Polycrystalline tungsten is a promising candidate as plasma-facing armour material in future fusion reactors. Below the DBTT tungsten shows brittle or semi-brittle behaviour with no or little plastic ...deformation before cleavage fracture. Moreover, tungsten has an inherent low fracture toughness as well as a large scatter within the strength properties. This requires a statistical treatment of the strength values. Therefore, four-point bending tests in the temperature range of room temperature (RT) to 400 °C were performed to allow the Weibull analysis in brittle failure regime and/or the Beremin model analysis in case of onset of plastic behaviour while approaching the DBTT temperature. The analyses are supported by FE simulations. Weibull and Beremin parameter values are determined from the simulation results using post-processing codes.
•Electroplating isa relevant technology for brazing of blanket and divertor parts.•Tungsten, Eurofer and steel joints successfully fabricated.•Reactive interlayers improve adherence and reduce ...failure risks.•Qualification of joints performed by thermomechanical testing and aging.•Shear strength of joints comparable with conventionally brazing of steels.
Fusion technology requires in the fields of first wall and divertor development reliable and adjusted joining processes of plasma facing tungsten to heat sinks or blanket structures. The components to be bonded will be fabricated from tungsten, steel or other alloys like copper. The parts have to be joined under functional and structural aspects considering the metallurgical interactions of alloys to be assembled and the filler materials. Application of conventional brazing showed lacks ranging from bad wetting of tungsten up to embrittlement of fillers and brazing zones. Thus, the deposition of reactive interlayers and filler components, e.g. Ni, Pd or Cu was initiated to overcome these metallurgical restrictions and to fabricate joints with aligned mechanical behavior.
This paper presents results concerning the joining of tungsten, Eurofer and stainless steel for blanket and divertor application by applying electroplating technology. Metallurgical and mechanical characterization by shear testing were performed to analyze the joints quality and application limits in dependence on testing temperature between room temperature and 873K and after thermal aging of up to 2000h. The tested interlayers Ni and Pd enhanced wetting and enabled the processing of reliable joints with a shear strength of more than 200MPa at RT.
► The effects of the neutron irradiation on the mechanical properties of RAFM steels are assessed up to 80dpa. ► The impacts of helium on the mechanical properties and swelling are estimated by ...reviewing helium simulating experiments. ► The recommendations for the operating conditions are given for fusion reactor First Wall and blanket structural materials.
The objective of the current work is assessment of the effects of neutron irradiation on the mechanical properties of blanket and divertor materials of a future fusion energy generation Demonstration Power Plant (DEMO). The emphasis is put on the review of the tensile, Charpy impact and fracture toughness properties of EUROFER97 and EUROFER ODS (9%Cr) steels irradiated up to a displacement damage dose of 80dpa in order to address (i) irradiation dose dependence of mechanical properties, (ii) irradiation temperature dependence of the mechanical properties, (iii) helium effects on the mechanical properties. The assessment will be used to give recommendations on the operating temperature range for the First Wall and helium cooled Breeding Blanket materials and to identify needs for structural materials R&D.
Nano/ultra-fine grained W and W-1wt%La2O3 (80nm–320nm) were fabricated by high-pressure torsion after 2 turns at 300°C/200°C, 400°C, 550°C and 700°C. Distinct textures were observed by using electron ...backscattering diffraction. Deformed W primarily revealed {112}〈111〉, {110}〈112〉 and {110}〈001〉 components, whereas, deformed W-1wt%La2O3 exhibited mainly γ-fiber at lower process temperatures. The texture anisotropy of (112)1−1−1 and (112)111− pair was also investigated. Results showed that the stacking fault energy and process temperature were crucial in determining texture. Dynamic recrystallization was retarded by La2O3 addition under given conditions.
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The ratcheting behavior of the 9%-Cr-1%-Mo ferritic–martensitic (FM) steel P91 is investigated by uniaxial strain- and stress-controlled cyclic loading tests at room temperature and 550 °C. ...Ratcheting rates under multiple loading conditions are measured to build the database of P91 for the further application in generation IV fission reactors. The unconventional asymmetry of stress under strain-controlled tests at room temperature predicts the non-zero ratcheting with zero mean stress, which is approved in the stress-controlled tests. A unified viscoplastic deformation model for RAFM steels is further modified to adapt the ratcheting behavior of P91. The current model describes cyclic softening in strain-controlled LCF tests very well. However it strongly overestimates the uniaxial ratcheting rates in stress-controlled tests, due to application of the Armstrong–Frederick dynamic recovery rule. Based on further analysis of back stresses (BS), a new rule for dynamic recovery is designed to fit the ratcheting rates under multiple loading conditions.
•Asymmetry of material strength of P91 under tension and compression at room temperature.•Symmetry of material strength under tension and compression at 550 °C.•Influences of peak stress, mean stress, stress rate and hold time to ratcheting behavior.•New proposed rule of dynamic recovery of kinematic hardening with two back-stress components.•Satisfactory agreement between material responses and simulation results.
Solid-state diffusion bonding is a selected joining technology to bond divertor components consisting of tungsten and EUROFER97 for application in fusion power plants. Due to the large mismatch in ...their coefficient of thermal expansions, which leads to serious thermally induced residual stresses after bonding, a thin vanadium plate is introduced as an interlayer. However, the diffusion of carbon originated from EUROFER97 in the vanadium interlayer during the bonding process can form a vanadium carbide layer, which has detrimental influences on the mechanical properties of the joint.
For optimal bonding results, the thickness of this layer and the residual stresses has to be decreased sufficiently without a significant reduction of material transport especially at the vanadium/tungsten interface, which can be achieved by varying the diffusion bonding temperature and duration. The investigation results show that at a sufficiently low bonding temperature of 700°C and a bonding duration of 4h, the joint reaches a reasonable high ductility and toughness especially at elevated test temperature of 550°C with elongation to fracture of 20% and mean absorbed Charpy impact energy of 2J (using miniaturized Charpy impact specimens). The strength of the bonded materials is about 332MPa at RT and 291MPa at 550°C. Furthermore, a low bonding temperature of 700°C can also help to avoid the grain coarsening and the alteration of the grain structure especially of the EUROFER97 close to the bond interface.
•A modeling approach for describing the mechanical behavior of steel Eurofer97 is presented. The simulated results are compared with test results.•The proposed constitutive model is proved able to ...predict fatigue lifetime and to describe the cyclic softening of Eurofer97 under strain-controlled low cycle fatigue tests.•The proposed model is at the same time proved able to describe the ratcheting behavior of Eurofer97 under stress-controlled cyclic loading. The effects of stress magnitude, stress ratio and stress rate on the ratcheting behavior are described by this model.•Various previous modeling approaches for 9Cr-ferritic-martensitic steels are collected and compared with the newly proposed model.
A constitutive model is proposed for Reduced-Activation Ferritic-Martensitic (RAFM) steel Eurofer97 at high temperatures by combining formulae from previous modeling approaches to describe and predict mechanical behaviors under various cyclic loading conditions. Two failure modes, fatigue fracture and over-accumulated strain (ratcheting) are able to be simulated by the combined constitutive model. Cyclic softening and the effect of magnitude, symmetry and rate of external loading on ratcheting behavior are also able to be described by the new model. Parameter values are fitted based on strain-controlled and stress-controlled isothermal uniaxial experiments on Eurofer97 batch two at 450°C and 550°C. The simulated results are presented to compare with corresponding experimental data, to show the performance of the new constitutive model.
The relative stability of 1/2 and dislocation loops, originating from anisotropic elasticity of alpha-Fe, was extensively exploited to explain experimentally observed variation in the relative ...population of the two loop types under varying irradiation temperatures. In this study, using cluster dynamics simulations taking into account the coevolution of C15 Laves phase structure (cluster) and loop reaction caused by defect one-dimensional (1-d) movement, we reveal new mechanisms that may underpin the variations in loop population of different Burgers vectors. We identify that C15 clusters have twofold roles in mediating loop evolution: (1) acting as buffers of self-interstitial atoms, (2) initiating loop nuclei via their collapses. The latter plays a critical role in nucleation of loops and subsequently affects loop evolution of both types. We show that the loop relative population undergoes further re-arrangement via a transfer reaction driven by loop long-range diffusion. Our findings essentially demonstrate that, apart from the loop stability, the relative population of two loop types is governed by at least two other factors: C15 cluster stability and loop 1-d movement.
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Plastic deformation during low-cycle fatigue (LCF) in equiatomic face-centered cubic (FCC) CoCrFeMnNi high-entropy alloys (HEAs) is accumulated by dislocation substructure formation, which leads to ...crack initiation. Whilst these substructures have been reported before, little has been done to clarify their formation mechanisms and the effects of strain amplitude, cycle number and grain orientation. In this study, cyclic deformation behavior and microstructural evolution of CoCrFeMnNi were examined for two different grain sizes at room temperature. Microstructural investigations by transmission electron microscopy showed that, while the dislocation structures at low strain amplitude (0.3%) mainly consisted of planar slip bands, at higher strain amplitudes (0.5% and 0.7%), wavy-substructures including veins, walls, labyrinth and cells prevailed. Slip mode also changes from initially planar-slip to wavy-slip with cycle numbers. Dislocations in veins, walls, labyrinth and cells are found to have different Burgers vectors, suggesting that apart from wavy-slip, multiple-slip also contributes to their formation. Moreover, distinct dislocation substructure in grains is dictated more by the constraints from neighboring grains rather than by their orientation. Additionally, the formation of various dislocation structures in a single grain is also linked to the constraint effects from the neighboring grains.
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For determination of fracture toughness in the brittle regime or ductile fracture in the upper shelf region, special standard specifications are in use e.g. ASTM E399 or ASTM E1820. Due to the ...rigorous size requirements for specimen testing, it is necessary to use big specimens. To circumvent this problem an approach based on finite element (FE) simulations using the cohesive zone model (CZM) is used. The parameters of the cohesive zone model have been determined using sub-size specimens. With the identified parameters, simulations of standard-size specimens have been performed to successfully predict fracture toughness of standard-size specimens in the brittle and ductile regimes. The objective is to establish small size testing technology for the determination of fracture toughness.
•Prediction of fracture toughness on standard-size specimens.•Valid fracture toughness based on sub-size specimens.•Triaxiality dependent cohesive zone model.•Approach works independent on fracture appearance (brittle, ductile).
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