Plastic deformation in advanced tungsten-based alloys for fusion applications studied by mechanical testing and TEM

https://doi.org/10.1016/j.ijrmhm.2020.105409Get rights and content

Highlights

  • Plastic deformation mechanisms differ in three studied groups of W alloys depending on alloying particles and production methods.

  • Pure commercial W grades exhibit the highest fracture stress due to the very large reduction area.

  • W-0.5 wt% ZrC alloy exhibits the highest tensile yield stress.

Abstract

In this work, we have assessed mechanical properties of several tungsten grades considered as perspective materials for applications in plasma facing components in the nuclear fusion environment, where the neutron irradiation damage is expected to cause embrittlement. In particular, the work focuses on two aspects: bending tests to deduce the onset of ductile deformation and microstructural analysis of the reference and plastically deformed materials. The microstructure in the reference state and the one induced by plastic deformation at 600 °C is studied by means of transmission electron microscopy (TEM). Six different types of tungsten-based materials were assessed: two commercial grades produced according to ITER specifications in Europe and China and four lab-scale grades utilizing different reinforcement options. The comparative assessment of tensile and bending strength was performed at 600 °C accompanied with a detailed TEM analysis. The deformation-induced microstructure was characterized and compared for all studied grades in terms of the dislocation density, heterogeneity, observation of pile-ups and tangles specifically near grain boundaries and/or strengthening particles. The obtained data will serve as reference information required to assess the impact of neutron irradiation.

Introduction

Development of nuclear fusion electricity production implies a number of technological and scientific challenges, where appropriate selection of materials is one of the major concerns. High temperature operation of materials and components is a must to ensure durable and stable plasma discharge required for the thermal energy generation and extraction to invoke electric energy gain [1]. Thus, refractory metals are naturally considered as main candidates for plasma-facing materials given high thermal conductivity, strength and melting point. The most perspective is tungsten (W) and therefore it is selected as candidate for divertor and first wall material for DEMO [[2], [3], [4]]. One of the critical problems in high temperature applications of refractory metals is mechanical performance in the low-temperature window, where refractory metals exhibit limited ductility due to relatively high ductile to brittle transition temperature (DBTT) 300–400 °C, see e.g. ref. [5,6]. Operation of W in ductile mode implies its application above the DBTT i.e. above 300–400 °C. However, upon the operation in fusion environment the DBTT will raise up due to the harsh particle flux, thermal cyclic fatigue and penetration of plasma species [7]. In addition to thermal loads, the irradiation by fast neutrons (14 MeV) will lead to generation of lattice damage across the whole plasma-facing components (PFC) and therefore it may cause brittle failure of tungsten and other materials which are not in direct contact with plasma. This is why the assessment of the neutron irradiation effects in tungsten is especially important at temperatures of 300–600 °C, where the irradiation damage should presumably make the material brittle. Physically, the neutron irradiation embrittlement is explained by the formation nano-scale irradiation defects (voids and loops) which obstruct plastic deformation and therefore don't allow for the ductile blunting of cracks, formed as a result of thermal shocks. Hence, understanding of elementary mechanisms driving plastic deformation, accumulation of stress concentration and eventual failure is important to assess the effect of neutron irradiation and, if understood, possibly propose some mitigation measures to suppress the irradiation embrittlement by specific microstructural or chemical engineering of advanced tungsten grades [8].

Currently, EUROfusion consortium carries a neutron irradiation campaign to investigate the thermo-mechanical properties of industrially available and advanced tungsten grades to assist down selection of different options on the way to develop plasma-facing components sufficiently tolerant to neutron irradiation [9]. The mechanical properties of the commercial tungsten in non-irradiated state, produced by hot rolling/forging (e.g. produced by Plansee and ALMT companies), have been studied earlier in refs. [[10], [11], [12], [13], [14], [15]] including investigation of the effect of texture, annealing and cold-rolling processing. Overall, it is agreed that successful 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. For that purpose, advanced W-based grades are under development to improve low- and high-temperature performance. For example, particle-reinforced tungsten and reduced grain size material can suppress grain growth, improve strength of grain boundary as well as fracture toughness of the material, and reduce DBTT (see reviews [16,17]). 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. [18,19]). By dedicated alloying with zirconium-carbide (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 [20]. Preliminary mechanical and high heat flux (HHF) assessment demonstrated that this material exhibits as low DBTT as 100 °C and can sufficiently withstand HHF loads up to the power density of 0.66 GW/m2 [20], simulated by electron beam. Thus, the next step is to investigate the impact of the neutron damage. As of now, there is a number of perspective lab-scale tungsten grades with improved mechanical properties whose performance under neutron irradiation is yet to be explored.

This work is dedicated to systematic characterization of bending strength and plasticity of several tungsten grades which are included in the screening irradiation programme. Here, we investigate properties of several materials in a non-irradiated i.e. reference state. The samples are fabricated and tested according to the single specification rule and inspected using the same lab equipment to provide one-to-one comparison. The equivalent sample geometries are used to investigate the effect of neutron irradiation.

This study focuses on mechanical properties of W in the low-temperature application range for PFC (i.e. around 600 °C), where the irradiation induced embrittlement is considered to play a crucial role. The bending and tensile tests are performed at 600 °C. The flexural strain is used as an indicator of the capacity of the material to experience ductile deformation. As proposed by Lassila et al. [21], the ductile-brittle transition temperature (DBTT) is in the range where the flexural bending strain reaches 5%. Using plastically deformed samples, a parametric study of the microstructure is performed by means of transmission electron microscopy (TEM). In particular, the reference microstructure and the one developed in the materials tested at 600 °C (deformation to rupture) is investigated in full details including characterization of dislocations, strengthening particles and morphology of sub-grains. The obtained results for mechanical and microstructural properties will serve as reference dataset required to assess the impact of the neutron irradiation effect.

Section snippets

Materials and experimental methods

As mentioned above, six types of tungsten material grades were studied in this work. Those were selected following the advance of the European fusion material research programme presented in ref. [22]. Two commercial grades produced in Europe and China according to ITER specification were included as industrial references. These two materials will be referred to as Plansee ITER specification W (IGP) and AT&M ITER specification W (CFETR), respectively. Other four research grades were developed,

Reference grain microstructure and mechanical properties

The microstructure of grains, as was studied in our earlier work [26], is presented as Inverse Pole Figures mapped by Bruker Quantax software in Fig. 1. The nominal chemical composition, which is provided by manufacturer, and the effective grain size, as determined by the OIM software, are provided in Table 1. As one can see, the IGP and CFETR clearly exhibit the texture and elongated grains aligned with the rolling direction (LD). IGP consists of carrot-like grains, while CFETR has

Summary and concluding remarks

To summarize, we have studied parametrically the mechanical response of six tungsten products under tensile and bending deformation at 600 °C. These tungsten grades were produced specifically for the nuclear fusion applications with a purpose of improving the operational temperature window by either reducing DBTT and/or increasing the recrystallization resistance. The selected materials are included in the currently running neutron irradiation programme, and therefore the obtained here data

Declaration of Competing Interest

None.

Acknowledgements

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

References (29)

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