The Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 (at.%) bulk metallic glass composites with various crystallization fractions were prepared by pretreating the bulk metallic glassy samples with pulsing current, and then by isothermal annealing at near initial crystallization temperature for different periods of time. The precipitations and crystallization fractions were studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC), and their effects on mechanical properties of the composite were studied by microhardness, uniaxial compression test and scanning electron microscopy (SEM). The experimental results show that the primary precipitate is quasicrystalline phase and other metastable phases including Be 2 Zr, Zr 2 Cu and FCC would precipitate subsequently. In the initial crystallization process, in which the crystallization fraction increases from 0 to 8.2%, both fracture strength and plastic strain increase, with the maximum plastic strain up to 6.4%. When the crystallization fraction is larger than 8.2%, the fracture strength and the plastic strain decrease sharply. Furthermore, the alloy with low crystallization fraction is fractured by shearing, while for high crystallization fraction it is fractured by splitting and cleavage. The results show that the mechanical properties of the glassy alloy could be optimized by controlling the processing parameters. bulk metallic glass, pulsing current, crystallization fraction, plasticity, shear band PACS: 64.70.Pf, 81.40.Lm, 81.30.Mh, 61.50.KsBulk metallic glasses (BMGs) possess extraordinary properties, such as high strength and strong corrosion resistance, which has attracted much attention from scientists in the area of materials and physics [1][2][3]. Usually the strengths of BMGs come up to theoretical values, much higher than those of the crystalline alloys with the same chemical compositions [1-4]. But BMGs exhibit very limited global plasticity since they deform by shearing through the formation of shear bands that tend to form highly localized shear bands, resulting in crack formation and premature fracture [1][2][3][4][5]. Recently, however, investigations have reported that some BMGs exhibit large plastic strains in compression due to the generation of multiple shear bands [6][7][8][9][10][11]. This implies possibility to enhance the plasticity of BMGs through effectively hindering the highly localized shear along the primary shear bands and promoting the activation of shear bands. The reported results show that one way for improving the ductility of BMGs is to introduce second phases to BMGs by in-situ formation [6] or by isothermal annealing [7], which might benefit the generation of shear bands. For instance, Sun et al. [10] reported that 12% compressive plastic strain could be achieved by in-situ forming spherical crystals within the glassy matrix, while Hofmann et al. [11] reported that Zr-Ti-based bulk metallic glass matrix composites (BMGCs) display tensile plasticity of 8.6%-13.1%. However, effects of different second phases and their cryst...

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Work hardening is a well-known phenomenon occurring in crystalline metals during deformation, which has been widely used to increase the strength of metals although their ductility is usually reduced simultaneously. Here we report that the plastic strain of Zr 41 Ti 14 Cu 12.5 Ni 10 Be 22.5 (at.%) bulk metallic glasses has been increased from 0.3% for the as-cast sample to 2.5%-8.0% for samples that have experienced pre-deformation under constrained conditions. The pre-deformed glassy alloys possess more free volume and abundant introduced shear bands, which are believed to promote the activation of shear bands in post-deformation and result in an increase in plasticity. The orientation of the pre-introduced shear bands relative to the loading direction will affect the deformation behavior of pre-deformed samples. The present results show that pre-deformation of this glassy alloy will result in work toughening. This work toughening effect can be removed by isothermal annealing at a sub-T g (glass transition) temperature, which causes annihilation of free volume and healing of shear bands. It is well known that most bulk metallic glasses (BMGs) possess super-high strength but very low global plasticity. The accepted reason for the limited plasticity of BMGs is that BMGs possess a tendency to form highly-localized shear bands which result in early fracture of the materials. It is obvious that deformation behavior of BMGs is quite different from crystalline metals, in which plastic deformation corresponds to the generation and motion of large numbers of dislocations. As the dislocation density increases with the plastic deformation, the interactions among dislocations are also increased, resulting in an increase in the resistance to dislocation movement. Then, further deformation requires an increase in the applied stress. This is the so-called "strain hardening" or "work hardening". However, since there are no dislocations in glassy alloys, absolutely no strain hardening resulting from interactions among dislocations can be observed in BMGs. So it is very difficult for glassy alloys to deform uniformly. Instead, highly localized shear bands [1] form easily during the deformation process, resulting in limited global plasticity of BMG. So to enhance the plasticity of BMGs, it is necessary to promote the activation of multiple shear bands [2-10] and hinder the localization and rapid propagation of shear bands [11][12][13][14][15][16][17][18][19]. Indeed much important progress has been made in enhancing the ductility of BMGs by various ways [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], such as by introducing second phase particles on the nanoscale/microscale [2][3][4][5] or solid solution particles [6] which restrict the severe localization and rapid propagation of shear bands, the as-prepared BMG composites could exhibit a global plasticity of about 7% [7]. Because the partially relieved local heating and hindrance of the propagation of shear bands, both the strength and the ductility of a Zr-bas...

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