Crystallization of undercooled liquid spinodals: Part II
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Crystallization of undercooled liquid spinodals: Part II K.L. Lee and H.W. Kui Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (Received 22 December 1997; accepted 10 June 1999)
We demonstrated in “Phase separation in undercooled molten Pd80Si20: Part I” that when a molten Pd80Si20 ingot is undercooled into its undercooling regimen with ⌬T 艌 220 K (⌬T ⳱ T1 − T, where T1 is the liquidus and T is the temperature of the undercooled melt), liquid-state phase separation by spinodal decomposition occurs. On crystallization, one of the metastable liquid spinodals becomes Pd3Si, whereas the other one turns into Pd9Si2. In both cases, Pd particles precipitate out. Microstructural analysis indicates the Pd3Si subnetwork forms first. It then acts as a seed for the subsequent crystallization of the remaining undercooled melt, which finally forms the Pd9Si2 dendrites. As crystallization proceeds, latent heat and volume contraction bring about morphological changes.
I. INTRODUCTION 1
We showed in our previous report “Phase separation in undercooled molten Pd80Si20: Part I” that when molten Pd80Si20 is undercooled to a temperature T substantially below its liquidus T1, the homogeneous liquid splits into two liquids, i.e., L → L1 + L2. The extent in temperature of the system below T1 is called undercooling ⌬T defined as ⌬T ⳱ T1 − T. The liquid-state phase separation (LPS) can occur through two different mechanisms, namely, liquid-state nucleation and growth (LNG) and liquidstate spinodal decomposition (LSD). The latter process produces a system of two metastable liquid networks or spinodals that interpenetrate each other. This system consists of plentiful interfaces. A study of the crystallization process of such a system is quite limited.2– 4 In this article, we report the crystallization behavior of undercooled Pd80Si20 liquid spinodals. II. EXPERIMENTAL
To prepare a Pd80Si20 ingot, elemental Pd and Si granules, weighed in the right proportion, were put in a clean fused silica tube. Alloying was brought about by radio frequency (rf) induction heating under Ar atmosphere. Our previous report1 showed that by a fluxing technique, molten Pd80Si20 could be undercooled to a large ⌬T before interrupted by crystallization. In this study, the fluxing technique was again used, and anhydrous B2O3 was also used as the fluxing agent. In the experiment, a raw Pd80Si20 ingot and anhydrous B2O3 flux were put into another clean fused silica tube. The tube was then evacuated by a mechanical pump to ∼10−3 torr. This J. Mater. Res., Vol. 14, No. 9, Sep 1999
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condition was maintained for the entire experiment. The system, i.e., the ingot and the B2O3 flux that resided at the bottom of the tube, were heated up by a torch to ∼1300 K. The heat treatment was applied for ∼4 h to facilitate the removal of impurities from the molten ingot. After the high-temperature fluxing, the fused silica tube was transferred to a
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