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    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
    Abstract292)   HTML4)    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.

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    A GENERAL METHOD TO INTRODUCE PRE-CRACK IN BULK METALLIC GLASSES FOR PLANE STRAIN FRACTURE TOUGHNESS TEST 1)
    Xie Yiling, Liu Ze
    Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (2): 392-399.   DOI: 10.6052/0459-1879-19-377
    Abstract311)   HTML4)    PDF(pc) (19766KB)(74)       Save

    We report a simple, low-cost but robust method to introduce pre-crack in bulk metallic glasses (BMGS) for plane strain fracture toughness test. In recent years, bulk metallic glasses have shown potential and promising engineering application due to their excellent properties such as high elasticity, high strength, wear resistance and soft magnetism. As an important material parameter for engineering application, fracture toughness has also attracted great attention in the BMG community. However, there is still challenge on the fracture toughness test of BMGs because of the metastable nature and limited maximum castable size of BMGs. On the one hand, the casting induced thermal history difference and the defects such as internal micropores and impurity inclusions in BMGs, and the way of crack prefabrication will significantly affect the reliability of the fracture toughness test. On the other hand, limited by the sample size, most of the measured fracture toughness of BMGs are not the plane strain fracture toughness, resulting in the reported values showing large deviation, even for the same amorphous alloy. In this work, pre-notched BMG samples were thermoplastically compressed at the notched region to form ideal crack by creep flow. As an example, the fracture toughness of Zr-based amorphous alloy is tested by this method. The experimental results show that as the sample thickness increasing, the measured toughness decreases quickly and tends to a constant value. It should be pointed out that in the experiments, the local thermoplastic compression is designed to obtain a neck shaped pre-crack, which makes the minimum thickness of the specimen being far less than the required thickness by the plane strain fracture toughness testing standard.

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    SHEAR-BAND DYNAMICS IN METALLIC GLASSES 1)
    Dong Jie, Wang Yutian, Hu Jing, Sun Baoan, Wang Weihua, Bai Haiyang
    Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (2): 379-391.   DOI: 10.6052/0459-1879-19-378
    Abstract294)   HTML11)    PDF(pc) (13388KB)(139)       Save

    Shear banding is a localized plastic deformation form that wildly exist in amorphous systems, and plays the decisive role in controlling the failure and plasticity. Due to the localization in space and instantaneous in motions, clarifying the map of shear bands is still a challenge mission. For traditional amorphous systems as rocks, colloid, oxide glasses and polymers, the studies on shear bands are impeded due to the poor mechanical properties or complex structures of these systems. In recent years, the developments of metallic glasses (MGs) overcome this barrier with the abundant mechanical experiments on shear bands in laboratories, which greatly promotes the insights to shear bands. In addition, the dynamics of shear bands have a significant influence to the mechanical properties and behaviors of MGs, which is also of significant importance for understanding the deformation mechanisms of MGs. The insights for the structural origin, morphology, affect area, properties and motions of shear bands has made great progress. With the investigations on MGs, shear mands are found inhomogenous in space distribution and non-steady in time. The behaviors of shear bands show a similarity with that of the complex systems in nature as well as physic areas. This article mainly reviews our recent studies on the complex behaviors of shear bands in MGs, including the serration follow behaviors and the self-organizing criticality (SOC) behaviors of shear bands. The models that quantify these behaviors of shear bands, such as stick-slip model, machine-sample coupling model, are also introduced. The review also identified the key questions remaining to be answered, and presents an outlook for the field.

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    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
    Abstract269)   HTML11)    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.

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    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
    Abstract417)   HTML17)    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.

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    MECHNICAL PROPERTIES AND BEHAVIORS OF HIGH ENTROPY ALLOYS 1)
    Li Jianguo,Huang Ruirui,Zhang Qian,Li Xiaoyan
    Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (2): 333-359.   DOI: 10.6052/0459-1879-20-009
    Abstract740)   HTML45)    PDF(pc) (85411KB)(742)       Save

    High-entropy alloys (HEAs) are a class of new metallic materials that have revolutionized alloy design over the past ten years. Unlike conventional alloys with one and rarely two base elements, HEAs contain multiple principal elements (at least four principal elements) with equal or nearly equal atomic concertation to promote the formation of simple solid solution phases. Due to the presence of multiple principal elements, multiple deformation mechanisms (including dislocation activities, deformation twinning, and phase transformation) activate during deformation of HEAS. Therefore, HEAs usually exhibited many excellent mechanical properties, such as ultrahigh hardness, high tensile strength, good ductility, high thermal softening resistance, remarkable irradiation resistance, and good wear resistance. HEAs are thought to be the most promising structure materials and have attracted tremendous attention over worldwide in the fields of solid mechanics and material sciences. In this review paper, we first briefly introduce the unique and complicated microstructural features of HEAs, i.e. HEAs have both chemically short-range orderings and severe lattice distortion. Then, we review the recent experimental studies on mechanical properties, behaviors and deformation mechanisms of HEAs with face-centered cubic, body-centered cubic, hexagonal close-packed, dual or meta-stable phases. We also mainly emphasize some effective strengthening and toughening strategies, including solid solution, grain refinement, second phase or precipitation. We further summarize some advanced atomistic simulations/modelling on microstructures, mechanical properties and deformation of various HEAs. Finally, we address a list of open problems and challenges for the future studies about design, fabrication and mechanics of HEAs, and provide some important mechanistic insights into design and fabrication of HEAs with excellent mechanical properties and performances.

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    REVIEW ON THE DEFORMATION BEHAVIOR AND CONSTITUTIVE EQUATIONS OF METALLIC GLASS MATRIX COMPOSITES 1)
    Zhang Juan, Kang Guozheng, Rao Wei
    Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (2): 318-332.   DOI: 10.6052/0459-1879-20-038
    Abstract351)   HTML15)    PDF(pc) (25179KB)(179)    PDF(mobile) (4835KB)(19)    Save

    Metallic glass and metallic glass matrix composites have good application prospects because of their excellent mechanical properties, and now more and more researches have been conducting on them. The deformation behavior, toughening mechanism and constitutive relationship of metallic glass matrix composites are summarized and reviewed in this paper, based on the existing research results in literature by other groups and the latest work done by the authors. Firstly, the research progress in the deformation behavior, failure mechanism and constitutive relation of metallic glass in recent decades is briefly reviewed. Then, the state-of-the-arts in the deformation behavior and failure mechanism of metallic glass matrix composites are introduced from the aspects of experiments and numerical simulation, and the plastic deformation, toughening mechanism and their correspondent influencing factors of metallic glass matrix composites are summarized. Furthermore, the existing studies on the constitutive equations of metallic glass matrix composites are reviewed, with emphasis on the application of homogenization method in this field. In addition, a two-stepped homogenization method proposed by the authors is introduced in more details as a representative approach, and then the constitutive model established on the two-stepped homogenization method and with a help of a failure criterion obtained by introducing a concentration of nano-voids as an internal variable is addressed. The deformation and failure behavior of metallic glass matrix composites are predicted reasonably by the proposed constitutive model. Finally, the research progress of this field is briefly summarized, and some future topics are suggested.

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    DOES STRUCTURE DETERMINE PROPERTY IN AMORPHOUS SOLIDS? 1)
    Wang Yunjiang, Wei Dan, Han Dong, Yang Jie, Jiang Mingqiang, Dai Lanhong
    Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (2): 303-317.   DOI: 10.6052/0459-1879-19-368
    Abstract599)   HTML38)    PDF(pc) (4835KB)(428)    PDF(mobile) (4835KB)(68)    Save

    The mechanical properties and plastic deformation mechanisms of crystalline solids are mainly determined by their structural defects, e.g., the motion of the versatile dislocations. However, how structures determine properties in non-crystalline solids remains as a major unsolved issue in both solid mechanics, materials sciences, as well as condensed matter physics. Structure determines property is the traditional paradigm of materials science. Following this rule, there are vast experimental characterizations, theoretical studies, and computer simulations appeared in the literature, trying to establish a one-to-one correspondence between a specific structural feature with a unique dynamic property in the general amorphous solids. However, up to date, people gain very little understanding of the structure-property relationships in amorphous solids, not to mention whether there exists any hidden rule behind the structure-property relationships. For this purpose, we focus on the unique features of deformations mechanisms in amorphous solids as well as their microstructure characteristics. Thorough proper samplings of the activation energies of the excitation of these structural parameters by an advanced molecular dynamics technique, we are trying to quantitatively assess the validity of simple short-range structures and medium- to long-range structures in determination of their properties. This is done by examination of the possible correlation between parameters of structures with their activation energies, which implies the level of difficulty in activation of the events. By this we find that the hidden governing rule of structure-property relationship in amorphous solids involves a critical role of spatial autocorrelation length of the specific structural parameter. Constraint is more relevant than geometry itself. If only one structural descriptor presents spatial autocorrelation length up to sub nanometer level, it might effectively predict the mechanical property of amorphous solids; otherwise, the short-range local structures lacking such correlation length fails to predict property. Furthermore, we present a general metric to assess the utilities of structures in determining functions of the amorphous solids, which can be served as a screening rule to seeking for effective structures in amorphous solids.

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    Chinese Journal of Theoretical and Applied Mechanics    2020, 52 (2): 301-302.  
    Abstract321)   HTML37)    PDF(pc) (77KB)(131)       Save
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