Effect of Mg Content on the Thermodynamics of Interface Reaction in Cf/Al Composite

The interface of carbon-fiber-reinforced aluminum matrix (C-f/Al) composites fabricated by the pressure infiltration method with different Mg contents (0, 3.2, 4.5, 6.5, and 8.5 wt pct) was observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The Gibbs free energy of Al4C3 and Al3Mg2 was calculated by an extended Miedema model and a Wilson equation. The effect of Mg on the critical nuclear size of Al4C3 was discussed also. The size and number of Al4C3 decreased and transformed from needle-like to blocky with Mg content, and no Al4C3 but blocky beta phase (Al3Mg2) was observed at the C-Al interface in the C-f/Al-8.5Mg composite. Increased Mg content in the Al matrix would increase the Gibbs free energy of Al4C3 but decrease that of the Al3Mg2 phase. The critical value of the Mg content, above which the formation of Al3Mg2 would be easier than Al4C3, was 8.8 wt pct when the C activity was 0.0013, which agreed well with the TEM observation. The critical nucleation size of Al4C3 increased slowly within a low Mg content (less than 8.5 wt pct) and then abruptly after that, indicating the formation of Al4C3 is inhibited significantly in thermodynamic environments. The calculated in the C-f/Al-6.5Mg composite was 26.3 nm, which was consistent with the HRTEM observation (24 nm).