Defects Activation in the La₅₅Al₂₅Ni₅Cu₁₀Co₅ Medium-Entropy Metallic Glass Under Ultrasonic Vibration

Flow defects are considered to be fundamental for determining the mechanical behavior of metallic glasses (MGs), especially plastic deformability. Characterizing or operating such defects is of great challenge due to the absence of the long-range order of MGs. In this work, the nanoindentation behaviors of ultrasonic-vibrated La₅₅Al₂₅Ni₅Cu₁₀Co₅ medium-entropy (Δ<i>S_conf</i>= 1.21R) MG with different energy states of 5, 10, 15, and 30 J were systematically investigated. It is found that the hardness and elastic modulus remarkably decrease after ultrasonic treatment in comparison with the as-cast sample, particularly at high energy states of 15 and 30 J, which is mainly ascribed to the enhancement of microstructural heterogeneity. With the enhance of applied ultrasonic energy from 5 to 15 J, the nanoindentation displacement of the La₅₅Al₂₅Ni₅Cu₁₀Co₅ MG reaches a maximum value of around 89 nm, and then slightly declines at an energy state of 30 J. To exploit the deformation mechanism of samples with various energy states, a generalized Maxwell-Voigt model was used to detect the defects activation of samples under different applied energies during the holding period. The corresponding characteristic relaxation time spectra show that the secondary peak intensity exhibits the highest value at 15 J under a high loading rate of 10 mN/s, indicating the largest number of defects that relate to the long relaxation time were activated in the La₅₅Al₂₅Ni₅Cu₁₀Co₅ MG, which is responsible for its large nanoindentation displacement. Meanwhile, the STZs calculation indicates that the samples with higher energy states exhibit larger STZs compared with those of samples with low energy states, which facilitate plastic deformation of MGs and meanwhile result in the large nanoindentation displacement. Based on the analysis of microstructure, the La₅₅Al₂₅Ni₅Cu₁₀Co₅ MG at high energy states such as 15 and 30 J, especially at 15 J, exhibits the lowest local structural order among these studied MGs, suggesting the corresponding STZs activation is easier from the potential energy landscape viewpoints, which eventually results in large nanoindentation displacement. Besides, ²⁷Al nuclear magnetic resonance (NMR) experiments were conducted to demonstrate the remarkably changes of Knight shift under different applied energies, which indicates that the distribution of local cluster around ²⁷Al sites is also changed by ultrasonic vibration. Our work realizes the tunable deformability of the La₅₅Al₂₅Ni₅Cu₁₀Co₅ MG by ultrasonic vibration with various applied energies and paves an avenue for disclosing its plastic deformation mechanism at different energy states.