Alloy Phase Formation in Isolated Nanometer-sized Particles in the Au-Sn and Sn-Bi Systems
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Alloy Phase Formation in Isolated Nanometer-sized Particles in the Au-Sn and Sn-Bi Systems Jung-Goo Lee1, Hirotaro Mori1 and Hidehiro Yasuda2 1 Research Center for Ultra-high Voltage Electron Microscopy, Osaka University, 2-1Yamadaoka, Suita, Osaka 565-0871, Japan 2 Department of Mechanical Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan ABSTRACT Alloy formation in nanometer-sized particles has been studied as a function of the particle size by in-situ transmission electron microscopy using particles in the Au-Sn and Sn-Bi systems. When the size of particles is larger than a critical value, essentially similar phase equilibrium was observed in nanometer-sized particles and bulk materials in the both systems. But a solid amorphous phase was formed over a compositional range near the eutectic composition when the size of particles was smaller than about 7 nm in diameter in the former system, whereas a fluid amorphous phase was formed when the size of particles is smaller than about 10 nm in diameter in the latter system. These amorphous phases directly changed into liquid phases upon heating and solidified into amorphous phases upon subsequent cooling. The formation of these thermodynamically stable amorphous phases can be attributed to the the suppression of the eutectic temperature (Teu) associated with size reduction being so large that Teu is below the glass transition temperature (Tg). INTRODUCTION Since Duwez et al. obtained the first amorphous alloy in 1960 by the liquid quenching technique [1,2], a number of techniques have been developed to produce amorphous alloys. Included in the techniques are condensation of metal vapors onto a cooled substrate [3], mechanical alloying [4-6], low temperature annealing of multi-layered films of components A and B with a negative heat of mixing [7,8], hydrogen absorption [9,10], mechanical grinding or severe plastic deformation of intermetallics [11,12] and irradiation with high-energy particles [13-15]. Amorphous materials produced by these techniques are all in a non-equilibrium state and therefore are thermodynamically metastable compared to the corresponding crystalline counterpart(s). On the other hand, it was found that in nanometer-sized particles rapid spontaneous alloying takes place even at ambient temperature in several binary systems [16-20] and in some binary systems a crystalline-to-amorphous (C→A) transition takes place by spontaneous alloying. This unique phase transition can be ascribed to the finite size effect on the eutectic temperature, Teu, in binary alloy systems being so strong that in nanometer-sized alloy particles Teu can be lowered down to a temperature even below the glass transition temperature, Tg, which is a situation which never takes place in bulk materials. As a result of such a situation, in the particular case of the In-Sn system where room temperature (RT) at which observations were carried out, lies in such an order as RT>Tg>Teu, a crystalline-to-liquid (C→L) transition has been observed by simply adding s
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