Assessment of mechanical properties of SPS-produced tungsten including effect of neutron irradiation

Highlights

• Fracture toughness of SPS tungsten before and after irradiation is assessed.

• neutron irradiation at end-of-life PFC component increases DBTT by 300 °C.

• SPS tungsten exhibit similar performance as powder metallurgy tungsten.

Abstract

Production and supply of tungsten for the first wall fusion application is becoming an important aspect given the progress of ITER construction. Exploration of advanced routes alternative to the conventional powder metallurgy is currently undertaken. In this work we have assessed a potential of the spark plasma sintering (SPS) production route to deliver well controlled microstructure, chemistry and mechanical properties of bulk tungsten as a first step. SPS-produced tungsten was sintered at 2000 °C and was characterized in terms of mechanical properties, namely: tensile, three point bending and fracture toughness data in the temperaure range of 250–600 °C. Then, neutron irradiation was performed at 600 °C and the change of the fracture toughness was measured after irradiation together with the characterization of the fracture surface. The results are compared with those obtained for the commerically produced swaged tungsten irradiated and tested in equivalent conditions. The obtained results show that SPS technology offers the production of bulk tungsten with a good potential for further optimization (by e.g. swaging/rolling). Neutron irradiation causes the reduction of the fracture toughness comparable to the one induced in the commercially produced tungsten.

Introduction

Development of materials and composites for components to operate in nuclear fusion environment remains one of the hottest topics on the agenda of fusion electricity roadmap. In particular, harsh operation environment involving large thermal gradients, high number of thermal cycles and flux of fast neutrons inevitably leads to degradation of thermal, physical and mechanical properties of materials facing the plasma [1]. Following a thorough assessment, tungsten has been selected as first wall material for the divertor of ITER [2] and DEMO [3]. In the case of ITER lifespan, the total power load of divertor PFC is targeted as ~0.3 MW a m-2, which is equivalent to the irradiation dose of 1.35 dpa (displacement per atom) [4]. The PFC of ITER is planned for replacement three times during the life span, thus the equivalent end-of-operation irradiation dose is ~0.3–0.5 dpa during the D-T phase [5].

Compared to ITER, the operational conditions of DEMO are more demanding. The first wall armor and diverter will experience longer pulses (>2 h), higher heat flux load (MW/m²) and correspondingly a higher total neutron fluence (up to 5 dpa in the strike point) [3]. While various routes may be applied to improve/optimize the material (e.g. alloying, secondary phase inclusion, fiber/laminar strengthening, additive manufacturing, etc.), in general the design of component must account for a number of constrains such as: efficient power exhaust (commercially viable), operational temperature window (compatible with coolant option), acceptable nuclear waste (economy, license), compatibility with plasma (low erosion, no plasma pollution), compatibility with fuel cycle (retention, breathing) and finally operation lifetime [6]. The latter is inherently linked to both safety and commercial viability aspects of the fusion nuclear power reactors. One of particular aspects that needs to be addressed for tungsten (W) as plasma-facing material is its mechanical properties, which are affected by neutron irradiation [5].

Overall, it is agreed that efficient application of W in fusion reactors will be determined by a best compromise between reduced DBTT and enhanced fracture toughness as well as high recrystallization temperature [7]. This is why advanced W-based grades as well as new manufacturing routes are under development to improve low- and high-temperature performance. For example, particle-reinforced tungsten and reduced grain size material can suppress the grain growth, improve the strength of grain boundary as well as fracture toughness of the material, and reduce DBTT (see reviews [6,8]). Fiber-reinforced tungsten allows one to overcome the intrinsic brittleness of tungsten and its susceptibility to embrittlement induced under operation, as W fibers arrest and deflect the propagating cracks (see e.g. [9,10]). By dedicated alloying with zirconium‑carbon (ZrC) nano-sized particles as well as by applying the powder metallurgical process, it was possible to reduce the free oxygen occupying grain boundaries and successfully fabricate bulk plate of W-0.5wt.%ZrC alloy [11].

Production of tungsten grades relevant for ITER applications consists of several steps, including preparation of green compact at room temperature, conventional sintering at temperatures above 2200 °C, subsequent thermo-mechanical treatment (rolling or forging) and final thermal treatment for stress relief. Such a multi-step process is time demanding and increases the price of the final product. Therefore, tungsten-based material development and investigation of alternative production routes is undergoing with the aim to simplify the production route and improve the mechanical response of the material. Several promising manufacturing routes are currently investigated, each offering specific advantages. Among these methods are powder/metal injection molding delivering net-shaped compacts with the help of polymer binders [12], thermal spray methods producing coatings over large areas (see e.g. [13,14]). Most recently, cold spray was used to produce dense tungsten coatings with minor Ti binder, offering the advantage of spraying in ambient air without apparent tungsten oxidation and excellent adhesion to the substrate material (J.Cizek and M. Vilemova, to be published soon). The powder consolidation method used in this study belongs to the family of sintering methods. The advantages of the SPS process are fast heating rates and very short hold times at the sintering temperatures (in the range of several minutes). Therefore, fine grain structure of the original powder can be preserved. During the sintering, uniaxial pressure can be applied and various sintering atmospheres can be used to influence the resulting microstructure and purity of the compacts.

Fracture toughness (FT) is one of the most important and relevant material characteristic, describing material response and behaviour in the presence of crack. According to the material resistance to crack propagation, brittle and ductile materials are distinguished. For many materials, including tungsten, particular temperature can be determined, above which the material responds in ductile way. This temperature is often denoted as ductile to brittle transition temperature (DBTT). For tungsten, the DBTT and generally the behaviour strongly depends on the fabrication route, related microstructure, impurities and thermal history (e.g. recrystallization temperature is of greatest importance).

In the case of commercially pure tungsten, it has been shown that at room temperature the FT value is about 5–10 MPa√m and it rises up to 20–25 MPa√m [[15], [16], [17]] as temperature exceeds 300–350 °C which can be seen as transition from ductile to brittle fracture mode in polycrystalline tungsten, following the results of tensile tests and subsequent fracture surface analysis [18].

Without any post-treatment, the DBTT of commercially pure tungsten usually falls in the region of 300–600 °C or higher [[19], [20], [21], [22]]. Given thermo-mechanical treatment (e.g. hot/cold rolling, swaging, forging etc.), the DBTT can be decreased down to as low as room temperature (RT) for tungsten in the form of foil/thin sheet or down to 80–350 °C for bulk products [[23], [24], [25], [26], [27], [28], [29]]. Following the above cited works, fracture toughness of bulk polycrystalline tungsten without post production treatment is about 5–10 MPa√m in the brittle region and it increases to ~20 MPa√m in the transition region. After rolling, the values were similar, only shifted to the lower temperatures [28]. In case of ultra-fine grain (UFG) tungsten foil [30], in brittle region fracture toughness K_Q reached values of about 10 MPa√m with increase to around 50 MPa√m in the ductile region. Of particular example of the reduction of DBTT is the work of Bonnekoh, Hoffmann and Reiser [24], in which the DBTT was measured to be as low as −65 ± 15 °C, which was achieved through multiple rolling steps introducing significant deformation and reducing the thickness as well as grain size.

Needless to say that mechanical properties of tungsten (and other metals), including fracture toughness and DBTT, can be affected significantly by neutron irradiation. The resulting microstructure consisting of point defects, dislocation loops, voids and precipitates is responsible for the degradation of the mechanical properties. Recently, hardness and tensile tests of single-grain tungsten samples after irradiation to 0.4 dpa at 690–800 °C were performed in [31]. Whereas unirradiated samples exhibited ductility starting at 300 °C, this shifted to above 500 °C after the irradiation [32]. Steichen et al. measured DBTT shift of about 165 °C (from 65 °C to 230 °C) of cold-pressed and sintered tungsten specimens irradiated to fluence of 0.9 × 10²² n/cm² [33]. Three point bending tests of tungsten (irradiated on the spallation neutron source) were performed by Habainy et al. [34]. The samples were irradiated by high-energy protons and spallation neutrons at temperatures up to 500 °C and damage up to 3.5 dpa. Whereas DBTT of unirradiated specimens was around 350 °C, all irradiated specimens exhibited brittle fracture at test temperatures up to 500 °C. Finally, Gorynin et al. have studied mechanical properties of tungsten under neutron irradiation up to 2.2 × 10²² n/cm² by performing irradiation at 350, 500 and 800 °C on BOR60 reactor and measuring tensile properties. Fully brittle deformation was observed after irradiation at 500 °C and below. As of today, the information on the change of the fracture toughness after neutron irradiation is not available in open literature up to the best knowledge of authors.

In this work, we perform a preliminary assessment of the mechanical properties of tungsten produced by SPS technology. Given a number of advantages of SPS including industrial scalability, cost effectiveness, flexibility for shape/alloying, we assess the mechanical properties of pure W produced by SPS in the temperature range up to 600 °C including neutron irradiation. The choice of 600 °C is driven by constrains originating from (i) the irradiation-induced void swelling above 650 °C [35,36] and compatibility with structural steels in location of joints [1]. The purpose of this work is to explore nominal performance of SPS-produced tungsten (i.e. without thermo-mechanical optimization) and compare the results with those available for commercially pure tungsten. The results include tensile, bending and fracture toughness tests in the as-received condition and fracture toughness tests for the material irradiated at 600 °C up to 0.24 dpa, which is close to the half end-of-life fluence of tungsten PFC on ITER device (i.e. replacement of the PFC is scheduled at 0.4 dpa). The results are compared with data recently obtained for commercially pure W (produced by AT&M) irradiated and tested in the equivalent conditions [37].