A first-principles study of the structural, mechanical and electronic properties of precipitates of Al₂Cu in Al-Cu alloys

The properties of precipitates are important in understanding the strengthening mechanism via precipitation during heat treatment and the aging process in Al-Cu based alloys, where the formation of precipitates is sensitive to temperature and pressure. Here we report a first-principles investigation of the effect of temperature and pressure on the structural stability, elastic constants and formation free energy for precipitates of Al₂Cu, as well as their mechanical properties. Based on the formation enthalpy of Guinier-Preston (GP(I)) zones, the size of the GP(I) zone is predicted to be about 1.4 nm in diameter, which is in good agreement with experimental observations. The formation enthalpies of the precipitates are all negative, suggesting that they are all thermodynamically stable. The present calculations reveal that entropy plays an important role in stabilizing θ-Al₂Cu compared with θ_C′-Al₂Cu. The formation free energies of θ′′-Al₃Cu, θ_C′-Al₂Cu, θ_D′-Al₅Cu₃ and θ_t′-Al₁₁Cu₇ increase with temperature, while those of θ′-Al₂Cu, θ_O′-Al₂Cu and θ-Al₂Cu decrease. The same trend is observed with the effect of pressure. The calculated elastic constants for the considered precipitation phases indicate that they are all mechanically stable and anisotropic, except θ_C′-Al₂Cu. θ_D′-Al₅Cu₃ has the highest Vicker's hardness. The electronic structures are also calculated to gain insight into the bonding characteristics. The present results can help in understanding the formation of precipitates by different treatment processes.