Effect of neutron irradiation to 0.7 and 1.4 dpa on the tensile mechanical properties and microstructure of EUROFER97 steel

Several grades of reduced-activation ferritic-martensitic (RAFM) steels have been proposed for fusion applications (e.g., blanket first wall) since the 1990s all over the world. Four batches of the European reference RAFM steel EUROFER97 have been produced since 1998. The RCC-MRx design code, developed, among others, for fusion reactors, currently contains a provisional section dedicated to EUROFER97, encompassing properties of the first two batches, whereas minimum three batches are required for a full qualification and final inclusion of a material into RCC-MRx. The EUROfusion project coordinates efforts to broaden the knowledge of EUROFER97 properties relevant for fusion reactors ITER and DEMO, preparing them for closing the database gaps in RCC-MRx and aggregating them in the DEMO material property handbook (MPH). Its purpose is to provide average and minimum curves of required properties according to the DEMO engineering design and manufacturing needs. The present work reports mechanical properties and fractographic analysis of batch 4 of neutron-irradiated EUROFER97 for the first time. The measured strength and ductility are in line with the data already aggregated in the MPH. SEM investigation confirms that the dimple fracture is retained in the material after neutron irradiation up to 1.4 dpa in the temperature range 25…550 °C

Effect of neutron irradiation on ductility of tungsten foils developed for tungsten-copper laminates

Severe plastic deformation of tungsten (W) is known to be an efficient way to reduce its inherently high ductile-to-brittle transition temperature (DBTT), what is essential for its use in components of a fusion reactor. Thin rolled W foils possess superior mechanical behaviour at room temperature (RT), as demonstrated in previous works. It was then proposed to expand the beneficial mechanical properties of the foil to bulk by fabricating tungsten-copper (W-Cu) laminate composites, which can be used for structural applications. Neutron irradiation in HFIR resulted in embrittlement of the laminate already after 0.016 dpa, with the W foil determining the composite behaviour. In this work, for the first time, we investigate the effect of neutron irradiation on individual W foil, and determine the resulting DBTT shift with the help of cantilever bend tests, using bulk W and the W-Cu composite as reference. The W foil and the bulk samples were irradiated to 0.15 dpa at 400 °C in the BR-2 reactor in Mol (Belgium). We also hypothesise that diffusion of Cu atoms into W could modify the response to irradiation in these materials. We substantiate it with complementary density functional theory (DFT) ab initio calculations to analyse the Cu-vacancy and Cu-self-interstitial interaction, which helps to elucidate co-alignment of the fluxes of point defects and Cu solutes in W matrix. Irradiated foil was found to retain its ductility at RT. No significant irradiation hardening or DBTT shift were detected in the irradiated W foil compared to the bulk W. The different irradiation effect on embrittlement in individual foils and in the laminate may be attributed to the irradiation-assisted diffusion of Cu solutes in W foil, which could form intermetallic phases and affect the accumulation of lattice defects

Crystal plasticity modelling of thermomechanical fatigue in ITER relevant tungsten

This work contributes to a better understanding of the micromechanics of tungsten during cyclic heat loads of plasma-facing components in the ITER fusion reactor. Colossal energy will have to be extracted from the reaction chamber and the temperature of the walls will oscillate together with periodical changes of plasma intensity. This research addresses thermomechanical fatigue, i.e. microscopic cracking due to repeated cycles of thermal expansion and compression. The effect of neutron irradiation damage is not accounted for, but the proposed model applies to thermal shock tests, which are used for the qualification of ITER-relevant tungsten grades. The computational model relies on crystal plasticity theory in order to account for the anisotropy of individual grains. The sensitivity of the mechanical response to strain rate and temperature is reproduced by considering thermally activated dislocation slip. The influence of internal stresses during repeated elastic-plastic transients is investigated using a simplified modelling of the kinematic hardening due to dislocation pile ups at grain boundaries. The model is implemented as a user-defined material law for the Abaqus finite element code, allowing crystal plasticity based finite element modelling (CPFEM). Fatigue indicators are defined to predict the onset of damage whereas cohesive elements are used to simulate the propagation of intergranular cracks. Based on an inverse finite element analysis, it is shown that experimental tensile test data can be reproduced with a limited set of parameters and then applied to as-received and recrystallized tungsten up to 1700 °C. Recrystallized tungsten shows significant asymmetry in the mechanical response during the heating and cooling phases. CPFEM predicts substantial inter- and intragranular heterogeneity and the fatigue indicator values probed at grain boundaries depend largely on the amplitude of backstresses. The occurrence of intergranular cracks is influenced by grain shape as well as the orientation with regard to the thermal flux. Experimentally observed foil delamination is properly reproduced only when accounting for crystalline anisotropy. The model predictions agree qualitatively with the outcome of thermal shock tests.(FSA - Sciences de l'ingénieur) -- UCL, 201

Modelling strain hardening during cyclic thermal shock tests of tungsten

An original model is proposed in order to simulate elastic-plastic transients inside tungsten subjected to cyclic thermal loads expected due to plasma instabilities called “edge-localized modes” in ITER. The model assumes that plasticity is achieved by thermally-activated dislocation motion and it accounts for both isotropic and kinematic hardening. Their relative contributions to the material response are tuned in order to reproduce uniaxial tensile tests performed at different temperatures and different strain rates in various tungsten grades. The model is designed for application as a user-defined material law in fully implicit finite element simulation of thermomechanical loads. The first predictions of the build-up of residual stresses are observed to be qualitatively in line with experimental trends

Plastic deformation of ITER specification tungsten: Temperature and strain rate dependent constitutive law deduced by inverse finite element analysis

In this work, we have derived a constitutive law describing the elasto-plastic response of tungsten by applying an inverse finite element analysis (IFEA) to grasp the deformation well beyond the onset of deformation instability in tensile tests. A model based on the Kocks-Mecking representation of thermally-activated dislocation-mediated plasticity was applied to characterise the mechanical response of tungsten compliant with the ITER specification. The developed model accurately describes the temperature and strain rate dependent tensile properties in the temperature range 250–600 ◦C. The capability to extrapolate the hardening law to a higher temperature and strain rate range is demonstrated. A particular advantage of the developed method is its applicability to neutron irradiated materials, for which the uniform elongation is often very low or even negligible

Finite element analysis of heat load of tungsten relevant to ITER conditions

A computational procedure is proposed in order to predict the initiation of intergranular cracks in tungsten with ITER specification microstructure (i.e. characterised by elongated micrometresized grains). Damage is caused by a cyclic heat load, which emerges from plasma instabilities during operation of thermonuclear devices. First, a macroscopic thermo-mechanical simulation is performed in order to obtain temperature- and strain field in the material. The strain path is recorded at a selected point of interest of the macroscopic specimen, and is then applied at the microscopic level to a finite element mesh of a polycrystal. In the microscopic simulation, the stress state at the grain boundaries serves as the marker of cracking initiation. The simulated heat load cycle is a representative of edge-localized modes, which are anticipated during normal operations of ITER. Normal stresses at the grain boundary interfaces were shown to strongly depend on the direction of grain orientation with respect to the heat flux direction and to attain higher values if the flux is perpendicular to the elongated grains, where it apparently promotes crack initiation

Displacement cascades in Fe Ni Mn Cu alloys: RVP model alloys

Primary damage due to displacement cascades (10-100 keV) has been assessed in Fe-1%Mn-1%Ni-0.5%Cu and its binary alloys by molecular dynamics (MD), using a recent interatomic potential, specially developed to address features of the Fe-Mn-Ni-Cu system in the dilute limit. The latter system represents the model matrix for reactor pressure vessel steels. The applied potential reproduces major interaction features of the solutes with point defects in the binary, ternary and quaternary dilute alloys. As compared to pure Fe, the addition of one type of a solute or all solutes together does not change the major characteristics of primary damage. However, the chemical structure of the self-interstitial defects is strongly sensitive to the presence and distribution of Mn and Cu in the matrix. 20 keV cascades were also studied in the Fe-Ni-Mn-Cu matrix containing (100) dislocation loops (with density of 1024 m-3 and size 2 nm). Two solute distributions were investigated, namely: a random one and one obtained by Metropolis Monte Carlo simulations from our previous work. The presence of the loops did not affect the defect production efficiency but slightly reduced the fraction of isolated self-interstitials and vacancies. The cascade event led to the transformation of the loops into 1/2(111) glissile configurations with a success rate of 10% in the matrix with random solute distribution, while all the pre-created loops remain stable if the alloy's distribution was applied using the Monte-Carlo method. This suggests that solute segregation to loops "stabilizes" the pre-existing loops against transformation or migration induced by collision cascades

Criteria of instability of copper and aluminium perfect crystals subjected to elastic deformation in the temperature range 0 – 400 K

Polycrystalline metals have flow stress two to three orders of magnitude lower than the theoretical shear strength estimated by Frenkel model. This significant strength difference is primarily due to the presence of defects, such as dislocations and grain boundaries. However, it was experimentally found that defect-free nanoscale objects (whiskers, nanopillars, etc.) can exhibit strength close to the theoretical limit. With the development of nanotechnology, interest in the study of the theoretical strength of metals and alloys has grown significantly. It is important to find reliable criteria of lattice instability when homogeneous nucleation of defects begins during deformation of an ideal crystal lattice. Note that the Frenkel estimation does not take into account thermal vibrations of atoms and attempts are being made to take into account the effect of temperature on the theoretical strength of defect-free crystals. In this paper, using molecular dynamics simulation, we study shear deformation in the direction of for single crystals of copper and aluminum in the temperature range from 0 to 400 K. Lattice instability was evaluated using two criteria: (i) macroscopic criterion, which is related to the loss of positive definiteness of the stiffness tensor, and (ii) a microscopic criterion related to the formation of a stacking fault, which leads to a drop of the applied shear stress. It was demonstrated that both criteria are consistent at low temperatures, but the macroscopic criterion is less reliable at higher temperatures