#### Project Articles

Theme Articles on Mechanical Behaviors of Disordered Solids

 Select STRUCTURE AND TOUGHNESS MODULATION OF A Zr$_{52.5}$Cu$_{17.9}$Ni$_{14.6}$Al$_{10}$Ti$_{5}$ METALLIC GLASS BY SURFACE MECHANICAL ATTRITION TREATMENT 1) Chen Ken, Huang Bo, Wang Qing, Wang Gang Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (2): 400-407.   DOI: 10.6052/0459-1879-20-030 Abstract （292）   HTML （4）    PDF（pc） （17792KB）（97）       Save As a new type of structural material, the toughness of metallic glasses (MGs) needs to be further improved. The methods of improving the toughness of MGs include introducing dendrite phase, tuning their compositions to change the Poisson's ratios in order to affect the formation and spread of shear bands and cracks, {etc}. In this paper, we use the method of surface mechanical treatment to alter the microstructure and toughness of MGs. A Zr$_{52.5}$Cu$_{17.9}$Ni$_{14.6}$Al$_{10}$Ti$_{5}$ (at. %) MG (Vit105) plate was prepared by arc melting in vacuum and centrifugal casting system for thin plates in the metastable state. Surface mechanical attrition treatment (SMAT) is introduced to form nanoscale local crystal-like ordered structure in Vit105. Through differential scanning calorimetry and nano-indentation experiments, we find that the relaxation enthalpy near the surface of the SMAT-treated Vit105 plate is reduced, and its microstructure is more homogenous and stable The analysis by Vickers hardness tester shows that the hardness of the regions near the surface is increased and the hardness values distribute more narrowly after the SMAT treatment. Three-point bending fracture experiment reveals that notch toughness of the plate is also improved by SMAT. By SMAT treatment, the notch toughness increases from $70.7\pm 4.7$ MPa$\cdot$m$^{1/2}$ to $112.8\pm 3.7$ MPa$\cdot$m$^{1/2}$. Meanwhile, the density of shear bands becomes larger near the fracture surface as compared to the untreated sample. The enhancement of the toughness of Vit105 plate treated by SMAT originates from the promotion of the formation of shear bands. Our studies show that surface mechanical treatment leads to the formation of local crystal-like ordered structure in MGs with the enhancement of structural homogeneity. The hardness and toughness of MGs are improved, being associated with the formation of profusive shear bands. As a novel approach of improving the properties of materials, surface mechanical treatment has a broad application prospect in future.
 Select PREDICTION OF SHEAR TRANSFORMATION ZONES IN METALLIC GLASSES BASED ON LAPLACIAN OF ATOMIC VOLUME 1) Shi Ronghao, Xiao Pan, Yang Rong Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (2): 369-378.   DOI: 10.6052/0459-1879-19-369 Abstract （269）   HTML （11）    PDF（pc） （20609KB）（81）       Save Shear transformation zone (STZ), as a basic characteristic unit of plastic events in metallic glasses (MGs), has been widely accepted by researchers, but the source of its origin and activation mechanism are still controversial. Deformation behaviours of Cu$_{64}$Zr$_{36}$ MGs under simple shear loadings are investigated using molecular simulation method in this paper. The results indicate that the activation locations of STZ are related to the initial configuration of MGs. Though the field of atomic volume and its gradient are a direct representation of the local atomic structural heterogeneity of MGs, they lack an obvious correlation to the regions of STZ activation. A new local structural parameter $\xi$ is proposed in this paper based on the initial configuration of MG to predict the potential regions of STZ. $\xi$ is the product of two factors: the Laplacian of atomic volume field (AVF) and the absolute difference between components of the gradient of AVF. Vectors of the AVF gradient present a distribution pattern of pointing inside if the Laplacian of AVF is negatively large, representing the localized soft regions in MGs. The absolute difference of AVF gradient components is used to select different patterns of the AVF gradient distribution. Furthermore, the relationship among structural parameter $\xi$, nonaffine displacement and shear localization is established, revealing that only certain patterns of AVF gradient distribution would lead to nonaffine displacements field strengthening shear localization, which is more likely to result in activation of STZs. The correlation analysis shows that the averaged spatial correlation index of $\xi$ and STZ is larger than 78%, so $\xi$ can be used as an effective parameter for predicting the activation regions of STZs in MGs. Moreover, the ideology of using Laplacian of local AVF in predicting potential STZ regions in MGs would bridge the analysis between atomic simulations of MGs, the mechanism of STZ activations and the traditional mechanical theory.
 Select DYNAMIC RELAXATION CHARACTERISTICS AND HIGH TEMPERATURE FLOW BEHAVIOR OF ZR-BASED BULK METALLIC GLASS 1) Hao Qi, Qiao Jichao, Jean-Marc Pelletier Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (2): 360-368.   DOI: 10.6052/0459-1879-20-004 Abstract （417）   HTML （17）    PDF（pc） （699KB）（209）       Save Dynamic mechanical relaxation processes of amorphous alloys are very important to understand plastic deformation, glass transition phenomenon, diffusion behavior and crystallization. How to establish the correlation between mechanical properties and mechanical relaxation modes is one of key issues. In the current research, with the help of dynamic mechanical analysis (DMA), dynamic mechanical behavior of Zr$_{50}$Cu$_{40}$Al$_{10}$ bulk metallic glass from room temperature to supercooled liquid region was probed. In parallel, based on the uniaxial tensile tests, high-temperature flow behavior of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass around glass transition temperature were investigated. Dynamic mechanical behavior and high temperature deformation behavior were discussed in the framework of quasi-point defects theory. The results demonstrated that main $\alpha$ relaxation process of metallic glass can be well described by the quasi-point defects theory. Based on internal friction of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass, activation energy of elementary movement of atoms $U_\beta$ is 0.63 eV. In addition, correlation factor $\chi$ corresponding to concentration of the quasi-point defects in solid glass remains almost constant below the glass transition temperature. When the temperature above the glass transition temperature, the correlation factor $\chi$ increases by increasing the temperature (below the crystallization temperature). Finally, high temperature flow behavior in tensile mode near the glass transition temperature of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass was studied. The normalized viscosity decreases with increasing strain rate at low temperatures or high strain rates, indicating a non-Newtonian flow behavior. Whereas Newtonian flow behavior is observed at higher temperatures and lower strain rates. The apparent viscosity is affected by temperature and strain rate. High-temperature flow behavior of Zr$_{50}$Cu$_{40}$Al$_{10}$ metallic glass was described by stretched exponential function and free volume theory. Specifically, experimental master curve of the high temperature flow behavior of metallic glass is in good agreement with the prediction of the quasi-point defects theory, which provides a new insight on understanding of viscous effects during high temperature deformation of solid glasses.