Coherent phase stability in Al-Zn and Al-Cu fcc alloys: The role of the instability of fcc Zn

The coherent phase stability of fcc-based Al-Zn and Al-Cu alloys is studied theoretically by first-principles total energy calculations, a mixed-space cluster expansion approach, and Monte Carlo thermodynamic simulations. We find that a large portion of the differences between Al-Zn and Al-Cu can be explained by the differences between fcc-Zn and fcc-Cu: While Zn is stable in the hcp structure, fcc-Zn shows an instability when deformed rhombohedrally along (111). In contrast, fcc-Cu is the stable form of Cu and is elastically extremely soft when deformed along (100). These elastically soft directions of the constituents permeate the phase stability of the alloys: (111) superlattices are the lowest energy coherent structures in Al-Zn, while (100) superlattices are stable coherent phases in Al-Cu. The short-range order of both Al-rich solid solutions show clustering tendencies, with the diffuse intensity due to short-range order in Al-Zn and Al-Cu showing streaks along (111) and (100), respectively. The mixing enthalpies and coherent phase boundaries are also calculated and found to be in good agreement with experimental data, where available.