Unprecedented plastic flow channel in γ- B28 through ultrasoft bonds: A challenge to superhardness

A longstanding controversy remains whether γ-B28 is intrinsically superhard or not, i.e., Hv≥40GPa. Here we perform comprehensive investigations on the mechanical properties of γ-B28 to reveal the plasticity and failure mode of γ-B28 through the unique combination of microindentation experiment, the ideal strength approach, and the ab initio informed Peierls-Nabarro model. A low load-invariant hardness of ∼30 GPa is found for both polycrystalline and monocrystalline γ-B28. By carefully checking the strength anisotropy and strain facilitated phonon instability, a surprising ideal strength of 23.1 GPa is revealed along the (001)[010] slip system for γ-B28, together with an inferior Peierls stress of 3.2 GPa, both of which are close to those of B6O and ZrB12 yet much lower than those of diamond and c-BN. These results suggest that γ-B28 could not be intrinsically superhard. Atomistic simulation and electronic structure analysis uncover an unprecedented plastic flow channel through the specific ultrasoft bonding, which causes a dramatic softening of γ-B28. These findings highlight an approach to quantifying the realistic hardness by means of two plasticity descriptors beyond the elastic limit, i.e., the ideal strength approach and the Peierls-Nabarro model.