| Impurity is one important kind of defects in materials. The properties of electronic transmission and magnetism are affected by the presence of impurities in materials. Impurities also compose the complex with other defects, such as dislocations, grain boundaries, etc., and apparently affect the mechanical properties of materials, such as ductility, strength. It can even control the ground state structure of materials. Investigating the effects of impurities and finding the explanation at microscopic level has not only great scientific meanings, but also the worth on application and techniques, because these investigations can efficiently accelerate the design on high-performance alloys and improve the properties of widely-used materials. In this dissertation, by employing the state-of-art first principles quantum mechanical calculated methods, we systematically investigate the effects of impurities in kinds of defects systems, and explain the microscopic mechanism based on the analysis on the energy and electronic structures. According to the curve of binding energy verses displacements of atoms, it is found that C or N impede the propagation of {001}[110] crack inα-Fe when they lie at the front of the crack, while O accelerates the propagation, and has detrimental effects on the ductility ofα-Fe. According to the results of charge density distribution, density of states (DOS), and interatomic interaction, we also find that it is the anisotropic bonding character that controls the effects of the impurity on the crack propagation: the bonds of C (N)-Fe pair which is vertical to the crack plane is much larger than that of the C (N)-Fe pair which is parallel to the crack plane, while it is not the case of O. According to Rice-Wang model, when segregating toα-FeΣ5 [001] (010) grain boundary, Re is a cohesive enhancer, and strengthens the interaction of the Fe-Fe pairs which crosses the interface. The effect of Re can be attributed to the large value of chemical potential of Re based on the analysis on the formation energy and the chemical potential of both Re and Fe. We also investigate the effects of co-segregation of two kinds of impurities on the grain boundary, and find that though Ti and B are both cohesive enhancers, the co-segregation of (Ti+B) does not strengthen the grain boundary further. Ti can completely reduce the detrimental effect of O. Furthermore, based on the segregation energy, it is found that Ti can efficiently prohibit the segregation of O to the grain boundary, and thus improve the ductility ofα-Fe. The segregation energy is agood auxiliary method to Rice-Wang model. According to experiments, Al substitutes Si in MoSi2 to form Mo(Si1-x, Alx)2, when the x exceeds the 10 at.%, Mo(Si1-x, Alx)2 will performs C1 1b→C40phase transition, based on group theory and crystal field theory, we analyze the DOS of Mo(Si1-x, Alx)2 with different x, and find that the concentration of valence electrons is changed with x, and the Fermi level as well. DOS of C1 1band C 40structures will split into different sub-bands because of the different symmetry of structures. The stabilities of these two structures depend on the relative positions between the Fermi level and sub-bands. We provide a new criterion to judge the stabilities of different structures, and explain the ground-state structure of the most of the early transition metal disilicides. Furthermore, we predict the feasibility of controlling the microscopic structure by adding the quaternary component according to the criterion which is presented in the work.
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