First-principles study of materials for advanced energy technology

Sammanfattning: The present thesis addresses promising material solutions for fusion reactors from a theoretical point of view. We focus on two specific systems: W-based alloys used as plasma-facing materials and reduced activation ferritic/martensitic (RAFM) steels used as structural materials of breeding-blanket. We aim to systematically investigate the alloying effects on the micro-mechanical properties of these body-centered cubic (bcc) solid solutions. The all-electron exact muffin-tin orbitals (EMTO) method in combination with the coherent-potential approximation (CPA) are the main tools for our theoretical studies. The knowledge of the elastic parameters and their solute-induced changes is important for alloy design and for a multi-scale modeling approach to the mechanical properties. We also explore the planar faults in the present alloys by studying the surface and unstable stacking fault energies. In part one, the effect of neutron transmutation elements on the elastic properties of the W-based alloys are calculated. Both Re and Os solute atoms shrink the lattice constant, which lead to increasing bulk modulus as the amount of Re or Os increases. The polycrystalline shear and Young’s moduli of W1−x−yRexOsy (0 ≤ x, y ≤0.06) enhance with the addition of Re but decrease with increasing Os. From the variations of the Cauchy pressure, Poisson ratio, Pugh ratio B/G, and the ratio of cleavage energy to shear modulus for the dominant slip system, we conclude that the intrinsic ductility of the alloy increases with increasing Re and Os content. The classical Labusch-Nabarro model for solid-solution hardening predicts that strengthening effects in W1−yOsy is larger than those in W1−xRex. We use the energy difference between the face centered cubic (fcc) and bcc structures to estimate the alloying effect on the ideal tensile strength in the [001] direction. Within a simple empirical equation, we find that the melting temperature of W-Re-Os alloy decrease with Re and Os addition. In part two, we investigate the micro-mechanical properties of the main alloy phases of three reduced activation ferritic/martensitic (RAFM) steels: CLAM/CLF-1, F82H, and EUROFER97. Being the main building blocks of the RAFM steels, first the lattice parameters, elastic properties, surface energy and unstable stacking fault energy of ferromagnetic α-Fe and Fe91Cr9 are calculated for reference. For quantitative understanding, we present a detailed analysis of the calculated individual alloying effects of V, Cr, Mn, and W on the elastic properties of Fe91Cr9. A linear superposition of these individual rates on the elastic properties of RAFM steels is shown to reproduce well the values from ab initio calculations. The composition dependence of the elastic constants is decomposed into electronic and volumetric contributions and analyzed. Finally, the intrinsic ductility is evaluated through Rice’s phenomenological theory by using the ratio of surface and unstable stacking fault energies. The results are consistent with those obtained by the common empirical criteria.