An atomic-scale insight into the effects of hydrogen microalloying on the glass-forming ability and ductility of Zr-based bulk metallic glasses

Notable improvements in both the glass-forming ability (GFA) and ambient temperature compressive ductility of Zr-based metallic glasses due to hydrogen microalloying have been reported by Granata et al. [1]. These two aspects of amorphous materials are considered to be mutually opposing properties in a metallic glass-forming system, usually explained via structural short-range order, local symmetry, structural free-volume and bonding character. To fundamentally understand why hydrogen simultaneously enhances GFA and ductility, ab initio molecular dynamic simulations were carried out on a Zr64Cu22Al12 alloy to assess how this element affects the local atomic configuration, electronic structure, chemical bonding and atomic transport processes. It is revealed that, while the addition of 1.5 at.% H decreases the icosahedral order of the supercooled liquid over the entire temperature range studied, it decreases the diffusivity of the metallic constituents over the temperature range where heterogeneous nucleation is expected to occur. In this tradeoff between local structural stability and depressed crystal nucleation dynamics, the latter dominates in the H-containing alloy and the capacity for glass formation is enhanced. In addition, H was found to increase iconicity of the base glassy alloy, promote heterogeneity in the local structure and increase metallicity. In the competition between these opposing tendencies of chemical bonding and local structure to determine mechanical properties, the latter prevails and explains the experimentally-observed improvement in compressive ductility. (C) 2016 Elsevier B.V. All rights reserved.