Danielle Duggins

How the toughness in metallic glasses depends on topological and chemical heterogeneity

Significance This article reports and explains how the cavitation in metallic glasses is controlled by topological structure as well as chemical heterogeneity. It is shown that in the tough metal–metalloid Pd-Si metallic glass, cavitation initiation is controlled by both chemical separation and particular types of low coordination number (LCN) Pd-centered polyhedra. In contrast, cavitation in the brittle metal-metal Cu-Zr metallic glass is only governed by topological factors. A high-energy barrier to form LCN polyhedra and the process of chemical separation during cavitation initiation are believed to contribute to a higher metallic glass toughness, thereby allowing a larger plastic strain to fracture. To gain insight into the large toughness variability observed between metallic glasses (MGs), we examine the origin of fracture toughness through bending experiments and molecular dynamics (MD) simulations for two binary MGs: Pd82Si18 and Cu46Zr54. The bending experiments show that Pd82Si18 is considerably tougher than Cu46Zr54, and the higher toughness of Pd82Si18 is attributed to an ability to deform plastically in the absence of crack nucleation through cavitation. The MD simulations study the initial stages of cavitation in both materials and extract the critical factors controlling cavitation. We find that for the tougher Pd82Si18, cavitation is governed by chemical inhomogeneity in addition to topological structures. In contrast, no such chemical correlations are observed in the more brittle Cu46Zr54, where topological low coordination number polyhedra are still observed around the critical cavity. As such, chemical inhomogeneity leads to more difficult cavitation initiation in Pd82Si18 than in Cu46Zr54, leading to a higher toughness. The absence of chemical separation during cavitation initiation in Cu46Zr54 decreases the energy barrier for a cavitation event, leading to lower toughness.