Effect of Mg content on the thermodynamics of interface reaction in C _f/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 Al ₄C ₃ and Al ₃Mg ₂ was calculated by an extended Miedema model and a Wilson equation. The effect of Mg on the critical nuclear size of Al ₄C ₃ was discussed also. The size and number of Al ₄C ₃ decreased and transformed from needle-like to blocky with Mg content, and no Al ₄C ₃ but blocky β phase (Al ₃Mg ₂) 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 Al ₄C ₃ but decrease that of the Al ₃Mg ₂ phase. The critical value of the Mg content, above which the formation of Al ₃Mg ₂ would be easier than Al ₄C ₃, was 8.8 wt pct when the C activity was 0.0013, which agreed well with the TEM observation. The critical nucleation size of Al ₄C ₃ t*Al ₄C ₃ increased slowly within a low Mg content (less than 8.5 wt pct) and then abruptly after that, indicating the formation of Al ₄C ₃ is inhibited significantly in thermodynamic environments. The calculated tAl ₄C ₃ in the C _f/Al-6.5Mg composite was 26.3 nm, which was consistent with the HRTEM observation (24 nm).