By introducing the concept of high-entropy alloys (HEAs) and the more general compositionally complex alloys (CCAs), a whole new range of materials with individual combinations of properties has been proposed and is being researched successively. 
Precipitation hardening is largely responsible for the high-temperature strength of Ni- and Co-based superalloys. The microstructure of these alloys consists of a disordered FCC matrix (γ) and ordered coherent L₁₂ precipitates (γ’). The same approach has been used to design HEAs for high temperature applications based on the CoCrFeNi system. In this alloy system the formation of the γ′ precipitates is induced through the addition of Al and Ti. These so-called high entropy superalloys (HESAs) have the potential to exceed the high temperature strength and stability of Ni-based superalloys while also offering a lower density.

Laser powder deposition was applied to build wall structures with gradient composition in the Al-Co-Cr-Fe-Ni-Ti system. Besides studying the influence of Al and Ti on precipitation hardening, it is of interest to partially substitute the critical element Co by Ni and if possible Fe. Therefore, pre-alloyed CoCrFeNi powder was in situ mixed with a fixed content of Al and Ti powder and varying content of Ni or Ni₂Fe powder. 

The primary scope of the investigations was to study the influence of the aging temperature on the microstructure and the mechanical properties of the designed alloys. Thus, the alloys were precipitation hardened with different aging temperatures from 650 °C to 900 °C. The aged alloys were analysed using high-resolution scanning electron microscopy (SEM) including EDS and EBSD techniques. Micro-hardness testing was used to evaluate the influence of the heat treatment on the mechanical performance of the alloys.

The results demonstrated that not only the alloy composition, but also the aging temperature strongly influences the microstructure by changing the fraction of the phases and the precipitation size. Therefore, it is very beneficial to adjust the heat treatment and find the best parameters for the individual alloy compositions to maximize the mechanical performance of the HESAs.