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Title: Plasticity enhancement of bulk metallic glasses by metal electroplating
Authors: Chen, Wen
Degree: M.Phil.
Issue Date: 2012
Abstract: Bulk metallic glasses (BMGs), first synthesized during the 1970s, have stimulated great research interest because of their unique properties. In the past decades, although significant progress has been achieved in understanding the atomic structure and properties of these materials, poor room temperature deformability still hinders their wide commercial applications. In contrast to crystalline materials that deform through dislocation movements, the plastic deformation of BMGs at room temperature is highly localized in shear bands that proceed and crossover the specimen rapidly, resulting in limited plasticity. Many approaches have been so far developed for plasticity enhancement of BMGs, such as the in situ formation of crystalline phases, the achievement of a critical value of Poisson's ratio, and the imposition of geometric confinement. Among these approaches, metal electroplating has been proven to be feasible in toughening Zr-based BMGs. However, its effect on other types of BMGs, and a detailed analysis on the underlying mechanism for the plasticity enhancement by coating confinement have yet to be undertaken. In the present work, the effect of Ni coating on Zr₅₇Al₁₀Ni₈Cu₂₀Ti₅ and Fe₇₅Mo₅(P₅₀C₄₁.₆₇B₈.₃₃)₂₀ BMGs were further analyzed. Both of the BMGs have exhibited higher plastic strains than their uncoated counterparts, manifesting that the Ni coating is an effective way in toughening these types of BMG. The increased plasticity is believed to be attributed to the nucleation of profuse shear bands and the impedance of shear band propagationdue to the imposed confining effect of the Ni coating.
In the present work, a Cu/Ni bilayered coating was also electrodeposited onto a Zr₅₇Al₁₀Ni₈Cu₂₀Ti₅ BMG. It demonstrated for the first time that the effectiveness of geometric confinement in toughening BMGs by the bilayered coating outweighed that by a mono-layered Cu or Ni coating with equivalent thickness. The inner soft Cu layer in the Cu/Ni coating behaves as a good "crack buffer zone" to effectively absorb the elastic deformation energy released by BMGs; while the outer strong Ni layer and the strong Cu/Ni interface assisted in strengthening the geometric confinement. By combining the effect of geometric confinement and the reduction of friction at the boundaries, remarkable plasticity enhancement was also reported for the first time in the Cu-coating encapsulated Zr₅₇Al₁₀Ni₈Cu₂₀Ti₅ BMG. With the aid of the Cu coating as a lubricant, the elastic mismatch between the loading platens and the specimen was reduced, and the Cu coatings at two ends could generate a lateral pressure near the edges of the specimen. Accordingly, the lateral spreading of the material near the ends was promoted and the plasticity of the BMG was improved. Besides, a non-homogeneous microstructure was observed in the Cu coated BMGs during the deformation, which was also considered to contribute to the enhanced plasticity of BMGs. The findings of the current work not only lead to a better understanding of the geometric confinement effect, but also lay down a good foundation for further optimizing this technique for plasticity enhancement of BMGs.
Subjects: Metallic glasses -- Plastic properties.
Bulk solids.
Hong Kong Polytechnic University -- Dissertations
Pages: xiv, 124 leaves : ill. ; 30 cm.
Appears in Collections:Thesis

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