Études sur le carbure de bore iii. domaine d'existence de la phase carbure de bore

Abstract

We have studied the boron carbide phase homogeneity range in the boron-carbon system. The samples have been electron beam melted and characterised by electron microprobe analysis, metallographic techniques, X-ray diffraction and Knoop microhardness measurements. Metallographic investigations, together with the electron microprobe phase analysis, show that the boron carbide phase exists in the homogeneity range 9–10 and 20 at.%C. Boron carbide melts congruently and forms a eutectic between carbon rich compounds and graphite. For the boron-rich compounds we assume the existence of a peritectic transformation with a solid solution of carbon in the β-rhombohedral boron structure. When the carbon content increases, within the homogeneity range of boron carbide, X-ray data and Knoop microhardness measurements show that the crystal lattice dimensions decrease and the microhardness increases regularly with an extreme value of HK 200g of 2840 ± 60 kg mm −2 for 20 at.%C (after electrolytic etching).