Microstructural evolution during decomposition and crystallization of the Cu 60 Zr 20 Ti 20 amorphous alloy
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K. Varga Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, H-1525 Budapest, P.O.B. 49, Hungary
G. Heunen European Synchrotron Radiation Facilities (ESRF), Grenoble, France 38042
M.D. Baro´b) Department of Physics, Faculty of Sciences, Universitat Auto`noma Barcelona, Edifici Cc, 08193 Bellaterra, Barcelona, Spain (Received 30 June 2003; accepted 15 October 2003)
The effect of continuous heating and isothermal heat treatments on ductile Cu60Zr20Ti20 amorphous ribbons was monitored by differential scanning calorimetry, x-ray diffraction, synchrotron radiation transmission, and high-resolution transmission electron microscopy. Upon continuous heating, the alloy exhibited a glass transition, followed by a supercooled liquid region and two exothermic crystallization stages. Decomposition of the amorphous phase was also observed. The first crystallization stage resulted in the formation of a nanocomposite structure with hexagonal Cu51Zr14 particles embedded in the amorphous matrix, while in the second crystallization stage hexagonal Cu2TiZr-like phase was precipitated. The released enthalpies were 19 J/g and 30 J/g for each crystallization stage. Crystallization kinetics was studied by the classical nucleation theory. Deviations from the Johnson–Mehl–Avrami–Kolmogorov theory may be explained by the contribution of the decomposition of the amorphous matrix.
I. INTRODUCTION
The discovery of improved physical and mechanical properties of amorphous and nanocomposite metallic materials,1,2 with respect to the crystalline counterparts, has led to increased research in this area during the past two decades. The possibility of preparing amorphous metallic alloys at cooling rates of 1–100 K/s has generated great interest3 because this enables these alloys to be manufactured not only in the form of ribbons, but also in the form of bulk materials. To develop this possibility, a greater understanding of the glass-forming ability (GFA) of these systems is essential. To extend the application of structural bulk metallic glasses, it is important to combine high glass-forming ability and good mechanical properties. In the case of Cu-based bulk amorphous alloys, recent studies have
a)
Present address: Department of General Physics, Eo¨tvo¨s University, H-1518 Budapest, P.O.B. 32, Hungary. b) Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 19, No. 2, Feb 2004
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found that the addition of Ti to the Cu–Zr and Cu–Hf binary systems drastically reduces the melting temperature, which strongly correlates with good glass-forming ability. New Cu-based bulk amorphous alloys were formed in Cu–Zr–Ti and Cu–Hf–Ti ternary alloys by copper mold casting method.4 Furthermore, in the Cu– Zr–Ti system, the Cu60Zr20Ti20 composition exhibits the best GFA, with a reduced glass-transition temperature (Tg/Tl) of 0.62.5 This alloy has good mechanical properties (e.g., tensile fracture strength is higher than 2000 MPa); however
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