Materials for advanced energy technology from quantum-mechanical modeling
Time: Wed 2020-06-03 10.00
Subject area: Materials Science and Engineering
Doctoral student: Xiaojie Li , Tillämpad materialfysik
Opponent: Professor Tapio Rantala,
Supervisor: Professor Levente Vitos, Tillämpad materialfysik
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, Ferich) 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) is the main tool 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 Fe-based alloys.In part one, the effect of neutron transmutation elements on the elastic properties of the W-based alloys are calculated. Under intensive radiation, W transforms to Re/Os and thus there is a certain degree of Re/Os doping in the base alloy. 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 level. From the variations of the Cauchy pressure, Poisson’s 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. 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  direction.In part two, we choose three RAFM steels: CLAM/CLF-1, F82H, and EUROFER97 and investigate the micro-mechanical properties of the main alloy phases at low temperature (0 K). 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 they are analyzed separately. 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.In part three, the temperature dependence (T ≤ 1120 K) of the isothermal singlecrystal and polycrystalline elastic parameters of α-Fe and CLAM are reported by using a first-principles based modeling approach. The effect of temperature on the strongly temperature-dependent elastic constants C11 T and CT' is reproduced, as well as that on derived isotropic elastic moduli. Weak changes in C12 T and C44 T with temperature are obtained. The approach is applied to predict the temperature effect on the elastic parameters of three RAFM steels. Contributions due to loss of longrange magnetic order and the combined effect of volume expansion and entropy are found to be important in determining the temperature dependence of the elastic parameters in all the materials investigated.In part four, the (100) and (110) surface energies and surface segregation energies of Fe1−xbCrxb binary alloys, xb ≤ 15 at.%, are computed. These alloys form the basic building blocks of RAFM steels and thus their surface properties are of fundamental importance for the modeling the mechanical behavior. The implications of these results for the surface alloy phase diagram are discussed. The surface chemistry of Fe-Cr(100) is characterized by a transition from Cr depletion to Cr enrichment in a critical bulk Cr composition window of 6 < xb < 9 at.%. In contrast, a nearly homogeneous Cr concentration profile is energetically favorable in Fe-Cr(110) surface. The strongly suppressed surface-layer relaxation at both surfaces is shown to be of magnetic origin. The compressive, magnetic contribution to the surface relaxation stress is found to correlate well with the surface magnetic moment squared at both surface terminations. The surface electronic structures are used to explain the stability of the Cr surface magnetic moments against bulk Cr content.