Synthesis, Crystal Structure, and Properties of MgxB50C8 or Mgx(B12)4(CBC)2(C2)2 (x = 2.4−4)

Activating Mobile Dislocation in Boron Carbide at Room Temperature via Al Doping

Dislocation glide, deformation twinning, and phase transition are critical mechanisms resulting in irreversible plastic deformations of materials. Because of the lack of dislocation movement, superhard ceramics generally exhibit brittle failure at room temperature. Here, by employing molecular dynamics simulations using a machine-learning force field, we reveal several plastic deformation mechanisms in superhard boron carbide as a small amount of aluminum (Al) is doped. Under shear deformation, dislocation nucleation and glide occur in Al-doped boron carbide (B_{12}-CAlC) due to the breakage of weakened chain bonds rather than the disintegration of icosahedral clusters. The dislocation activities then cause twin boundaries to migrate, thereby mitigating amorphization and enhancing ductility. Furthermore, the mobile dislocation with the Burgers vector of b=⟨11[over ¯]0⟩{111} is observed in the tensile nanopillar, which is well consistent with the experiment. This Letter demonstrates that mobile dislocation could be activated in superstrong covalent materials through a simple doping strategy.

Synthesis and crystal structures of the new ternary borides Fe3Al2B2 and Ru9Al3B8 and the confirmation of Ru4Al3B2 and Ru9Al5B8−x (x≈2)

Single crystals of the new ternary borides Fe3Al2B2 and Ru9Al3B8 were obtained from the elements at 1900°C. Both compounds represent new structure types which combine well-known features of binary and ternary borides of transition metals in combination with aluminum. The crystal structure of Fe3Al2B2 (P2/m, Z=2, a=5.724, b=2.857, c=8.723 Å, β=98.57°) contains tetramers of face-sharing trigonal prisms BFe6 with a B4 unit in trans-configuration. The tetrameric units are separated by Al-atoms which occupy all remaining rectangular sites of the trigonal prisms. The structure can be derived from Fe2AlB2 by insertion of additional FeAl fragments in a bcc arrangement. The crystal structure of Ru9Al3B8 (P6̅2m, Z=1, a=9.078, c=2.913 Å) combines zig-zag chains of boron atoms made of face-sharing trigonal prisms BFe6 and isolated BFe6 units. Three of these chains are connected by common corners to rods running in direction [001]. The rods are linked to a three-dimensional framework by isolated prisms via common edges. Again, Al occupies the capping positions of the trigonal prisms. Ru9Al3B8 is the second representative for the combination of boron zig-zag chains and isolated B atoms. The existence of Ru4Al3B2 (P4/mmm, Z=2, a=8.515, c=2.924 Å) and Ru9Al5B8−x (P4/m, Z=1, a=8.741, c=2.923 Å) were confirmed and the crystal structures refined. High quality data reveal a stoichiometric composition for Ru4Al3B2, while in Ru9Al5B8−x there is a significant underoccupation (i.e. x≈2) of the central boron site within the B4 units. The crystal structures of all four compounds represent examples for the combination of CsCl and AlB2 fragments as they were frequently found for ternary borides of transition metals.