Phase equilibria at 1000 degrees C in the Al-C-Si-Ti quaternary system: an experimental approach

More than 50 powder mixtures with different compositions in the Al-C-Si-Ti quaternary system were cold-pressed, heated for 400 h at 1000 degrees C under 10(5) Pa argon (1 atm) and rapidly cooled. Phases present in the as-treated samples were characterized by X-ray diffraction, optical metallography, scanning electron microscopy and electron probe microanalysis. From the results obtained, a three-dimensional isobaric-isothermal section of the Al-C-Si-Ti phase diagram has been constructed, using the classical equilateral tetrahedron representation. Different two-dimensional sections through this tetrahedron and projections on its faces have also been drawn. These constructions are discussed with the aim to provide a comprehensive description of the phase equilibria that tend to be established in the Al-C-Si-Ti quaternary system at 1000 degrees C under a pressure of 1 arm. Among the most important results, one can mention the existence of liquid-solid phase equilibria between Al-Si base liquids and each of the solid phases Ti5Si3Cx, Ti3SiC2 and TiC. The Si contents of the Al-Si base conjugate liquids are, respectively, within the limits 8-10.5 at.%, 1-19 at.% and 0-11.5 at.%. Moreover, it is worth noting that Ti5Si3Cx conjugates with all the solid compounds of the system (Al4C3 and SiC excepted) and that the three-dimensional isobaric-isothermal section is separated into two distinct parts by a three-phase equilibrium Ti5Si3Cx-Al3Ti-TiC. The discussion also deals with the thermodynamic aspect of the high-temperature chemical interaction between silicon carbide SiC and Ti-Al alloys. (C) 1997 Elsevier Science S.A.