The Q phase is an important quaternary precipitate in aluminum-based alloys that contain Mg, Si and Cu. These alloys belong to the most promising materials in automotive and aircraft engineering. Their mechanical properties are related to their microstructure. The Q phase stands at the end of a complex precipitation sequence and is considered to yield significant strengthening effects. To tailor these mechanical characteristics via heat treatments, a precise knowledge of their precipitation properties is essential.
The stability of each precipitate can be predicted by the CALPHAD method, which takes different thermodynamic data sets as input and provides temperature and/or composition dependent phase stabilities of materials. Nowadays CALPHAD input is not only obtained from experiments, but wherever there is a difficulty in performing the experiment and a lack of data, powerful ab-initio tools are used to provide the necessary data.
In order to achieve an improved thermochemical parameter set of the Q phase, we have performed first-principles and phonon calculations and combined them with CALPHAD concepts such as the compound energy formalism to investigate its Gibbs energy and heat capacity. For a fair comparison with available experimental data, the impact of lattice vibrations on the site occupancy of sublattices has also been taken into account. Excellent agreement with experimental data and previously CALPHAD assessment demonstrates the high predictive power of our first-principles approach .