Chemical reactivity of aluminium with boron carbide

The chemical reactivity of boron carbide (B4C) with metallic aluminium (Al) was studied at temperatures ranging from 900 to 1273 K (627–1000 °C). Al–B4C powder mixtures were cold pressed, heated for 1–450 h under 105 Pa of purified argon and characterized by X-ray diffraction (XRD) optical metallography (OM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Whatever the temperature in the investigated range, B4C has been observed to react with solid or liquid Al. As long as the temperature is lower than 933 K (660 °C), i.e. as long as Al is in the solid state, interaction proceeds very slowly, giving rise to the formation of ternary carbide (Al3BC) and to diboride (AlB2). At temperatures higher or equal to 933 K, Al is in the liquid state and the reaction rate increases sharply. Up to 1141 ± 4 K (868 ± 4 °C), the reaction products are Al3BC and AlB2: at temperatures higher than 1141 K, Al3 BC is still formed while Al3B48C2 (β-AlB12) replaces AlB2. In the three cases, interaction proceeds via the same mechanism including, successively, an incubation period, saturation of aluminium in B and C, nucleation and growth by dissolution–precipitation of Al3BC and a C-poor boride and, finally, the passivation of B4C by Al3BC. These results are discussed in terms of solid–liquid phase equilibria in the Al–B–C ternary system, with reference to the binary invariant transformation: α-AlB12 + L ⇔ AlB2, which has been found to occur at 1165 ± 5 K (892 ± 5 °C).