Partial disordering and homogeneous melting in multicomponent systems

Melting is a topological order-to-disorder transition where the crystal becomes disordered and turns into a liquid. In pure systems, melting is related to positional disordering only, whereas in multicomponent systems, it is affected also by the chemical component. Here, we report a first investigation of this important open issue in homogeneous melting in a five component model system, or high entropy alloy, with a particular focus on the atomic mechanisms. We show that melting proceeds with several stages dictated by the low melting point component: Partial disordering starts at a much lower temperature below the bulk melting point with the low melting point element executing an exceedingly large atomic displacement. Instead causing melting, the displaced element catalyzes the formation of mobile atomic chains and loops that still conform to the crystalline lattice. With increasing temperature, other elements gradually participate in these highly correlated atomic configurations, causing their growth and proliferation, and eventual formation of the liquid phase. The detailed atomic process provides a direct support for the recently proposed melting mechanisms involving the atomic chains and loops, rather than the Lindemann critical vibrational displacements.