Stable and metastable phase equilibria in the chemical interaction between aluminium and silicon carbide

Abstract

An experimental investigation was carried out on the Al-C-Si ternary system under atmospheric pressure and at temperatures up to 1900 K. From the results obtained, a thermodynamic model based on stable and metastable phase equilibria in the Al-C-Si ternary system was set up in order to provide a general description of the chemical interaction between aluminium and SiC. According to this model, aluminium and SiC are in thermodynamic equilibrium at every temperature lower than 923 K. At 923±3 K, i.e. at 10 K below the melting point of pure aluminium, a quasiperitectic invariant transformation occurs in the Al-C-Si system. In this transformation, solid aluminium reacts with SiC to give Al₄C₃ and a ternary (Al-C-Si) liquid phase. The carbon content of this liquid phase is very low; its silicon content is 1.5±0.4 at%. From 923 to about 1620 K, aluminium partially reacts with an excess of SiC, leading to a metastable monovariant equilibrium involving SiC, Al₄C₃ and an aluminium-rich (Al-C-Si) ternary liquid phase, L. The carbon content of this liquid phase, L, remains very low whereas its silicon content increases with temperature from 1.5±0.4 at% at 923 K to 16.5±1 at% at 1620 K. In the temperature range 1670 to 1900 K, two other three-phased monovariant equilibria can be reached by reacting aluminium and SiC. These equilibria involve on the one hand SiC, Al₄SiC₄ and a liquid phase, L′, and on the other hand, Al₄SiC₄, Al₄C₃ and a liquid phase, L″. The former is a stable equilibrium, the latter is a metastable one. At temperatures higher than about 2200 K, the latter metastable equilibrium is replaced by two monovariant stable phase equilibria including the ternary carbide Al₈SiC₇.