The current magnetic confinement nuclear fusion power reactor concepts going beyond ITER are based on assumptions about the availability of materials with extreme mechanical, heat, and neutron load capacity. In Europe, the development of such structural and armour materials together with the necessary production, machining, and fabrication technologies is pursued within the EFDA long-term fusion materials programme. This paper reviews the progress of work within the programme in the area of tungsten and tungsten alloys. Results, conclusions, and future projections are summarized for each of the programme´s main subtopics, which are: (1) fabrication, (2) structural W materials, (3) W armour materials, and (4) materials science and modelling. It gives a detailed overview of the latest results on materials research, fabrication processes, joining options, high heat flux testing, plasticity studies, modelling, and validation experiments

The structure and phase stability of binary tungsten-vanadium and tungsten-tantalum alloys are investigated over a broad range of alloy compositions using ab initio and cluster expansion methods. The alloys are characterized by the negative enthalpy of mixing across the entire composition range. Complex intermetallic compounds are predicted by ab initio calculations as the lowest energy structures for both alloys. The effect of atomic relaxation on the enthalpy of mixing is almost negligible in W-V, but is substantial in W-Ta alloys. Canonical Monte Carlo simulations are used for predicting the order-disorder transition temperatures for both alloys. Differences in the short-range order between the two alloys are explained by the opposite signs of the second nearest-neighbour cluster interaction coefficients for W-V and W-Ta. Using the predicted ground-state structures, we evaluate the monovacancy formation energies and show that in W-Ta alloys they are highly sensitive to the alloy composition and the local environment of a vacancy site, varying from 3 to 5 eV. In the dilute tungsten alloy limit, a $\langle 111\rangle$ self-interstitial atom crowdion defect forms a configuration strongly bound to a vanadium solute atom, whereas interaction between the same defect and a tantalum solute atom is repulsive. Values of elastic constants computed for all the ground-state structures and several metastable cubic alloy structures are used for assessing the effect of alloying on mechanical properties. Values of the Young modulus and the Poisson ratio, as well as the empirical Rice-Thompson criterion, are applied to screening the alloys, to assess the effect of chemical composition on ductility.

The phase stability of fcc and bcc magnetic binary Fe-Cr, Fe-Ni, and Cr-Ni alloys, and ternary Fe-Cr-Ni alloys is investigated using a combination of density functional theory (DFT), cluster expansion (CE), and magnetic cluster expansion (MCE) approaches. Energies, magnetic moments, and volumes of more than 500 alloy structures have been evaluated using DFT, and the predicted most stable configurations are compared with experimental observations. Deviations from the Vegard law in fcc Fe-Cr-Ni alloys, resulting from the nonlinear variation of atomic magnetic moments as functions of alloy composition, are observed. The accuracy of the CE model is assessed against the DFT data, where for ternary Fe-Cr-Ni alloys the cross-validation error is found to be less than 12 meV/atom. A set of cluster interaction parameters is defined for each alloy, where it is used for predicting new ordered alloy structures. The fcc ${\mathrm{Fe}}_2\mathrm{CrNi}$ phase with ${\mathrm{Cu}}_2\mathrm{NiZn}$-like crystal structure is predicted to be the global ground state of ternary Fe-Cr-Ni alloys, with the lowest chemical ordering temperature of 650 K. DFT-based Monte Carlo (MC) simulations are applied to the investigation of order-disorder transitions in Fe-Cr-Ni alloys. The enthalpies of formation of ternary alloys predicted by MC simulations at 1600 K, combined with magnetic correction derived from MCE, are in excellent agreement with experimental values measured at 1565 K. The relative stability of fcc and bcc phases is assessed by comparing the free energies of alloy formation. The evaluation of the free energies involved the application of a dedicated algorithm for computing the configurational entropies of the alloys. Chemical order is analyzed, as a function of temperature and composition, in terms of the Warren-Cowley short-range order (SRO) parameters and effective chemical pairwise interactions. In addition to compositions close to binary intermetallic phases ${\mathrm{CrNi}}_2$, FeNi, ${\mathrm{FeNi}}_3$, and ${\mathrm{FeNi}}_8$, pronounced chemical order is found in fcc alloys near the center of the ternary alloy composition triangle. The calculated SRO parameters compare favorably with experimental data on binary and ternary alloys. Finite temperature magnetic properties of fcc Fe-Cr-Ni alloys are investigated using an MCE Hamiltonian parameterized using a DFT database of energies and magnetic moments computed for a large number of alloy configurations. MCE simulations show that the ordered ternary ${\mathrm{Fe}}_2\mathrm{CrNi}$ alloy phase remains magnetic up to 850–900 K due to the strong antiferromagnetic coupling between (Fe,Ni) and Cr atoms in the ternary Fe-Cr-Ni matrix.