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Irradiation damage is known to alter a material's microstructure due to the accumulation of high densities of defect clusters. Such irradiated microstructures change the mechanical response of the material due to dislocation-defect interactions, which leads to a host of issues such as hardening, swelling, irradiation creep, embrittlement, etc. Traditionally, the effect of irradiation on the mechanical response of materials is evaluated via tensile tests of pre-irradiated specimens at different doses and temperatures. From a modeling perspective, methods exist that simulate irradiation and deformation as separate processes, with the former based on kinetic transport theory and the latter on crystal plasticity (CP). Generally, these are connected by a state variable, usually in the form of a characteristic length scale, that represents the defect concentration and strength and its effect on dislocation-mediated slip. However, cases where deformation takes place during irradiation are also important despite being less common. In this paper, we develop a coupled CP and stochastic cluster dynamics (SCD) approach capable of treating all instances of irradiation/deformation in irradiated materials. We apply the methodology to tungsten crystals due to its importance as a high-temperature candidate structural material and to its extensive defect and mechanical data base. SCD evolves the defect microstructure stochastically, providing a statistically-averaged defect cluster spacing parameter that informs CP calculations of the material's mechanical deformation. The coupling is bi-directional in the sense that the SCD method updates the obstacle density and furnishes a resistance stress to the CP model, while CP feeds updated dislocation densities that act as defect sinks in the SCD calculation cycles. The coupling can be done sequentially, as in standard tensile tests of pre-irradiated materials, or concurrently, as in in situ straining tests during irradiation. We carry out simulations of realistic irradiation/deformation scenarios and highlight the differences between the present method and past works considering similar situations. [ABSTRACT FROM AUTHOR]

Fuel retention in plasma facing tungsten components is a critical phenomenon affecting the mechanical integrity and radiological safety of fusion reactors. It is known that hydrogen can become trapped in small defect clusters, internal surfaces, dislocations, and/or impurities, and so it is common practice to seed W subsurfaces with irradiation defects in an attempt to precondition the system to absorb hydrogen. The amount of H can later be tallied by performing careful thermal desorption tests where released temperature peaks are mapped to specific binding energies of hydrogen to defect clusters and/or microstructural features of the material. While this provides useful information about the potential trapping processes, modeling can play an important role in elucidating the detailed microscopic mechanisms that lead to hydrogen retention in damaged tungsten. In this paper, we develop a detailed kinetic model of hydrogen penetration and trapping inspired by recent experiments combining ion irradiation, hydrogen plasma exposure, and thermal desorption. We use the stochastic cluster dynamics method to solve the system of coupled partial differential equations representing the mean field description of the multispecies system. The model resolves the spatial distribution of defects and hydrogen clusters during the three processes carried out experimentally and is parameterized with information from atomistic calculations. We find that the calculated thermal desorption spectra are broadly characterized by three H emission regions: (i) a low temperature one where dislocations are the main contributors to the release peaks; (ii) an intermediate one governed by hydrogen release from small overpressurized clusters with multiple overlapping peaks, and (iii) a high temperature one defined by clean isolated emission peaks from large underpressurized bubbles. These three temperature intervals are seen to largely correlate with the depth at which the clusters are found. The relevance of the 'super abundant' vacancy mechanism is assessed, finding that its main role is to transfer more clusters from the intermediate to the high temperature regions as its relevance increases. We find this picture to be in very good agreement with the experiments, adding confidence to the predictive potential of the models and their useto understand irradiation damage and plasma exposure effects in plasma facing components. [ABSTRACT FROM AUTHOR]

The Portevin-Le Chatelier (PLC) effect is a phenomenon by which plastic slip in metallic materials becomes unstable, resulting in jerky flow and the onset of inhomogeneous deformation. The PLC effect is thought to be fundamentally caused by the dynamic interplay between dislocations and solute atoms. However, this interplay is almost always inaccessible experimentally due to the extremely fine length and time scales over which it occurs. In this paper, simulations of jerky flow in W-O interstitial solid solutions reveal three dynamic regimes emerging from the simulated strain rate-temperature space: one resembling standard solid solution strengthening, another one mimicking solute cloud formation, and a third one where dislocation/solute coevolution leads to jerky flow as a precursor of dynamic strain aging. The simulations are carried out in a stochastic framework that naturally captures rare events in a rigorous manner, providing atomistic resolution over diffusive time scales using no adjustable parameters. Understanding the plastic deformation mechanism within specific ranges of temperature and strain in metal alloys is of great technological importance. Here the authors report on dynamic simulations of dislocation–solute coevolution in tungsten crystals containing a few atomic parts per million of interstitial oxygen and their relation to unstable plastic flow. [ABSTRACT FROM AUTHOR]

Understanding and improving the mechanical properties of tungsten is a critical task for the materials fusion energy program. The plastic behavior in body-centered cubic (bcc) metals like tungsten is governed primarily by screw dislocations on the atomic scale and by ensembles and interactions of dislocations at larger scales. Modeling this behavior requires the application of methods capable of resolving each relevant scale. At the small scale, atomistic methods are used to study single dislocation properties, while at the coarse-scale, continuum models are used to cover the interactions between dislocations. In this work we present a multiscale model that comprises atomistic, kinetic Monte Carlo (kMC) and continuum-level crystal plasticity (CP) calculations. The function relating dislocation velocity to applied stress and temperature is obtained from the kMC model and it is used as the main source of constitutive information into a dislocation-based CP framework. The complete model is used to perform material point simulations of single-crystal tungsten strength. We explore the entire crystallographic orientation space of the standard triangle. Non-Schmid effects are inlcuded in the model by considering the twinning-antitwinning (T/AT) asymmetry in the kMC calculations. We consider the importance of 〈111〉{110} and 〈111〉{112} slip systems in the homologous temperature range from 0.08 Tm to 0.33 Tm, where Tm =3680 K is the melting point in tungsten. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Abstract: Due to its advantageous physical properties, tungsten (W) is being considered as a candidate structural material in fusion applications. In this paper, we perform stochastic cluster dynamics calculations of irradiation damage accumulation in pure W under fast neutron spectra up to doses of 1.5dpa in the 400–600°C interval. Our calculations suggest that He bubbles and dislocation loops accumulate under fusion conditions, but not under fast fission spectra. We study the temperature dependence of swelling and find that it is maximum in the 550–590°C temperature range, falling precipitously above 600°C. Swelling levels are very low, never surpassing a fraction of a percentage point. We also provide hardening estimates based on the accumulation of sessile dislocation loops under fusion conditions and show that they are moderate, ranging between 70 and 137MPa at 400°C. [Copyright &y& Elsevier]

We present calculations of secondary electron emission (SEE) yields in tungsten as a function of primary electron energies between 100 eV and 1 keV and incidence angles between 0 and 90 ° . We conduct a review of the established Monte Carlo methods to simulate multiple electron scattering in solids and select the best suited to study SEE in high-Z metals. We generate secondary electron yield and emission energy functions of the incident energy and angle and fit them to bivariate fitting functions using symbolic regression. We compare the numerical results with experimental data, with good agreement found. Our calculations are the first step towards studying SEE in nanoarchitected surfaces for electric propulsion chamber walls. [ABSTRACT FROM AUTHOR]

Simulations of neutron damage under fusion energy conditions must capture the effects of transmutation, both in terms of accurate chemical inventory buildup as well as the physics of the interactions between transmutation elements and irradiation defect clusters. In this work, we integrate neutronics, primary damage calculations, molecular dynamics results, Re transmutation calculations, and stochastic cluster dynamics simulations to study neutron damage in single-crystal tungsten to mimic divertor materials. To gauge the accuracy and validity of the simulations, we first study the material response under experimental conditions at the JOYO fast reactor in Japan and the High Flux Isotope Reactor at Oak Ridge National Laboratory, for which measurements of cluster densities and hardening levels up to 2 dpa exist. We then provide calculations under expected DEMO fusion conditions. Several key mechanisms involving Re atoms and defect clusters are found to govern the accumulation of irradiation damage in each case. We use established correlations to translate damage accumulation into hardening increases and compare our results to the experimental measurements. We find hardening increases in excess of 5000 MPa in all cases, which casts doubts about the integrity of W-based materials under long-term fusion exposure. [ABSTRACT FROM AUTHOR]

Abstract We use atomistic simulations to investigate grain boundary (GB) phase transitions in elemental body-centered cubic (bcc) metal tungsten. Motivated by recent modeling study of grain boundary phase transitions in [100] symmetric tilt boundaries in face-centered cubic (fcc) copper, we perform a systematic investigation of [100] and [110] symmetric tilt high-angle and low-angle boundaries in bcc tungsten. The structures of these boundaries have been investigated previously by atomistic simulations in several different bcc metals including tungsten using the γ -surface method, which has limitations. In this work we use a recently developed computational tool based on the USPEX structure prediction code to perform an evolutionary grand canonical search of GB structure at 0 K. For high-angle [100] tilt boundaries the ground states generated by the evolutionary algorithm agree with the predictions of the γ -surface method. For the [110] tilt boundaries, the search predicts novel high-density low-energy grain boundary structures and multiple grain boundary phases within the entire misorientation range. Molecular dynamics simulation demonstrate that the new structures are more stable at high temperature. We observe first-order grain boundary phase transitions and investigate how the structural multiplicity affects the mechanisms of the point defect absorption. Specifically, we demonstrate a two-step nucleation process, when initially the point defects are absorbed through a formation of a metastable GB structure with higher density, followed by a transformation of this structure into a GB interstitial loop or a different GB phase. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

We use a physically-based crystal plasticity model to predict the yield strength of body-centered cubic (bcc) tungsten single crystals subjected to uniaxial loading. Our model captures the thermally-activated character of screw dislocation motion and full non-Schmid effects, both of which are known to play critical roles in bcc plasticity. The model uses atomistic calculations as the sole source of constitutive information, with no parameter fitting of any kind to experimental data. Our results are in excellent agreement with experimental measurements of the yield stress as a function of temperature for a number of loading orientations. The validated methodology is employed to calculate the temperature and strain-rate dependence of the yield strength for 231 crystallographic orientations within the standard stereographic triangle. We extract the strain-rate sensitivity of W crystals at different temperatures, and finish with the calculation of yield surfaces under biaxial loading conditions that can be used to define effective yield criteria for engineering design models. [ABSTRACT FROM AUTHOR]

• Thermo-fracture and thermal shock effects in solid tungsten and open-cell tungsten foam. • Re-crystallization, grain growth and crack initiation and propagation in sold tungsten and W-foam. • Effect of self-constraint and compressive stress state on developing residual stresses, grain growth and crack formation in tungsten. • Effect of boundary condition and geometry on stress state and deformation of tungsten disks tested under high heat flux plasma arc-jet. • Nature of tungsten foam thermomechanical damage exposed to extreme-power (GW/m2) short duration laser tests and high-power (MW/m2) long duration arc-jet tests. We investigate here the effects of transient (cyclic) arc-jet plasma and laser heating on fracture behavior of W-foam and solid tungsten. The two key parameters that control the foam thermomechanical response are its density and mean cell size expressed in Pores Per Inch (PPI). Tungsten foam samples were fabricated with Chemical Vapor Deposition (CVD) with variety of PPI and relative density. These were tested under two types of qualitatively different conditions: (1) high-enthalpy arc-jet, and (2) high-power cyclic laser heating. None of the foam samples showed macroscopic through-thickness cracks. However, distributed micro-cracks were observed on ligaments and their triple junctions. Under the same loading conditions, W-foam and solid tungsten showed similar crack network pattern and characteristic length-scale. However, Crack Opening Displacement (COD) was twice as large in solid W as compared to W-foam. Foam samples that have been previously exposed to a low-pressure helium plasma showed significant changes in their surface forming nano-texture fuzz which was removed by subsequent testing in the arc-jet. Extensive fracture and re-crystallization were observed in the thin solid W disk that was fully-constrained from expansion. Thicker and fully-constrained solid W disks did not display recrystallization, grain growth, and extensive cracking. However, thicker disks that were free to expand showed some recrystallization and extensive through-thickness cracks due to less effective cooling and thus higher temperatures. Laser beam testing showed no visible damage formation at 0.19 GW/m 2 and 0.38 GW/m 2 for both low-density (23%) and high-density (43%) foams at low pulses (100-1000). Micro-cracks were observed after 10,000 pulses at 0.19 GW/m 2 in both foams, and in low-density foam after 100,000 at 0.38 GW/m 2. The nature of thermomechanical damage in W-foam exposed to extreme power (GW/m 2) short-duration laser pulses was found to be qualitatively similar to that of high power (MW/m 2) long-duration arc-jet. [ABSTRACT FROM AUTHOR]