Temperature-dependent enthalpy and entropy stabilization of solid solution phases in non-equiatomic CoCrFeNiTi high entropy alloys: Computational phase diagrams and thermodynamics

Research interest in multi-principal element high entropy alloys (HEAs) has increased drastically since the field was first formally introduced in 2004. Since then, HEAs have become important candidate materials for many key applications. However, despite the progress made in this field, there remains much ambiguity surrounding HEA phase stabilities. To that end, the calculation of phase diagrams (CALPHAD) method was used to construct extensive temperature-composition phase diagrams of the CoCrFeNi_xTi_2-x, Co_xCrFeNiTi_2-x, CoCrFe_xNiTi_2-x, and CoCr_xFeNiTi_2-x HEA systems. Due to its potentially favorable properties, the current work was focused on the single face-centered cubic (fcc) solid solution phase and an extensive thermodynamic analysis was carried out to examine the underlying thermodynamic factors of its stabilization. The mixing enthalpies and entropies of the alloys in the studied systems were calculated, where it was found that the single fcc solid solution phase can be either enthalpy- or entropy-stabilized depending on the temperature. The deviation of these quantities from the ideal solid solution thermodynamic behavior was considered, and it was found that close to and within the single fcc solid solution regions, the deviation is smallest in all systems. Furthermore, a preliminary exploration of the impact of interstitial nonmetals such as C, N, and O showed noticeable alteration of the phase equilibria of the studied systems. This work emphasizes the importance of exploring non-equiatomic compositions of HEAs as well as the necessity of a comprehensive thermodynamic analysis to understand HEAs phase stabilities.