Oxidation-induced copper segregation in Cu 60 Zr 30 Ti 10 bulk metallic glass
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Copper segregation in a subsurface layer during annealing of Cu60Zr30Ti10 bulk metallic glass at 773 K under oxygen atmosphere has been investigated by x-ray diffraction, Auger electron spectroscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy. The formation of metallic copper is strongly dependent on the annealing environment. Various oxides with metallic copper are formed after annealing in oxygen atmosphere, but only crystalline intermetallic phases are found under vacuum annealing. Besides, surface characterization results show that the sample annealed in oxygen and vacuum result in enrichment and depletion of Cu on the surface region, respectively.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE 1,2
Earlier investigations stated that metallic Cu or some Cu-rich islands embedded in ZrO23,4 are formed in Cu–Zr metallic glass ribbon after ageing in air. Oxidation of other Zr-based metallic glasses leading to the formation of ZrO2 with metal particles was also reported.1,5–7 Aoki et al.8 studied the oxidation behavior of Zr–Ni amorphous alloy under different gases and found that metallic Ni is formed when the sample is annealed in CO atmosphere. It suggested that the strong tendency for the formation of ZrO2 and ZrC resulted in the continuous increase in the Ni concentration and eventually the formation of metallic Ni. Surface segregation can usually be verified by surfaceanalysis techniques such as Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS).9–11 Our previous works12,13 on the oxidation behavior of Cu–Zr–Ti, Cu–Zr–Hf–Ti, and Cu–Zr–Ti–Y bulk metallic glasses all lead to the conclusion that metallic Cu formed in the samples after oxidation at 773 K for 10 h. The Cu segregation may be attributed to the low solubility of Cu in ZrO2 and fast diffusion of Cu due to its high mobility. Although Cu segregation in metallic glasses has been reported by many authors,2,9,10 a detailed investigation on the segregation mechanism is still lacking. The aim of this paper is to report the influence of oxygen on the Cu segregation in the Cu60Zr30Ti10 bulk metallic glass.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0120 J. Mater. Res., Vol. 21, No. 4, Apr 2006
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A Cu60Zr30Ti10 (nominal composition) bulk metallic glass sample was prepared by arc melting the mixtures of pure Cu, Zr, and Ti metals in a Ti-gettered argon atmosphere. Bulk metallic glass rods with 3-mm-diameters were then prepared by suction casting into a copper mold. The amorphous structure and oxide phases were identified using a Siemens D500 diffractometer (Munich, Germany) with CuK␣ ( ⳱ 0.15406 nm) radiation. Oxidation of samples was done in a Seiko Instrument TG/ DTA 220 (Tokyo, Japan) in oxygen environment with a constant flow rate of 500 ml/min. The samples were heated from room temperature to 773 K with a heating rate of 20 K/min and then held isothermall
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