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Description
Thermodynamic stability of metallic materials and their many properties are strongly affected by presence of defects in crystal lattices. An analysis of changes in interatomic bonding induced by defects is required to reveal the exact nature of defect-related phenomena. Ab initio atomistic simulations based on density functional theory are an ideal tool for such task. Information about “bond stiffness” for a particular pair of atoms can be obtained from phonon calculations by projections of the force constants on the unit vector along each bonding direction. A deeper insight into mutual chemical interaction between individual atoms can by provided by the analysis of crystal orbital Hamilton population (COHP). In the present work, we analyze the first- and second-nearest-neighbor interactions in (i) the vicinity of Sn substitutional impurity in Mg$_{2}$Ge intermetallic with antifluorite structure and (ii) Σ5(210) grain boundaries (GBs) in Ni$_{3}$Si intermetallic with Cu$_{3}$Au structure. In addition, we also analyze the effect of Al impurity segregated in Ni$_{3}$Si GB.
The bond stiffness of bonds between the Sn impurity atom and its first nearest neighbors, i.e. Sn-Mg bonds, is significantly higher than bond stiffness of Mg-Ge bonds in bulk Mg$_{2}$Ge. This increase corresponds very well to the reported higher heat capacity of Mg$_{2}$Sn compare to Mg$_{2}$Ge. The studied Ni$_{3}$Si GB variant containing both Ni and Si atoms at the interface is shown to be unstable with respect to a shear deformation. The COHP and bond stiffness analysis reveal that this instability originates in a weak interaction far from the GB interface between the Ni atoms in the 3rd plane and the atoms in the 4th, 5th, 6th plane. However, this bond weakening is a consequence of a very strong interaction between the Si atoms in the GB plane and Ni atoms in the 3rd plane of the GB interface. The described bond weakening was not observed in stable GB variant containing two Ni atoms at the interface and when Si atom at the interface is replaced by Al.
