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ISSN 1005-0302
CN 21-1315/TG
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      20 June 2014, Volume 30 Issue 6 Previous Issue    Next Issue
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    Dynamic Mechanical Relaxation in Bulk Metallic Glasses: A Review
    Qiao J.C., Pelletier J.M.
    J. Mater. Sci. Technol., 2014, 30 (6): 523-545.  DOI: 10.1016/j.jmst.2014.04.018
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    Metallic glasses have aroused considerable interest in the past decades because they exhibit fascinating properties. First, this article briefly outlines the mechanical, thermal properties and application of the metallic glasses. In addition, we focus on the dynamic mechanical relaxation behaviors, i.e. main (α) and secondary (β) relaxations, in metallic glasses. The mechanical relaxation behaviors are connected to the mechanical properties and physical properties in glassy materials. The main relaxation in glassy materials is related to the glass transition phenomenon and viscous flow. On the other hand, the β relaxation is linked to many fundamental issues in metallic glasses. In these materials relaxation processes are directly related to the plastic deformation mechanism. The mechanical relaxations, particularly, the β relaxation provides an excellent opportunity to design metallic glasses with desired physical and mechanical properties. We demonstrate the universal characteristics of main relaxation in metallic glasses. The phenomenological models and the physical theories are introduced to describe the main relaxation in metallic glasses. In parallel, we show the dependence of the α and β relaxations on the thermal treatments in metallic glasses. Finally, we analyze the correlation between the atomic mobility and the thermo-mechanical treatments in metallic glasses. On the one hand, the atomic mobility in metallic glasses is reduced by physical aging or crystallization. On the other hand, the atomic mobility in metallic glass is enhanced by deformation (i.e. compression and cold rolling). Importantly, to analyze the atomic mobility in amorphous materials, a physical theory is introduced. This model invokes the concept of quasi-point defects, which correspond to the density fluctuations in the glassy materials.
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    A Diagram for Glass Transition and Plastic Deformation in Model Metallic Glasses
    Gao X.Q., Wang W.H., Bai H.Y.
    J. Mater. Sci. Technol., 2014, 30 (6): 546-550.  DOI: 10.1016/j.jmst.2013.12.021
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    In this paper, we report a strain rate related glass transition in model SrCaYbMg(Li)Zn(Cu) metallic glasses at room temperature. A critical strain rate, equivalent to glass transition temperature, is found for the strain rate induced glassy state to liquid-like viscoplastic state translation. The results show that the observation time, equivalent to temperature and stress, is a key parameter for the transition between the glass and supercooled liquid states. A three-dimension glass transition diagram involved in time, temperature and stress in metallic glasses is established.
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    Correlation Between Local Atomic Symmetry and Mechanical Properties in Metallic Glasses
    Li M.Z.
    J. Mater. Sci. Technol., 2014, 30 (6): 551-559.  DOI: 10.1016/j.jmst.2014.05.001
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    The local atomic symmetry was investigated and discussed for understanding mechanical, vibrational and dynamical properties in metallic glasses. Local five-fold symmetry was defined based on the ratio of pentagons to the total number of faces in a Voronoi cluster analyzed by Voronoi tessellation method. It is found that the plastic deformation prefer to be initiated in the regions with less degree of local five-fold symmetry (LFFS) and propagate gradually toward the region with more degree of LFFS. On the other hand, the local structures having less degree of LFFS contribute more to the soft low-frequency modes, and thus the so-called boson peak, while those with more degree of LFFS participate more in moderate- and high-frequency modes in metallic glasses. The relationship between local atomic symmetry and structural heterogeneity, mechanical heterogeneity, and glass transition was also discussed. It is shown that local atomic symmetry could be a general structural indicator in metallic liquids and glasses for better understanding the structure-property relationship in amorphous alloys.
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    Structure Heterogeneity in Metallic Glass: Modeling and Experiment
    Ke H.B., Zeng J.F., Liu C.T., Yang Y.
    J. Mater. Sci. Technol., 2014, 30 (6): 560-565.  DOI: 10.1016/j.jmst.2013.11.014
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    Despite the great efforts dedicated to metallic glasses (MGs), their structure still remains a mystery to be understood. With comparison to the existing micromechanical models, such as the free-volume and shear transformation zone (STZ) models, we first discuss in this article our recently proposed ‘core-shell’ model, which contains a solid-like matrix and liquid-like inclusions. This serves as the theoretical basis to understand the structural heterogeneity in MGs in our analytical framework. After that, a scanning ultrafast nanoindentation technique is used to map out the structure heterogeneity in a Zr-based bulk metallic glass (BMG). With these ongoing research efforts, we hope that more research work could be stimulated in the pursuit of the structure-property relation in MGs.
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    Designing Bulk Metallic Glass Composites with Enhanced Formability and Plasticity
    Wu Y., Wang H., Liu X.J., Chen X.H., Hui X.D., Zhang Y., Lu Z.P.
    J. Mater. Sci. Technol., 2014, 30 (6): 566-575.  DOI: 10.1016/j.jmst.2014.03.028
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    To address the main stumbling-block of bulk metallic glasses (BMGs), i.e., room temperature brittleness, designing BMG matrix composites has been attracted extensive attention. Up to date, BMG composites in various alloy systems have been successfully developed by forming crystalline phases embedded in the amorphous matrix through either ex-situ or in-situ methods. In this paper, a brief review of our recent work in this topic will be presented and the novel approaches to improving composite formability and mechanical properties will also be highlighted. The main purpose of this manuscript is not to offer a comprehensive review of all the BMG composites, but instead focuses will be placed on illustrating recently developed advanced BMG composites including Fe-based BMG composite with no metalloids, Al-based BMG composite and BMG composites reinforced by the TRIP (transformation-induced plasticity) effects. The basic ideas and related mechanisms underlying the development of these novel BMG composites will be discussed.
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    Dry Sliding Tribological Properties of a Dendrite-reinforced Zr-based Bulk Metallic Glass Matrix Composite
    Yang Huijun, Liu Yong, Zhang Teng, Wang Hengpeng, Tang Bin, Qiao Junwei
    J. Mater. Sci. Technol., 2014, 30 (6): 576-583.  DOI: 10.1016/j.jmst.2014.05.004
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    In-situ dendrite-reinforced metallic glass matrix (MGM) composites with the composition of Zr58.5Ti14.3Nb5.2Cu6.1Ni4.9Be11.0 were prepared with a vacuum arc melter by the copper mold suction casting. Effect of different normal loads and sliding velocities on the tribological properties of MGM composites was studied. The results showed that the friction coefficient and wear rate of composites initially descended with increasing the normal load and reached a minimum of 0.339 and 1.826 × 10-4 mm3/(N m) at 10 N, respectively, then ascended. Similarly, the friction coefficient and wear rate of composites initially decreased with the increase in the sliding velocity and reached a minimum of 0.330 and 2.389 × 10-4 mm3/(N m) at 0.4 m/s and 0.3 m/s, respectively, then raised. The wear mechanism of composites was mainly adhesive wear accompanied by abrasive wear at lower normal load and sliding velocity. However, the wear mechanism of composites was abrasive wear and adhesive wear as well as delamination at higher normal load and sliding velocity due to the nucleation and propagation of surface and subsurface cracks during the wear process. The flake-like and particle-like wear debris was the dominant shapes of debris observed.
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    Microstructural Evolution and Mechanical Behaviour of Metastable Cu–Zr–Co Alloys
    Pauly S., Kosiba K., Gargarella P., Escher B., Song K.K., Wang G., Kühn U., Eckert J.
    J. Mater. Sci. Technol., 2014, 30 (6): 584-589.  DOI: 10.1016/j.jmst.2014.05.006
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    Three different Cu–Zr–Co alloys, namely Cu40Zr37.5Co22.5, Cu42.5Zr45Co12.5 and Cu49Zr49Co2, were obtained by rapid cooling. The microstructure and phase formation of as-cast rods with diameters of 2 mm are compared with those of the respective ingots. An increasing Co content stabilises the B2 CuZr phase and leads to the precipitation of a ternary Cu–Zr–Co phase. The variation of the cooling rate affects the size of the B2 dendrites as well as the volume fraction and the morphology of the interdendritic phases. The mechanical properties were determined in compression and all alloys show a certain plastic deformability despite the presence of several binary and ternary intermetallic phases. The deformation mechanisms are discussed on the basis of the microstructures and the constituent phases.
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    Large-sized CuZr-based Bulk Metallic Glass Composite with Enhanced Mechanical Properties
    Ding Junfeng, Liu Zengqian, Wang Hui, Zhang Tao
    J. Mater. Sci. Technol., 2014, 30 (6): 590-594.  DOI: 10.1016/j.jmst.2014.01.014
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    A large-sized CuZr-based bulk metallic glass (BMG) composite with enhanced mechanical properties is prepared successfully. With the addition of Ta to CuZr-based alloys, the critical composite size changes and the microstructure diversifies. The composite with 0.5 at.% Ta addition has the largest critical size with the microstructure of single CuZr(B2) phase uniformly dispersing in amorphous matrix. This composite exhibits good mechanical properties, i.e., large compressive plasticity and work-hardening ability, which should be attributed to the uniformly distributed CuZr(B2) phase. The increased critical size of CuZr-based BMG composite can be explained by the fact that proper Ta addition can suppress the precipitation of crystalline phases other than B2 CuZr phase during solidification. This study may be helpful for the fabrication of large-sized BMG composites with excellent mechanical properties.
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    Tensile Elastic Behavior of a Zr–Cu–Ag–Al Bulk Metallic Glass
    Cao Q.P., Jin J.B., Yu Q., Wang X.D., Zhang D.X., Jiang Y., Jiang J.Z.
    J. Mater. Sci. Technol., 2014, 30 (6): 595-598.  DOI: 10.1016/j.jmst.2013.12.020
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    Tensile elastic behavior of bulk Zr46(Cu4.5/5.5Ag1/5.5)46Al8 metallic glass was experimentally investigated. It exhibited linear and non-linear time-independent elastic deformation with a demarcative stress of approximately 500 MPa within the timescale in the present work, and repeated loading–unloading before yielding did not alter stress–strain relationship. The pure linear elastic strain limit is 0.6%, significantly lower than 2% as generally reported, but still much higher than 0.1% observed for typical crystalline alloys. Deviation from linear elastic behavior at stresses higher than 500 MPa is explained here as a macroscopic manifestation of local fluctuations in elastic strain, which becomes pronounced at stresses higher than the critical value. The occurrence of non-linear elasticity is possibly also related to the sinusoidal relationship between shear stress and atomic displacement.
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    Notch Effect of Materials: Strengthening or Weakening?
    Qu Ruitao, Zhang Peng, Zhang Zhefeng
    J. Mater. Sci. Technol., 2014, 30 (6): 599-608.  DOI: 10.1016/j.jmst.2014.04.014
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    Notch is a very important geometry with widespread applications in engineering structural components. Finding a universal equation to predict the effect of notch on strength of materials is of much significance for structural design and materials selection. In the present work, we tried to find this universal equation from experimental results of metallic glasses (MGs) and other materials as well as theoretical derivations based on a universal fracture criterion (Qu and Zhang, Sci. Rep. 3 (2013) 1117). Experimental results showed that the notch effect of the studied MG was affected by the notch geometry characterized by the stress concentration factor Kt. As Kt becomes smaller, the notch strength ratio (NSR, which is the ratio of nominal ultimate tensile strength (UTS) of the notched sample to UTS of the unnotched sample) increases. By comparing MGs with other materials like brittle ceramics and ductile crystalline metals, we find that when Kt is same, the NSR is larger for ductile metals but smaller for brittle ceramics. Theoretically, we derived a universal equation for notch effect on strength of materials: NSR=M/Kt, where M is a constant related to materials. This universal equation was found to be consistent with the experimental results.
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    Plastic Flow of a Cu50Zr45Ti5 Bulk Metallic Glass Composite
    Wang G., Pauly S., Gorantla S., Mattern N., Eckert J.
    J. Mater. Sci. Technol., 2014, 30 (6): 609-615.  DOI: 10.1016/j.jmst.2014.05.003
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    By modifying the cooling rate, a Cu50Zr45Ti5 alloy with various structures was developed. A fully glassy rod and specimens with different sizes and volume fractions of nanocrystals were produced. The relationship between the structure and mechanical properties of the Cu50Zr45Ti5 alloy was investigated. The different structures result in a transition of the deformation mechanism from being dominated by shear banding to being governed by dislocation action accompanied by shear band formation. These different plastic deformation mechanisms were discussed in the framework of self-organized critical behavior.
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    Onset and Direction of Shear Banding Instability in Metallic Glasses
    Chen Yan, Dai Lanhong
    J. Mater. Sci. Technol., 2014, 30 (6): 616-621.  DOI: 10.1016/j.jmst.2014.05.005
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    The shear banding instability occurs as the homogenous deformation in metallic glasses (MGs) develops to a critical point, at which the discontinuity in deformation rate is incipient across nano-scale shear bands. When and where the shear instability takes place is an important issue for understanding the shear band origin. However, such condition and direction of shear localization concerning the unique properties of MGs is still lacking for general stress state. In this paper, a new constitutive is introduced for MGs accounting for the pressure sensitivity, dilatancy and structural evolution; the shear banding is regarded as the appearance of instability in the constitutive description of inelastic deformation. Tying the bifurcation theory to the new constitutive, the general condition of deformation localization is derived. The shear band orientation corresponding to the easiest direction of shear instability is then obtained in dependence on pressure sensitivity, dilatancy and Poisson's ratio for MGs. The range of the predicted shear band angles is consistent with the experimental observations.
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    Fatigue Behaviors of a Ni-free ZrCuFeAlAg Bulk Metallic Glass in Simulated Body Fluid
    Wang Yimei, Liu Yan, Liu Lin
    J. Mater. Sci. Technol., 2014, 30 (6): 622-626.  DOI: 10.1016/j.jmst.2014.05.002
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    Fatigue behaviors of a biocompatible Ni-free Zr60.14Cu22.31Fe4.85Al9.7Ag3 Zr-based bulk metallic glass (BMG) have been studied under three-point-bending test in a simulated body fluid (SBF) at 37 °C and compared with those in air at room temperature (RT). The BMG shows a high fatigue limit of approximately 366 MPa in SBF, which was slightly lower than that in air (400 MPa). The fatigue cracks tended to initiate from the defects such as cast-pores, inclusions and corners of the samples and propagate in a similar path in SBF and in air. Three distinct regions, i.e. a crack-initiation region, a stable crack-growth region and an unstable fast-fracture region were clearly observed on the fatigue-fractured surface. Although pitting occurred at the defects where crack initiated, it does not affect significantly the fatigue life of the BMG, because the lifetime in the present BMG is mainly determined by crack propagation. The high corrosion-fatigue limit of the studied BMG results from its excellent corrosion resistance in SBF and intrinsically good toughness.
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    A Damage-tolerant Bulk Metallic Glass at Liquid-nitrogen Temperature
    Yi Jun, Seifi S. Mohsen, Wang Weihua, Lewandowski John J.
    J. Mater. Sci. Technol., 2014, 30 (6): 627-630.  DOI: 10.1016/j.jmst.2014.04.017
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    Tensile tests and notch toughness tests were conducted on Zr61Ti2Cu25Al12 glass (ZT) at room temperature and liquid-nitrogen temperature. The tensile strength of ZT was improved from 1.63 GPa at room temperature to 1.72 GPa at liquid-nitrogen temperature. Micro-notches with a root radius of 1–3 μm were introduced to test the notch toughness of ZT at room temperature and liquid-nitrogen temperature. The test results revealed that the notch toughness of ZT at liquid-nitrogen temperature is comparable to that of ZT at room temperature. The combination of high yield strength and notch toughness of ZT at liquid-nitrogen temperature is comparable to that of the best cryogenic engineering materials.
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ISSN: 1005-0302
CN: 21-1315/TG
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