| 其他摘要 | In contrast to crystalline materials, metallic glasses have significant advantages such as high strength, high specific strength, high elastic limit and good wear resistant property. However, lack of dislocation and hence the strain hardening capability make bulk metallic glasses (BMGs) prone to catastrophic fracture, they show essentially no macroscopic uniform plastic deformation due to the unstable prolongation of the shear band at room temperature, which restrict the wide applicaton of the BMGs as structral materials. Recent investigations have shown that some BMGs could sustain a certain extent of plastic deformation in compression at room temperature, which can be attributed to their resistance to the rapidly prolongation of shear band or cracks and the following catastrophic failure. But the deformation mechanism and the origin of the plasticity of BMGs were not well understood so far.
The plasticity of BMG is closely related to the shear deformation behavior, this is significant to the potential application of BMG as structral materials. In the present thesis, the mechanical property and deformation behavior of three typical BMG systems, Zr-, Fe- and Al-based BMGs, were investigated systematically under uniaxial tension or compression, to establish the relationship between the mechanical property and the shear deformation of BMGs under uniaxial loading mode. The main results are summaried as follow:
1. The tensile fracture of Zr-based BMG is dominated by shear behavior, no significant plastic strains were observed from the tensile strain-stress curves. The similarity in tensile and compressive strengths of the Zr48Cu45Al7 BMG indicates that the fracture mode of BMGs is evidently different from common brittle materials; the tensile fracture is due to the unstable prolongation of the shear band after the shear stress increases to the critical value. The tensile strength of the Zr48Cu45Al7 BMG was found to show a narrow distribution; the Weibull analysis shows that the BMG has a Weibull modulus of 36.5 in tension, which is much higher than those of common brittle materials, indicating the high reliability of the BMG.
2. The tensile properties of the Zr47.5Cu47.5Al5 BMG composites containing the CuZr crystalline phase are determined by the content of the crystalline phase. As the content of the crystalline phase increases, the fracture strength of the sample decreases and the plasticity increases gradually. The tensile strain-stress curve of the composite sample with less than 5% crystalline phase content is similar to that of the fully glassy sample; the composite sample with 20% crystalline phase content exhibits a yield strength of 730 MPa, a fracture strength of 1046 MPa as well as a plastic strain of 2.0%, the tensile strain-stress curve of the sample shows strain hardening behavior; obvious strain hardening behavior can be observed in the samples containing more CuZr crystalline phase, which exhibit a tensile plasticity of about 4% and a lower fracture strength of about 500 MPa.
3. The glass forming compositions predicted by the structure model of BMGs based on the atomic cluster efficient packing are very close to the practical compositions in the Fe-Nb-B and Fe-Zr-B alloy systems. The optimum glass-forming alloys in Fe-Nb-B, Fe-Zr-B and Fe-(Nb,Zr)-B alloy systems are Fe71Nb6B23, Fe77Zr4B19 and Fe71(Nb0.8Zr0.2)6B23, respectively; the corresponding critical size of glass-forming is 1.5 mm, <1 mm and 2 mm, respectively. The addition of Nb produces a deeper eutectic reaction and steeper liquidus line, therefore better glass forming ability than that of Zr in the Fe–B systems. The best glass-forming alloy in the whole Fe–(Nb, Zr)–B system, Fe71(Nb0.8Zr0.2)6B23, deviates from the straight line linking the two best glass-forming alloys in the ternary systems. The glass formation in the whole system agrees well with the phase selection criterion.
4. The addition of Nb which holds a high Poisson’s ratio can improve the plasticity of Fe-based BMG significantly, simultaneously facilitates the formation of the networklike structure of (Fe,M)23B6, leading to the ultrahigh strength of Fe-based BMG. A Fe71Nb6B23 BMG with a plastic strain of 1.6%, as well as a record strength of 4.85 GPa is designed; the BMG can sustain stable prolongation of the shear band in compression. The fracture surface of the Fe71Nb6B23 BMG clearly demonstrates a typical vein pattern related to the plastic flow in the shear plane together with periodical nanoscale corrugations in mirrorlike fracture surfaces. From the relationship between the fracture features and the fracture toughness of the BMG, it is indicated that the periodical nanoscale corrugation is not closely related to the intrinsic deformation mechanism of BMG.
5. The newly developed Al-based BMGs exhibit a strength of about 1050-1140 MPa under uniaxial compression, holding an ultrahigh specific strength of about 3.4 ´ 105 Nm kg-1. The stiffness of the testing machine shows a strong influence on the deformation mechanism of the Al-based BMGs under compression. In a low machine stiffness condition, the Al-based BMGs exhibit macroscopic brittle fracture, failing by unstable prolongation of one dominant shear band with no obvious plastic deformation. In a high machine stiffness condition, the Al-based BMGs can avoid unstable failure; the deformation mechanism for Al86Ni6Y4.5Co2La1.5 and Al86Ni7Y4.5Co1La1.5 alloy is stable prolongation of one dominant shear band; while Al86Ni7Y5Co1La1 alloy primarily stable deforms by simultaneous operation of uniform multiple shear bands. Serrated flow is observed during the slip of the sample along the dominant shear plane, showing the sliding striations. The deformation mechanism of the Al-based BMGs is sensitive to the strain rate; as the strain rate increasing, the sliding rate along the dominate shear plane increases, the degree of sliding striations in the dominant shear plane is reduced; for Al86Ni7Y5Co1La1 alloy, the capability of stable uniform plastic deformation is restricted under high strain rate, the deformation mechanism of the alloy turns to stable prolongation of one dominant shear band. |
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