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

Abstract

A longstanding controversy remains whether $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$ is intrinsically superhard or not, i.e., ${H}_{v}\ensuremath{\ge}40\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Here we perform comprehensive investigations on the mechanical properties of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$ to reveal the plasticity and failure mode of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$ 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 \ensuremath{\sim}30 GPa is found for both polycrystalline and monocrystalline $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$. 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 $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$, together with an inferior Peierls stress of 3.2 GPa, both of which are close to those of ${\mathrm{B}}_{6}\mathrm{O}$ and $\mathrm{Zr}{\mathrm{B}}_{12}$ yet much lower than those of diamond and c-BN. These results suggest that $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$ 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 $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{B}}_{28}$. 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.