Understanding the serrated flow and Johari-Goldstein relaxation of metallic glasses

• Abnormal serrated flow in a La-based metallic glass is observed
• The origin of the serrated flow of metallic glasses are systematically elucidated
• The nature of Johari-Goldstein relaxation in metallic glasses is illustrated

An increasing tendency of serrated flow with increasing loading rate of a La-based (La) metallic glass (MG) with pronounced Johari-Goldstein relaxation in nanoindentation is investigated, which is abnormal compared to precedent observations where the serrated flow normally decays with increasing loading rate as discovered in the referenced Zr-based (Zr) and Pd-based (Pd) MGs. Based on systematic analysis, a unified dimensionless number is proposed to elucidate the different origins of the abnormal serrated flow of the La MG and the normal serrated flow of the Zr and Pd MGs. The abnormal serrated flow behavior of the La MG provides critical implications in understanding the role of Johari-Goldstein relaxation in the flow of metallic glasses.