Comparison of thermodynamic databases for 3xx and 6xxx aluminum alloys

Abstract

Computational thermodynamics, or Calculation of Phase Diagram (CALPHAD) methods have proven useful in applications to modeling a variety of alloy properties. However, the methods are only as accurate as the thermodynamic databases they use, and two commercial thermodynamic databases exist for aluminum alloys: Thermotech and Computherm. In order to provide a critical comparison of these databases, we used both the databases to calculate equilibrium solid-state phase fractions and phase diagram isothermal sections of several industrial aluminum alloys: a 319-type and 356 cast alloys, as well as the wrought alloys 6022 and 6111. All of these alloys may be generically described as being based on the Al-Mg-Si-Cu quaternary with other additions such as Fe, Mn, and Zn. Although many of the results are consistent between the two databases, several qualitative and quantitative differences were observed. Many of these differences are found to be due to the intermetallic compounds involving Fe, Mn, Cr, and Zn. On the other hand, thermodynamics involving only phases from the Al-Mg-Si-Cu quaternary show good agreement between the databases, although some small differences still exist, particularly involving the quaternary Q phase. To understand and assess these differences, formation enthalpies and reaction energies from the databases were compared against density functional first-principles energetics. These comparisons indicate possible avenues for future improvements of Al-alloy thermodynamic databases. Finally, we demonstrate an interesting correlation between the calculated phase fractions and the measured yield strengths across this wide family of 3xx cast and 6xxx wrought alloys.