Investigating the impact of minor alloying on shear transformation zones and strain rate sensitivity in Pd-based bulk metallic glasses via nanoindentation, correlation symmetry analysis and atom probe tomography

Abstract

Minor additions of Co or Fe significantly affect the plastic behavior of the ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{40}{\mathrm{P}}_{20}$ bulk metallic glass (BMG) (Nollmann et al., 2016). Co enhances plasticity, while Fe addition leads to early catastrophic failure. Structural analyses via variable resolution fluctuation electron microscopy showed that minor alloying profoundly affect the glassy structures beyond the typically 2 nm range (Hilke et al., 2020). Combining nano-mechanical measurements, 4D-STEM angular correlation symmetry analysis, and atom probe tomography (APT), we explore how microalloying impacts structure–property relationships in these BMGs. High-precision nanoindentation revealed two populations of shear transformation zones (STZs), each with fewer than 100 atoms. The alloy with higher plasticity had smaller STZs, whereas the less ductile alloy showed larger STZs. All samples exhibited negative strain rate sensitivity, with the Fe-containing alloy showing the highest. 4D-STEM analysis indicated increased 5-fold symmetry in the Fe-containing alloy, whereas the Co containing BMG displayed reduced 5-fold symmetry similar to the amount of 4- and 6-fold symmetries. APT mass spectra showed no oxygen species, ruling out oxidation effects. Clustering analysis of nearest-neighbor atom distributions indicated no significant Fe–Fe or Co–Co clustering, suggesting uniform dispersion. Overall, these findings shed light on how microalloying influences the microstructure, strain rate sensitivity, and macroscopic plasticity in BMGs, advancing our understanding of deformation mechanisms in BMGs.