Thermodynamic Stability, Phase Formation, and Structural Evolution of Al-Zn-Cu Alloys: A Combined Theoretical and Experimental Investigation

Abstract

The thermodynamic characteristics are of significant importance for evaluating phase stability and understanding the relative stability of binary and ternary systems. This study investigates the thermodynamic and structural properties of Al-Zn-Cu alloys, with a particular focus on phase stability, enthalpy of formation. A semi-empirical model (Medema model) was used to estimate the enthalpies of these systems. The chemical enthalpy analysis of the Al-Cu, Zn-Cu, and Al-Zn binary systems reveals that Cu-Al and Cu-Zn exhibit negative enthalpy values, indicating a strong thermodynamic driving force for alloy formation, while the Al-Zn system displays positive enthalpy values, suggesting limited solubility. Elastic enthalpy calculations highlight significant lattice strain in the Al-Cu system due to atomic size mismatches, followed by Zn-Cu, while the Al-Zn system exhibits minimal elastic effects. Analysis of phase stability through enthalpy of formation calculations shows that intermediate compositions between Al and Cu along with Zn-rich areas achieve maximum thermodynamic stability. The empirical results show that after five hours of mechanical alloying of the Al₈₀Zn₁₀Cu₁₀ system yields a stable α-Al (Zn, Cu) solid solution with a face-centered cubic (FCC) structure formed and matches well with theoretical expectations. The nanoparticles exhibit irregularly shaped and a propensity to agglomerate together as a result of cold-welding during milling, according to FESEM (Field Emission Scanning Electron Microscopy) photos. The rod-like structures at the nanoscale indicate the formation of a secondary phase, which improves the hardness and wear resistance. The particle size distribution shows a successful decrease to an average of 33.112 nm with a consistent size spread, and EDX analysis validates the predicted elements composition.