Local atomic structure of CoB-based glassy alloys: Ab initio molecular dynamics simulations

The first principle molecular dynamics simulations based on the density functional theory were performed to study the local structure of CoB-based glassy alloys. It was evidenced that the B-centered non-distorted (regular) Kasper polyhedra with a coordination number (CN) of 9-10, i.e. <0, 3, 6, 0> and <0, 2, 8, 0>, are primary structure-forming clusters in CoB-based glassy alloys. The formation of these regular clusters can be attributed to the efficient dense packing as well as the strong chemical interaction indicated by the results of chemical short range order parameter and electronic charge density. Compared with the Co65B35 glassy alloy, the Ta-bearing Co57B35Ta8 alloy has a higher fraction of Z9 and Z10 (Z = CN) Kasper polyhedra. A higher fraction of these regular Kasper polyhedra and their efficient and stable packing over medium range are likely to constitute the relatively stable regions which may compete with the corresponding crystals, and thus reduce atomic diffusivity and increase the viscosity of liquid/glassy structures. This result is in favor of higher thermal stability and better resistance to plastic flow. Therefore, the Co57B35Ta8 glassy alloy possesses a larger glass-forming ability and a higher mechanical strength than the Ta-free glassy alloy.