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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 x Ti2−x , Co x CrFeNiTi2−x , CoCrFe x NiTi2−x , and CoCr x FeNiTi2−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.

In the current research, the mechanical and tribological behavior, and structural evolution of AA2124-3 and 5-wt.% graphene (G) composites prepared by a combination of ball milling and hot extrusion were investigated. Properties such as macro and micro-hardness, nanohardness, tensile and lattice strain were characterized. Wear rates, coefficient of friction (COF) as well as nanoscratch testing were used to investigate the self-lubricating tribological behavior. X-ray diffraction, optical and scanning electron microscopy were employed. Hardness of the AA2124-G composite increased by 133 and 102%, the wear rates decreased by 34 and 25%, and the COF decreased by 25 and 9%, for the 3 and 5 wt.% G composites, respectively compared to the plain alloy extrudates. The AA2124-3 wt.% G composite displayed the least scratch depth and width and highest nanohardness. The uniform distribution of the 3-wt.% G-particles within the Al-matrices formed uniform G-tribofilm, which enhanced the tribological behavior of the composite.