| Bulk metallic glasses(BMGs) are a new class of structural materials possessing many unique mechanical properties due to their long-range disorder structure. Such as large elastic limit and hardness, and high resistance to corrosion as well as wear resistance. They have attracted intensive attentions for potential applications in structural and functional areas. In general, these excellent properties are always followed by the poor plasticity in room temperature. In the present study, it presents that the deformation of bulk metallic glass is different from traditional crystalline materials. The deformation is dependent on the propagating of shear bands forming during the plastic process. BMGs under mechanical loading undergo highly local shearing without any barrier, which can lead to significant softening in shear bands and cause catastrophic failure. The catastrophic failure has severely limited the application of BMGs as structural materials upon loading at room temperature.In this study, three kinds of coatings were deposited on the bulk metallic glass substrate. One is the copper film plated on the Zr52.5Cu17.9Ni14.6Al10Ti5(Vit.105) substrate by electroplating, and investigated the effect of copper coatings on its compressive and the serrated flow behavior at room temperature; Then, electroless method was adopted to plate the Ni-P amorphous alloy on the Zr41.2Ti13.8Cu12.5Ni10Be22.5(Vit.1) matrix. The effect of the amorphous film on its compressive properties in the room temperature is studied. Then, Multilayer films of Ni and Ni-P was together plated on the Zr41.2Ti13.8Cu12.5Ni10Be22.5(Vit.1) matrix. The effect of multilayer films on the compressive properties is investigated and the different effects from the monolayer is compared.According to the study, it is concluded as follows:(1) The bare Vit.105 pillars and the coated Vit.105 pillars reveal that the plasticity is improved by the copper film under the compressive tests. The plastic strain was 6.1 % for the coated pillars with a coating thickness of 161.1 μm, which is 3.59 times of 1.7 % of the bare Vit.105 pillars. It is found that the thicker coating presents better plasticity through comparing the different thickness of films. The deformation of the copper films dissipated the strain energy and limited the propagation of shear bands, which led to the plasticity improvement of the bulk metallic glasses.(2) The serration process is characterized by repeated cycles of a stress drop followed by an elastic loading, and the serration flow of the samples certified the plasticity improvement by the Cu-coatings.(3) Using the chemical plating method, Ni-P amorphous films were coated on the surface of Zr41.2Ti13.8Cu12.5Ni10Be22.5(Vit.1) BMG with different p H values and film thicknesses. P content is apparently influenced by the p H values. The P content is decreased with the increasing p H. According to XRD results, the different p H values influence the structure of the Ni-P coatings.(4) Depositing the Ni-P film made bulk metallic glasses form the special amorphous / amorphous structure and exhibit large plasticity. The plastic strain is 10.6 % for the coated pillars with a coating thickness of 86.5 μm, which is 5.9 times of 1.8 % of the bare Vit.1 pillars. Due to the well bonding and the high radial confinement stress provided by Ni-P coating. It leads to more homogeneous formation of a high density of shear bands at the Ni-P/Vit.1 interface.(5) Combining the electroless and electroplating, the multilayer films were deposited on the surface of Zr41.2Ti13.8Cu12.5Ni10Be22.5(Vit.1) BMG. The Ni-P/Ni/Vit.1 pillars with amorphous-crystalline-amorphous sandwiched structure. The results of the nanoindentation experiment indicated it formed a“hard-soft-hard” composite. As a result, Ni-P/Ni/Vit.1 pillars have the largest engineering plastic strain of 21.3 ± 1.0 % due to “uniform plastic deformation”, about 11.2 times of 1.9 % of the bare Vit.1 pillars. The result is better than any monolayer’ function to the plasticity improvement in bulk metallic glasses. |
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