Hugoniot measurements on the highly dense, pure B4C polycrystal were performed by the inclined-mirror method to study the elastoplastic transition and to search phase transition. In inclined-mirror streak photographs, the smoothly jagged structure was observed at the free-surface shape in the plastic region. The Hugoniot-elastic limit (HEL) has been determined to be approximately 19.5GPa. In the plastic region, a kink was observed at a particle velocity of around 1.26km∕s. The shock velocity (US)–particle velocity (UP) Hugoniot relations in the plastic region were given by US=3.7+5.4UPkm∕s in the Up range of 0.54–1.26km∕s and US=9.61+0.73UPkm∕s in the Up range of 1.26–4.3km∕s. The S value (0.73) in US=C0+SUP above UP=1.26km∕s is significantly small compared with the result of Vogler et al. [J. Appl. Phys. 95, 4173 (2004)], and was much smaller than those of many oxides and nitrides. This material behaved as an elastoisotropic solid above the HEL and showed a large and linear change in the pressure-density plot above 38GPa (UP=1.26km∕s), which indicated the onset of a phase transition.

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