Thermodynamics of the effect of alloying on phase formation during crystallization of aluminum matrix composites with exogenous reinforcement

Abstract

A thermodynamic assessment of the effect of alloying elements (Si, Mg, Cu, Ti) on phase formation processes during the production and liquid-phase processing of cast aluminum matrix composite materials with exogenous reinforcement (Al–SiC, Al–B₄C) was carried out. It was shown that without suppressing Al–Si–C and Al₄C₃ carbide formation in the range of carbon concentrations from 0 to 4.5 wt.%, the equilibrium phase composition of Al–SiC composites in the solid state at 423 to 575 °C lies in the (Al) + Si + Al₄SiC₄ three-phase region, and the Al₄SiC₄ ternary carbide is replaced by the Al₈SiC₇ compound at a temperature below 423 °C. SiC and B₄C phases in Al–SiC–Cu and Al–B₄C–Cu systems are stable in the entire crystallization range and do not interact with aluminum or copper. In the Al–SiC–Mg system, the crystallization of composites containing more than 0.58 wt.% magnesium ends in the (Al) + Al₃Mg₂ + SiC + Mg₂Si four-phase region. In the Al–SiC–Ti system, the end of crystallization is observed in the (Al) + Al₃Ti + SiC three-phase region. In the Al–B₄C system, once Al₄C₃ phase formation is suppressed, aluminum borides are formed with a deviation from the concentrations of elements providing 10 vol.% B₄C towards boron increase and free carbon is formed with a deviation towards boron decrease. Under equilibrium conditions, Al–B₄C–Si system crystallization ends in the (Al) + B₄C + AlB₁₂ + Al₈SiC₇ four-phase region (at a silicon content of up to 0.67 wt.%, and in the (Al) + Si + AlB₁₂ + Al₈SiC₇ region at a higher silicon content. In the Al–B₄C–Ti system, crystallization ends in the (Al) + TiB₂ + B₄C three-phase region at a titanium content of less than 0.42 wt.%.