Crystalline layer on the surface of Zr-based bulk metallic glasses

The structure, stability and microstructure of Zr65Cu17.5Ni10Al7.5 and Zr50Cu18Ni17Ti5Al10 bulk amorphous alloys was studied by X-ray diffraction, differential scanning calorimetry, and scanning electron microscopy. A thin crystalline surface layer was observed on both bulk metallic glasses. This layer consists of zirconium carbide and some crystalline phases. The thickness of the layer varies between 5 and 10 μm, it depends on the location in the sample. The origin of the formation of the crystalline layer is discussed. The formation of the crystalline layer is connected with carbon deposition on the sample surface during the production of the metallic glass, and with select carbide formation. The resulting change of the concentration of the alloy components near the surface due to the carbide formation leads to a shift of the alloy composition from the amorphization region and results in further crystallization of near surface regions.