Boron carbide under torsional stress at 5 GPa combined to density functional theory (DFT) calculations: evidence of the formation of chain vacancies in the plastic regime driven with the rotating tomography Paris Edinburgh cell (RoToPEC)

The behavior of boron carbide under stress/deformation has been a long-standing puzzle with, on the one hand, outstanding static mechanical properties, and on the other one, a shear strength in the shocked state that rapidly decreases beyond the Hugoniot elastic limit, resulting in premature failure of the material as the shock stress reaches a threshold value of 20 GPa. Several explanations have been put forward: phase transition, occurrence of shear bands containing amorphous solid, formation of chain vacancies followed by carbon-carbon bond reformation at high pressure. 

In the present work, damage and point defects generated with the RoToPEC have been identified at ambient pressure by energy dispersive X-ray microdiffraction, Raman spectroscopy, and DFT calculations of atomic structures and phonon frequencies.

We show that apart from the signals due to B4C, new peaks appear in both characterisation methods, and that most of them can be attributed to boron atom vacancies in the intericosahedral chains of boron carbide, as predicted in Ref. [1]. Some of the Raman spectra also show peaks that have been attributed to amorphous boron carbide in the literature. Deformed boron carbide thus shows small inclusions of clusters of B4C, with chain vacancies, and/or small zones interpreted as amorphous zones [2].