Formation of Nanostructured Alloys by Liquid Phase Spinodal Decomposition
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FORMATION OF NANOSTRUCTURED ALLOYS BY LIQUID PHASE SPINODAL DECOMPOSITION W.H. Guo and H.W. Kui Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, P.R. China ABSTRACT A molten Pd82 Si18 alloy, free of potent heterophase impurities and enclosed in a fused silica tube of conical shape, was quenched in water. Four regions classified according to their microstructures can be distinguished. They are: region I, amorphous; region II, a mixture of an amorphous nanostructure and a nanocrystal; region III, a mixture of two kinds of nanocrystals; region IV, spinodal networks; and region V, conventional eutectic structure. The detailed microstructures in each region are discussed.
INTRODUCTION The notion of nanostructured material, with average grain size, r, in the range of 1 < r < 100 nm, was first put forward by Turnbull [1] and Gleiter [2]. It was first put into practice by Birringer et al. [3]. The synthesis technique involved the preparation of nanometer sized powders, which were then pressed together under high pressure to form a compact in disk shape of typical diameter ~1 cm and thickness 2-3 mm. But there are plenty of pore inclusions. Later, warm compaction [4] was introduced, attempting to improve the density of the compact to its theoretical value. By annealing an amorphous metal at an elevated temperature (near its glass transition temperature, Tg), it was found that nanocrystals emerged in the amorphous matrix. Schneider et al. [5] suggested that there is a chemical decomposition in the annealed specimen. One of the decomposed phases then crystallizes preferentially, producing nanocrystals in the amorphous matrix. Pores are absent now, but the grain size distribution can be wide. Preliminary studies indicate that nanostructured alloys possess attractive mechanical [6, 7] and magnetic properties [8]. There is therefore an urgent need for a new synthesis technique to manufacture nanostructures that are pore-free, of uniform grain size, and bulk in size. Recently, it was found that when undercooled below its melting temeperature, a eutectic melt would undergo liquid state spinodal decomposition, becoming metastable liquid networks of wavelength λ [9, 10, 11]. Upon crystallization for those cases of λ < 100 nm, nanostructures result [12, 13]. The details are described below. BACKGROUND Consider a binary alloy, in which the like species prefer to stay together. According to Gibbs free energy G, which is given by G = H – TS, where H is the enthalpy, T is the temperature and S is the entropy, there are two equilibrium phases at low temperatures for the enthalpy term H dominates there. On the other hand, at L5.4.1
high temperatures the entropic term S forces the mixing up of the different atoms to make the alloy homogeneous. By definition, the like species of a eutectic alloy attract each other more than that between unlike ones. In general, the free energy of the liquid phase of a eutectic alloy in the whole composition range is described by a curve that is concave upward for tempera
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