The melting of boron carbide and the homogeneity range of the boron carbide phase

Abstract

Various mixtures of boron and carbon were electron beam melted. The samples were characterized by electron microprobe analysis, metallographic techniques, X-ray diffraction and Knoop microhardness measurements. The metallographic investigations, together with the electron microprobe phase analysis, showed that the boron carbide phase exists in the homogeneity range 9 – 20 at.% C (with a relative accuracy of 3 – 5%). Boron carbide melts congruently and forms a eutectic between carbon-rich compounds and graphite. For the boron-rich compounds we assumed the existence of a peritectic transformation with a solid solution of carbon in the β rhombohedral boron structure. We studied the crystal lattice dimensions and the microhardness of boron carbide in the above homogeneity range. As the carbon content increased, the crystal lattice dimensions decreased and the microhardness increased regularly with an extreme value of HK 200 g = 2840 +- 60 kg mm −2 for 20 at.% C (B 4C) (after electrolytic etching).