Rapidly solidified structure of alloys with up to eight equal-molar elements-a simulation by molecular dynamics

Alloys with equal-molar elements (so-called high-entropy alloys) designed since 1995 are unique in many aspects and possess extraordinary structural and functional properties. In this study, structure evolutions were simulated for alloys with two to eight equal-molar elements Ni, Al, Cu, Co, Ti, V, Zn, Zr (in sequence) as being molten, rapidly solidified (at 2 x 1013 K s-1), and annealed at 900 K, respectively. The simulation was done using molecular dynamics with tight-binding potential energy and the Verlet algorithm, taking into consideration the difference in crystal structure and atomic size of the constituent elements. The obtained radial distribution function (RDF) was quantitatively analyzed. Three factors obviously dominating are the number of elements (n), the size of constituent elements, and temperature. For n less than four the melt-quenched alloys tend to form amorphous structure; however, when n is five and more, the alloys show a liquid-like solidified structure. The annealing at 900 K results in a higher degree of order for amorphous structure in alloys with n≤6, while it makes alloys with n≥7 more random than those of their original quenched state. The hard ball model is proposed to explain the evolution of amorphous and liquid-like structures. The mechanism of the annealing effect is elucidated by the competition between the dense packing (for lowering enthalpy energy) and the randomness of atoms (for increasing TΔS) driven by thermal energy