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arXiv · 2610.25295

Instability-driven segregation of V group transition metals on HEB$_2$ grain boundaries influencing enhanced mechanical properties

Daniil Alekseev·Viktor S. Baidyshev·Alexander G. Kvashnin
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Abstract

Five metals share a common sublattice in the (Hf,Ta,Ti,Nb,Zr)B$_2$ (HEB$_2$). Most high-entropy materials are produced as polycrystalline materials in the form of sintered ceramics. The question of which metal species are located at grain boundaries is still unknown. Here, we calculated the grain-boundary energy $γ_{GB}$ of four types of boundaries in five binary diborides and HEB$_2$ using density functional theory (DFT). We analyzed the bonding at these boundaries using COHP and sampled cation segregation at six boundaries of HEB$_2$, ordered and general, using Monte Carlo / molecular dynamics with a MACE potential that was fine-tuned for this system. According to our results $γ_{GB}$ splits by group: the group V diborides NbB$_2$ and TaB$_2$ form boundaries with about half the energy of the group IV at three of the four types of GBs, ranging from 0.8 to 2.7 compared to 2.5--4.8~J/m$^2$. COHP shows a significant difference at the Fermi level, namely the Me--B bonds of NbB$_2$ and TaB$_2$ are antibonding at Fermi energy ($E_F$) in the bulk but bonding at the GB, while their integrated strength hardly changes. In HEB$_2$ the segregation follows a similar pattern: Nb and Ta are enriched at five of the six grain boundaries, including general boundaries, up to 23--35~at.\% at 2000~K, although the size of the atoms would favor Zr and Hf in the expanded sites. A segregated $Σ193$ grain boundary can withstand 26 against 20~GPa for a random boundary in tension at 300~K. The grain boundaries of HEB$_2$ are composed of group V metals, as suggested by the boundary energies of binaries compounds and their bonding at the Fermi level. Grain boundaries of the (Hf,Ta,Ti,Nb,Zr)B$_2$ preferentially include Nb and Ta. This lowers the grain boundary energy and strengthens the boundaries. This consists of the lower boundary energy and changes Fermi level bonding in NbB$_2$ and TaB$_2$.

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Daniil Alekseev, Viktor S. Baidyshev, Alexander G. Kvashnin. 2026-10-18. Instability-driven segregation of V group transition metals on HEB$_2$ grain boundaries influencing enhanced mechanical properties. https://arxiv.org/abs/2610.25295

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