Ab initio study of vacancies and impurities on grain boundaries in fcc Ni

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Year of publication 2019
Type Conference abstract
MU Faculty or unit

Faculty of Science

Description A detailed ab initio study of fcc Ni supercells containing lattice defects such as sigma 5(210) tilt grain boundaries (GB), impurities X (X = Al, Si) and vacancies (Va) is presented. The calculations were performed using the density functional theory by means of the VASP code and Projector-Augmented plane Wave potentials including the equilibration of structure. It turns out that the distance between the structure defects plays an crucial role in the finding of the equilibrium arrangement as the close defects may be subject to „recombination“, as it was found in case of some Ni59Si(i)+Va configurations. Here, the vacancy is occupied by interstitial Si atom resulting in the vacancy annihilation. In structures with larger distances between the Si atom and vacancy, the energy barriers prevent the migration of Si atom towards the vacancy. Comparing the volume per atom (Vat) of bulk material and material with GB, the Vat increases by the GB introduction. This effect is quantified by the excess free volume which is only slightly influenced by addition of the impurity. Analysing the interlayer distances in supercells containing the GB and impurity, a big increase in distance between the GB and layer 2 and a significant decrease in distance between the layers 2 and 3 is found in all studied configurations. The distances between next layers converge with small oscillations to the bulk value. In configurations containing the GB, impurity and Va in various layers, the largest decrease in Vat always corresponds to the most easily but still unfavourably formed vacancy. The vacancy in the GB Ni119+VaL2, GB Ni117Al2(s)+VaL2, GB Ni117Si2(s)+VaL2 and GB Ni117Si2(s)+VaL2 structure causes the attraction of Ni atoms lying in the nearest GB layer. Despite of the changes in the arrangement of the atoms in the GB X Va structures, delocalisation or instability of Va is not observed. All above mentioned structures are also studied from the energetic and magnetic point of view. The effect of arrangement and interaction of GB, impurity and Va on the magnetic moment and stability of the structures is analysed.
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