Atomic Scale Microscopy of Zr-based Bulk Metallic Glasses Processed by Various Routes

Abstract

Bulk metallic glasses (BMGs) exhibit a rare combination of strength and toughness that is difficult to achieve by other materials. These properties make them favourable for a diverse range of engineering applications. However, their disordered amorphous structure invokes catastrophic failure with shear bands localisation, limiting their industrial development as structural materials. Moreover, it is not yet clear how to quantitatively link their microstructural features to processing and mechanical properties. The aim of this thesis was to quantitatively analyse the structural features contributing to local hardness variations in thermomechanically processed zirconium (Zr)-based BMGs. Advanced atom probe tomography (APT) techniques were used to observe structural and chemical changes in these BMGs. APT operational parameters were optimised and tested for robust data outcomes. APT cluster analysis was effectively utilised in the characterisation of nanoscale heterogeneities in the BMG microstructure. The chemical composition of the nanoscale heterogeneities was roughly Zr27Cu29Al21Ni19Nb4 (at. %) in Zr63.96Cu13.36Ni10.29Al11.04Nb1.25 (at. %), and Zr22Cu29Al17Ni23Ti9 (at. %) in Zr52.5Cu17.9Ni14.6Al10Ti5 (at. %). Their chemistry was experimentally reported herein for the first time. Additionally, an ab-initio molecular dynamic (AIMD) simulation was used to simulate the atomistic distribution in a Zr-based BMG. Clusters observed in APT assigned as MRO regions were found synonymous to the shear band nucleation zones. Beyond the novel methodological rigor introduced here, the findings provide a new, independent validation of the inverse correlation between local hardness and size of the MRO regions, with their chemical compositions, providing a novel handle on the quest for understanding microstructure- property-processing relationship in BMGs.