Fig. 5b shows the hardness as a function of depth for the same indentations; a clear hardening for the implanted sample is observed. The plastic zone around a nanoindent is complicated and its exact shape and size are difficult to measure experimentally, meaning that it is difficult to determine when the measured hardness and modulus begin to be influenced by the underlying unimplanted material
Discussion
The increase in hardness in pure tungsten self-ion-implanted at 300 °C saturates below 0.4 dpa with a relative increase in hardness of 0.7–0.9 GPa (an increase of ∼12% over the unimplanted material’s hardness). This is in good correlation with the data produced by TEM studies of the dislocation damage structures, which show the same damage level at 0.4 dpa and 1.2 dpa. In W–5 wt%Re, damage saturation is not seen in the hardness data. While the hardness is constant between 0.07 dpa and 1.2 dpa (with a
Conclusions
Ion implanted pure tungsten shows a small increase in hardness at 0.07 dpa (of 0.25 GPa from a base value of 7.62 GPa) followed by a saturation in hardness increase (with an increase of 0.8 GPa) in the dose range 0.4–33 dpa. This correlates well with TEM studies which show no increase in dislocation loop density between 0.4 and 1.2 dpa and only a small increase up to 33 dpa. Tungsten 5 wt% rhenium shows a similar increase in hardness of 0.85 Gpa between damage levels of 0.07 dpa and 1.2 dpa, from a base
Acknowledgments
DEJA thanks Culham Centre for Fusion Energy for funding via a Research Fellowship at St. Edmund Hall, Oxford. EAM acknowledges financial support from the Royal Society (Dorothy Hodgkin fellowship). XY acknowledges support from a China Scholarship Council for research studentship. All authors acknowledge support from EPSRC Grants EP/H018921/1, EP/G004676/1, and EP/F004451/1, and the support of staff at the National Ion Beam Centre, University of Surrey, UK.
References (45)
- et al.
Fusion Engineering and Design
(2000) Journal of Nuclear Materials
(2011)- et al.
Journal of Nuclear Materials
(2007) - et al.
Journal of Nuclear Materials
(2009) - et al.
International Journal of Refractory Metals & Hard Materials
(2010) - et al.
Journal of the Less-Common Metals
(1961) - et al.
Acta Metallurgica
(1984) - et al.
Acta Metallurgica
(1985) - et al.
Materials Today
(2009) - et al.
Journal of Nuclear Materials
(2008)
Journal of Nuclear Materials
Nuclear Instruments & Methods in Physics Research Section B – Beam Interactions with Materials and Atoms
Journal of Nuclear Materials
Journal of Nuclear Materials
Journal of Nuclear Materials
Journal of Nuclear Materials
Journal of Nuclear Materials
Journal of Nuclear Materials
Nuclear Instruments & Methods in Physics Research Section B – Beam Interactions with Materials and Atoms
Nuclear Instruments & Methods in Physics Research Section B – Beam Interactions with Materials and Atoms
MRS Bulletin
Fusion Science and Technology
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