Manipulating defects in metallic glasses via ultrasonic treatment

Flow defects play an important role in shaping the mechanical behaviors of metallic glasses (MGs), especially their plastic deformation. Characterizing or manipulating these defects in MGs is of a great challenge due to the absence of the long-range order. In this work, we systematically investigated the defect activation mechanism of the La₅₅Al₂₅Ni₅Cu₁₀Co₅ medium-entropy MG under ultrasonic treatment (UT) from perspectives of order and electronic structure. The finding reveals that the ordered configurations inherited from the liquid state intend to unravel after UT, meanwhile the atomic packing rejuvenates to a more disordered loose state even though the structural ordering interrupts at high injected energies, which ultimately contributes to the strong defect relaxation with longer characteristic times as evidenced by a generalized Maxwell-Voigt model. The ²⁷Al nuclear magnetic resonance (NMR) confirms the redistribution of local clusters around Al sites during UT, along with the bonding characteristics evolving from a covalent-like to predominantly metallic state, and then towards covalent-like nature again accompanied with ordering. Simultaneously activated defects enhance the mechanical heterogeneity with the remarkable decrease of hardness and elastic modulus, thereby more significant plastic flow under multiscale defect activation. Our work provides new perspectives for dexterously modulating the plastic deformability of MGs and potentially sheds new light on the deformation mechanism of amorphous materials.