The chemical behaviour of carbon fibres in magnesium base Mg-Al alloys

The chemical interaction between carbon fibres and Mg-rich Mg-Al alloys was studied at 723–1273 K using optical metallography, scanning electron microscopy and electron probe microanalysis. In a first stage, carbon fibres were heated at 723, 1000 and 1273 K with Mg-Al alloys of different compositions. Two carbide phases were identified at 1000 and 1273 K: an Al4C3 type phase with up to 6 wt.%Mg present in solid solution and a new ternary carbide with the chemical formula Al2MgC2 existing under two hexagonal crystalline varieties. In a second stage, the solid-liquid phase equilibria in the Al-C-Mg ternary system were experimentally established at 1000 K. At that temperature, all the Mg-Al alloys containing more than 19 ± 2 wt.%Al were observed to be in equilibrium with the Al4C3 type phase whereas Mg-Al alloys containing from 0.6 to 19 wt.%Al were found in equilibrium with the ternary carbide Al2MgC2. As for the Mg-Al alloys with an Al content lower than or equal to 0.6 ± 0.2 wt.%, they appeared to be in equilibrium with carbon. These thermodynamic principles being established, the extent, morphology and composition of the reaction zones formed in out of equilibrium conditions at the interface between carbon fibres and Mg-rich Mg-Al alloys were characterized. Attempts were made to determine the influence of different factors such as the fibre nature, the alloy composition, the heating time and the heating temperature on the formation and growth of the ternary carbide Al2MgC2. The observed changes were interpreted in terms of reaction mechanism and kinetics.