For material science, controllable tailoring of properties is one of its driving forces. It is being achieved by designing materials, developing techniques of fabrication which is important for extreme environments applications. Currently, significant research focus is put on promising high entropy alloys (HEAs), and their modification. Especially surface treatment such as plasma nitriding of HEAs can lead to new and surprising outcomes. There is a few papers investigating nitriding of HEA. However, none of them deal with bulk material constituted by strong (Ti) and weak (Co, Cr, Fe, Ni) nitride formers as well as covalent bonding (Al) to nitrogen. 
In this work, we conducted plasma nitriding of bulk HEA AlCoCrFeNiTi0,2 to investigate the change in microstructure and mechanical properties in surface layer. Nano-indentation and micro-pillar compression was applied to test changes in local mechanical response (nano-hardness) and strength, correspondingly. FIB-milled cross-section as well as top surface were characterized with the EDS and EBSD. Also phase transformation was measured by XRD. Plasma nitriding of bulk HEA lead to complex microstructure of surface layer. Mechanical properties of the modified layer exhibited high scattering. This results are related to the porosity as well as many different precipitates. However, we were able to find areas of significant hardening. Nitriding also caused the segregation of metallic elements. The layer formed at the surface is impoverished in Al and shows uniform distribution of the remaining elements. We tentatively attributed high hardness results to the areas of thick uniform distribution without local porosity. XRD patterns confirmed the formation of multiple nitrides.
These results are promising for better understanding of nitriding of dual-phased, eutectic HEAs and we hope to develop the technology to minimize negative effects and significantly increase mechanical properties of surface layer.