Speaker
Description
Introduction. Grain boundaries (GB) represent an important class of two-dimensional extended defects and macroscopic strength of polycrystalline materials depends strongly on GB cohesion. It was found that the impurities in ppm concentration can drastically change material properties. For example, in magnetic materials they can significantly change magnetic moments in the GB region. Intergranular embrittlement which is usually associated with segregation of impurities on the GB can result in a dramatic reduction of the ductility and strength. Here we present an ab initio study of $\Sigma$5(210) grain boundary in Ni$_3$Fe compound with two different interface stoichiometries, $\Sigma5(210)^{\mathrm{Fe,Ni}}$ with both Fe and Ni atoms at the GB and $\Sigma5(210)^{\mathrm{Ni,Ni}}$ with Ni atoms only. We consider both clean GB and the GB with segregated Al and Si.
Summary. Our calculations show that the $\Sigma5(210)^{\mathrm{Fe,Ni}}$ has the GB energy of 1.34 J/m$^2$ and an additional volume per unit GB area of 0.50 $\unicode{xC5}$ while the $\Sigma5(210)^{\mathrm{Ni,Ni}}$ exhibits a little bit higher GB energy of 1.43 J/m$^2$ and an additional volume per unit GB area of 0.51 $\unicode{xC5}$. Here we are not able to decide which interface stoichiometry, $\Sigma5(210)^{\mathrm{Fe,Ni}}$ or $\Sigma5(210)^{\mathrm{Ni,Ni}}$, is more stable. The energy difference between them is only 4.5 meV/atom in favor of the $\Sigma5(210)^{\mathrm{Fe,Ni}}$ configuration. Whereas magnetic moments of Ni atoms at the clean GBs are slightly decreased with respect to the bulk (less than by 0.1 $\mu_{\mathrm{B}}$), there is a slight enhancement (by 0.11 $\mu_{\mathrm{B}}$) of magnetic moment of Fe atoms at the clean $\Sigma5(210)^{\mathrm{Fe,Ni}}$ GB. On the other hand, in the neighborhood of $\Sigma5(210)^{\mathrm{Ni,Ni}}$ GB the magnetic moments of Fe atoms are decreased by about 0.18 $\mu_{\mathrm{B}}$. Segregated Al and Si impurities reduce the magnetic moments of Fe and Ni atoms in the region of GB up to 0.4 $\mu_{\mathrm{B}}$ with respect to the bulk.
Acknowledgement. This research has been financially supported by the Ministry of Education, Youth and Sports of the Czech Republic under the Project CEITEC 2020 (Project No. LQ1601), by the Czech Science Foundation (Project No. GA16-24711S) and by the Academy of Sciences of the Czech Republic (Institutional Project No. RVO:68081723). Computational resources were provided by the Ministry of Education, Youth and Sports of the Czech Republic under the Project IT4Innovations National Supercomputer Center (Project No. LM2015070) within the program Projects of Large Research, Development and Innovations Infrastructures.
