Formation of ordered/disordered nanoparticles in FCC high entropy alloys

This study comprises the microstructure characterization of four as-cast CoCrFeNiAl0.3, CoCrFeNiTi0.3, CoCrFeNiMo0.3 and CoCrFeNiAl0.3Mo0.1 FCC high entropy alloys. Massive nanoparticles are observed in the CoCrFeNiAl0.3, CoCrFeNiTi0.3 and CoCrFeNiAl0.3Mo0.1 alloys, whereas no nanoparticle is found in the CoCrFeNiMo0.3 alloy. These nanoparticles are multi-elemental solid solutions instead of inter-metallic compounds, and the crystal structure possesses both the ordered L12 and the disordered FCC structures. It is suggested that the formation of nanoparticles is dominated by the effects of the high mixing entropy, the sluggish cooperative diffusion of atoms, and the relatively negative mixing enthalpies of the atomic pairs between Al (or Ti) and other compositional elements. Furthermore, the formation of L12 ordered nanoparticles should fulfill the criteria of alloy composition having both more than 25 at.% of Ni (Ni could be partially substituted by Co) and at least 6.25 at.% of Al or Ti.