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Ab initio based study of magneto-chemo-structural coupling in complex alloys
Lattice imperfections (or defects) are ubiquitous in materials. Understanding the properties of various defects is crucial to improve materials' performance. In magnetic materials, as it is shown in this work, complicated interactions between the structural aspects of defects and magnetism are present. While comprehending these interactions at the atomic scale is highly relevant, the theoretical treatment of high-temperature paramagnetic (magnetically disordered) states, in particular, poses serious challenges. In the present thesis, a new, efficient first-principles method is introduced for the accurate computation of atomic relaxations in magnetically disordered systems with defects. The method is based on the spin-space averaging technique, and is general by construction, i.e., it is applicable to any magnetically disordered system with defects. Firstly, the strength of the method is benchmarked for the vacancy defect in body-centered cubic (bcc) Fe by computing vacancy formation and migration energies. The impact of proper atomic relaxations is found to be significant and is compared with other approximate schemes, followed by a discussion on the influence of thermal expansion. In the following stage, the relaxation method is applied to address vacancy-mediated diffusion in the dilute FeMn system, which is decisive for steels' performance. Mn is found to diffuse very fast relative to Fe in the ferromagnetic state and much slower in the paramagnetic state. The presence of Mn is shown to reduce the acceleration of diffusion coeffcients across the magnetic order-disorder transition. These findings are linked to the strong impact of disordering on chemical interactions. Further, the remarkable impact of magnetic states on the Mn segregation behavior at the grain boundaries is presented. A new phenomenon, where the grain boundary and bulk simultaneously exist in different magnetic states, is brought forward. Throughout the discussion, interplays of structure, chemistry and magnetism are identified. The effect of ...
Ab initio based study of magneto-chemo-structural coupling in complex alloys
Lattice imperfections (or defects) are ubiquitous in materials. Understanding the properties of various defects is crucial to improve materials' performance. In magnetic materials, as it is shown in this work, complicated interactions between the structural aspects of defects and magnetism are present. While comprehending these interactions at the atomic scale is highly relevant, the theoretical treatment of high-temperature paramagnetic (magnetically disordered) states, in particular, poses serious challenges. In the present thesis, a new, efficient first-principles method is introduced for the accurate computation of atomic relaxations in magnetically disordered systems with defects. The method is based on the spin-space averaging technique, and is general by construction, i.e., it is applicable to any magnetically disordered system with defects. Firstly, the strength of the method is benchmarked for the vacancy defect in body-centered cubic (bcc) Fe by computing vacancy formation and migration energies. The impact of proper atomic relaxations is found to be significant and is compared with other approximate schemes, followed by a discussion on the influence of thermal expansion. In the following stage, the relaxation method is applied to address vacancy-mediated diffusion in the dilute FeMn system, which is decisive for steels' performance. Mn is found to diffuse very fast relative to Fe in the ferromagnetic state and much slower in the paramagnetic state. The presence of Mn is shown to reduce the acceleration of diffusion coeffcients across the magnetic order-disorder transition. These findings are linked to the strong impact of disordering on chemical interactions. Further, the remarkable impact of magnetic states on the Mn segregation behavior at the grain boundaries is presented. A new phenomenon, where the grain boundary and bulk simultaneously exist in different magnetic states, is brought forward. Throughout the discussion, interplays of structure, chemistry and magnetism are identified. The effect of ...
Ab initio based study of magneto-chemo-structural coupling in complex alloys
Hegde, Omkar Gopalkrishna (author) / Neugebauer, Jörg
2022-02-23
Theses
Electronic Resource
English
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