| Summary: | 博士 === 國立清華大學 === 工程與系統科學系 === 100 === Bulk-amorphous metallic alloys are a new class of materials that exhibit superior material properties, these unique features makes them perspective materials for various advanced engineering applications. In addition, some of the bulk metallic-glass (BMG) compositions have drawn much attention for their potential for hydrogen-energy-related applications, such as hydrogen purification/separation membranes. “Hydrogen in metals” has been a popular topic for material scientists and engineers for many decades. One of the main reasons is the infamous ability of hydrogen to degrade the mechanical properties of most metallic materials. Another reason is the potential of using metal hydrides for hydrogen-storage materials. For either perspective, it is very important to understand the effect of hydrogen on the mechanical behavior and structure of BMGs.
In the present research, the effect of hydrogen on the mechanical behavior and structure of Zr-based BMGs have been studied. The deformation of the Zr-based BMG was first examined. It is found that the elastic formation of BMG in macroscopic scale is not “elastic” in atomic scale. Through x-ray scattering and the anisotropic PDF analysis, it is shown that about 25% of the volume of a metallic glass is occupied by anelastic sites, which are soft and bear no static shear load. Just as other glasses, metallic glasses are fundamentally viscoelastic. The dissolved hydrogen was found to increase the hardness of the Zr-based BMG, roughly 40% right after hydrogen charging. The hydrogen will also embrittle the Zr-based BMG. The embrittlement is attributed to the rearrangement of the local atomic structure and the decohesion of atomic bond strength. The atomic pair-distribution-function (PDF) analysis and inelastic neutron scattering experiment reveals that hydrogen atoms preferentially occupy tetrahedral-like interstitial sites composed of mainly Zr atoms. The introduction of hydrogen atoms results in a ~ 15% volume expansion of the amorphous matrix. Such volume expansion produces a large residual strain at the hydrogen-charged area.
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