Segregation and aggregation behavior of impurity atoms at grain boundaries of BeO: A first-principles study

The effect of segregation behavior of common impurity atoms (including Mg, Al, Si, S, Ca, Fe) and transmutationatoms (H, He) on the performance of grain boundary (GB) in Beryllium oxide (BeO) has been systematically investigated by first-principles calculations. The uncharged vacancy defects play a dominant role in the formation of defects. Besides, it is also found that O vacancy is more easily formed in BeO bulk. Most impurity atoms prefer to occupy the substitutional site of Be atom, where Mg atom have the lowest solution energy of 1.18 eV. Of all impurity atoms, S atom is the most prone to dissolution in BeO bulk and have no obvious tendency of solution in different sites. Most impurity atoms tend to segregate towards GBs, which will significantly influence stability and strength of GBs. It is found that the segregation of all impurity atoms has a deterioration effect on the stability of GBs. The strengthening/embrittlement energies of the impurity atoms Al and Fe at ∑7[001](210) GB are -5.672 eV and -4.774 eV, respectively, which contribute to increasing the fracture strength of GB. On the contrary, the strengthening/embrittlement energy of Si atom at ∑31[001](610) GB is 2.645 eV, so that the segregation of Si atom leads to the embrittlement of ∑31[001](610) GB. Especially, the transmutation H and Heatoms are more prone to segregate towards GBs, and their strengthening/embrittlement energies at three GBs are all positive, which easily induce the embrittlement of GBs and the significant reduction of fracture toughness. Further, the aggregation of H/He atoms can aggravate the deterioration of microstructure and performance of GBs, which could be the source of swelling and powdering in BeO under neutron irradiation environment. These results provide important theoretical insights for the design and optimization of high-performance BeO ceramic, particularly in enhancing fracture toughness and preventing embrittlement.

本研究通过第一性原理计算，系统探究了常见杂质原子（包括Mg、Al、Si、S、Ca、Fe）与嬗变原子（transmutation atoms）对氧化铍（beryllium oxide，BeO）晶界（grain boundary，GB）性能的影响。中性空位缺陷在缺陷形成过程中占据主导地位。此外，研究同时发现氧化铍体相内氧空位的形成更为容易。多数杂质原子倾向于占据铍原子的取代位点，其中镁原子的固溶能最低，为1.18 eV。在所有杂质原子中，硫原子最易溶于氧化铍体相，且在不同位点的固溶倾向并不显著。多数杂质原子倾向于向晶界偏聚，这将显著影响晶界的稳定性与强度。研究发现，所有杂质原子的偏聚均会对晶界稳定性产生劣化作用。杂质原子Al与Fe在∑7[001](210)晶界处的强化/脆化能分别为-5.672 eV与-4.774 eV，这有助于提升晶界的断裂强度。与之相反，硅原子在∑31[001](610)晶界处的强化/脆化能为2.645 eV，因此硅原子的偏聚会导致∑31[001](610)晶界发生脆化。尤为值得注意的是，嬗变原子H与He更易向晶界偏聚，且它们在三种晶界处的强化/脆化能均为正值，极易引发晶界脆化与断裂韧性的显著降低。进一步而言，氢/氦原子的聚集会加剧晶界微观结构与性能的劣化，这可能是中子辐照环境下氧化铍产生肿胀与粉化的根源。上述研究结果为高性能氧化铍陶瓷的设计与优化提供了重要的理论参考，尤其在提升断裂韧性与防止晶界脆化方面具有关键指导价值。