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 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 β 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 \( \left( {t_{{{\text{Al}}_{4} {\text{C}}_{3} }}^{*} } \right) \) 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 \( t_{{{\text{Al}}_{4} {\text{C}}_{3} }}^{*} \) in the C_f/Al-6.5Mg composite was 26.3 nm, which was consistent with the HRTEM observation (24 nm).