Cu, CuCrZr alloy and W are three typical materials used in the plasma-facing components of fusion reactors, whose mechanical properties vary significantly. The small punch test (SPT) is used to evaluate the mechanical properties for materials in the nuclear industry due to its advantage of requiring minimal sample volumes. In this paper, the correlation between SPT and uniaxial tensile testing (UTT) is investigated for Cu, CuCrZr alloy and W. From the views of mechanical properties and fracture morphologies, materials demonstrate consistent behaviours between SPT and UTT, which proves that the SPT is an effective method to characterize the mechanical properties and fracture mechanism of Cu, CuCrZr alloy and W. Moreover, the finite element simulation results of SPT for five other copper alloys with different mechanical properties are added to establish the correlation equations of mechanical properties between SPT and UTT. One unified correlation equation between SPT and UTT can be established for the ductile materials, even if with largely different mechanical properties. However, the introduction of a brittle material W invalidates the correlation equation. This work provides a prospect of SPT to study the mechanical properties of materials used in plasma-facing components.

Small punch creep (SPC) testing represents an effective way to rapidly assess the creep performance of novel materials and potentially monitor degradation of in-service components. Recent progress in standardisation has also led to improvements in data analysis. However, estimation of equivalent uniaxial stresses is still somewhat challenging and has hindered wider usage of the small punch technique. In this study, the creep properties of two candidate materials for structural applications in future fusion reactors were assessed via SPC. These included the baseline structural material, Eurofer97, and a more recently developed 14Cr Oxide Dispersion Strengthened (ODS) steel (14YWT). Having been assessed at 550 °C, the 14YWT demonstrated superior creep life and significantly lower rates of deformation, but also exhibited reduced ductility. The Modified Chakrabarty (MCH) approach was employed to estimate equivalent uniaxial creep stresses. This methodology appeared to work well with the Eurofer97 but struggled when applied to 14YWT, making accurate estimation of the 14YWT performance difficult. Since the MCH approach was developed for ductile materials, its predictive capabilities may have been limited by the low ductility of 14YWT.

A dedicated procedure was developed to categorize the technology readiness of materials for specific DEMO in-vessel fusion reactor applications. This methodology was employed to assess the technological maturity of materials under development within the EUROfusion materials work package (WPMAT). This covers materials intended for structural, high heat flux, optical and dielectric applications in the European DEMO fusion reactor (breeder materials and barrier coatings are not covered here). The baseline materials have been assigned DEMO Material Technology Readiness Levels (MTRLs) of 4 (EUROFER97), 3 (conventional tungsten) and 4 (Copper-Chromium-Zirconium). In addition, a further 28 candidate materials (and groups of materials) were also assessed. These were generally assigned DEMO MTRLs in the range of 2-3. This process has highlighted the wide range of materials under development within WPMAT. However, it has also brought into focus the many challenges facing DEMO materials development. While the lack of technologically ready materials is clearly a source of risk to DEMO, the introduction of a biennial review of technology readiness within WPMAT is intended to facilitate more effective planning and targeted materials development, in line with the strategic plans of EUROfusion. This paper highlights the methodologies for fusion specific material technology readiness levels, their application for EU-DEMO and the effectiveness of these in strategic materials development.

The use of small specimen test techniques (SSTT) to determine the mechanical properties of irradiated materials has been studied over the past decades both in fission and fusion programs, but also to characterise and optimise new materials by nuclear and non-nuclear communities. Currently a number of activities are running that focus on the standardisation of SSTT to determine fracture toughness properties for fusion reactor materials (IAEA [1], EUROfusion [2], F4E [3]), and to support the long-term operation of light-water reactors (CRIEPI [4]). The determination of the T0 reference temperature (ASTM E1921 [5]) has been successfully achieved by testing small compact tension (C(T)) specimens (W = 8mm, B = 4mm) of non-irradiated and irradiated pressure vessel materials. However, some concerns exist regarding the use of the Master Curve (MC) on ferritic-martensitic steels, not only with SSTT but also with standard specimens. The main concern is the slope of the MC [6, 7], that seems to be steeper than the standard one. In this paper, the fracture toughness of Eurofer97 has been obtained by testing small C(T) specimens with the geometry selected in IFMIF-DONES (W = 9.2mm, B = 4.6mm) in the transition region. T0 has been determined and compared to the one obtained from 0.5T-C(T) specimens (both normalised to 1T). The scatter of the results has also been assessed to validate the scatter description of the MC.

A novel multicomponent alloy, V2.5Cr1.2WMoCo0.04, produced from elements expected to favour a BCC crystal structure, and to be suitable for high temperature environments, was fabricated by arc melting and found to exhibit a multiphase dendritic microstructure with W-rich dendrites and V–Cr segregated to the inter-dendritic cores. The as-cast alloy displayed an apparent single-phase XRD pattern. Following heat treatment at 1187 °C for 500 h the alloy transformed into three different distinct phases - BCC, orthorhombic, and tetragonal in crystal structure. This attests to the BCC crystal structure observed in the as-cast state being metastable. The radiation damage response was investigated through room temperature 5 MeV Au+ ion irradiation studies. Metastable as-cast V2.5Cr1.2WMoCo0.04 shows good resistance to radiation induced damage up to 40 displacements per atom (dpa). 96 wt% of the as-cast single-phase BCC crystal structure remained intact, as exhibited by grazing incidence X-ray diffraction (GI-XRD) patterns, whilst the remainder of the alloy transformed into an additional BCC crystal structure with a similar lattice parameter. The exceptional phase stability seen here is attributed to a combination of self-healing processes and the BCC structure, rather than a high configurational entropy, as has been suggested for some of these multicomponent “High Entropy Alloy” types. The importance of the stability of metastable high entropy alloy phases for behaviour under irradiation is for the first time highlighted and the findings thus challenge the current understanding of phase stability after irradiation of systems like the HEAs.

This study investigated the effect of surface finish on the deformation behavior of Eurofer-97 steel during room-temperature small punch testing. The surface finish of specimens was varied from coarse to fine (ranging from P240-grit silicon carbide paper to a polished finish using 0.06-mu m colloidal silica). Testing was conducted on 8-mm diameter by 0.5-mm-thick disks using a 1-mm-radius punch head. No significant change was observed in the elastic-plastic transition load, maximum load, displacement at maximum load, or displacement at failure. Similarly, in all cases, failure occurred in a ductile manner, yielding a dimpled fracture surface. Although friction and surface defects may influence the output of small punch tests, the effect of altering the specimen surface finish alone was negligible in this case, indicating this is not a dominant factor in small punch testing.

A new standard is currently being developed under the auspices of ECISS/TC 101 WG1 for the small punch testing technique for the estimation of both tensile and creep properties. Annex G of the new standard is covering the assessment and evaluation of small punch creep (SPC) data. The main challenge for estimating uniaxial creep properties from SPC data is the force to equivalent stress conversion between SPC and uniaxial creep tests. In this work a range of SPC assessment methodologies, benchmarked for the standard, are compared for verifying the best practice used in the standard. The estimated equivalent stresses for SPC are compared to uniaxial creep stresses at equal rupture times, using three alternative models. In-depth analyses are performed on SPC and uniaxial creep data for P92, F92 and 316 L steel tested within an inter-laboratory round robin. The formulation for SPC equivalent creep strain rate in the standard is also assessed.

Small punch creep testing (SPCT) is currently experiencing a resurgence of interest as a small-scale testing technique (SSTT) for a wide range of uses, including testing in-service components and materials with limited availability. SPCT is particularly pertinent for the fusion community since it is difficult to irradiate large volumes of material due to the high damage levels required and relatively low number of available facilities. An important aspect of the SPCT development that still requires investigation is the effect of the testing environment, which has been shown to impact the creep properties of materials when using standard testing techniques. This paper investigates the effect of using an air or an argon environment on the SPCT behaviour of the leading European fusion reactor material Eurofer97 at 550°C. The test environment was found to impact on the small punch creep behaviour: testing in an argon environment significantly increased time to failure and deflection at failure by a factor of approximately 30 and 7%, respectively. The test environment also appeared to affect the behaviour of the test via oxidation of the punch head which should be an important consideration in future testing.