CALPHAD-informed phase-field model for two-sublattice phases: η-phase precipitation in Al-Zn-Mg-Cu alloys
In this project we developed a phase-field model capable of describing multi-component and multi-sublattice ordered phases, by directly incorporating the compound energy CALPHAD formalism based on chemical potentials. We investigated the complex compositional pathway for the formation of the η-phase in Al-Zn-Mg-Cu alloys during commercial multi-stage artificial ageing treatments.
Corrosion resistance in high strength 7xxx aluminium alloys strongly depends on the substitutional occupancy of Zn by Cu and Al in the multi-sublattice structure of strengthening η-phase. Predicting compositional evolution in these alloys is thus critical for the design of new generation corrosion resistant variants. Using the integrated CALPHAD/phase-field model developed in DAMASK, we have systematically investigated the influence of alloy composition, solute diffusivity, and heat treatment parameters on the microstructural and compositional evolution of η-phase precipitates from a thermodynamic and kinetic perspective. Furthermore, the predicted results have been compared to electron probe microanalysis validation data.
The phase-field prediction shows that the compositional pathway of the η-phase formation during the ageing process in all the three alloys (AA7050, AA7010, AA7085 alloys) was characterised by the simultaneous enrichment of Cu and Al and gradual depletion of Zn in the η-phase. Based on the comparison of the η-phase growth kinetics and the matrix residual solute evolution, we found that Zn exhaustion mainly controlled the η-phase growth process during the early stage of ageing, resulting in fast η-phase growth kinetics, enrichment of Zn in the η-phase, and an excess in the residual Cu solute in the matrix. The gradual enrichment of Cu in the η-phase subsequently occurred during the later ageing stage and was in principle kinetically controlled, due to the slower diffusivity of Cu relative to Zn in the matrix. Two-stage ageing simulation implies that Zn in the matrix has been almost completely depleted after the first ageing step for 24 hours at 120 °C, while Cu remained highly supersaturated in the matrix. The second higher-temperature ageing stage at 180 °C thus substantially enhanced the incorporation of Cu atoms from the supersaturated matrix into the η-phase.
The model developed have been implemented into the free and open-source software package, DAMASK (Düsseldorf Advanced Material Simulation Kit).
Software available under https://git.damask.mpie.de.