Crystallization of Phase-Separated Pd 41.25 Ni 41.25 P 17.5 BMGs
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INTRODUCTION
AMORPHOUS phase separation was first observed by Chen and Turnbull[1] in their studies of the microstructure of annealed Pd-Au-Si glassy foils by transmission electron microscopy (TEM). It is unusual for Pd-Au-Si to undergo amorphous phase separation because the alloy system has a negative heat of mixing.[2,3] Later, Chou and Turnbull[4] confirmed that the observed amorphous phase separation is consistent with a spinodal mechanism by small-angle X-ray scattering. Since then, reports on phase separation in metallic glasses or bulk metallic glasses (BMGs) of negative heat of mixing have been conflicting.[5–17] The difficulty in settling this problem is that direct evidence is not available. As a result, it became a controversial issue for about 40 years. On the contrary, there is direct evidence of phase separation in some BMGs that are characterized to have at least two of the major constituent elements having a positive heat of mixing,[18–23] in agreement with Cahn’s prediction of spinodal decomposition.[2,3] Recently, Lan et al.[24] were able to prepare three different types of Pd41.25Ni41.25P17.5 BMGs. A-type specimens are homogeneous glasses. B-type specimens are glassy specimens, embedded with crystalline precipitates. C-type specimens are phase-separated glasses. The different glass states were confirmed by highresolution transmission electron microscopy (HREM), the method of high-angle annular dark field (HAADF) in scanning transmission mode, and energy-dispersive X-ray spectroscopy (EDX). It was also found that on heating an A-type specimen from room temperature to
Z.D. WU, Graduate Student, and H.W. KUI, Professor, are with the Department of Physics, Chinese University of Hong Kong, Shatin, N.T., Hong Kong, P.R. China. Contact e-mail: [email protected]. edu.hk S. LAN, Graduate Student, formerly with the Department of Physics, Chinese University of Hong Kong, is now Post-Doctoral Fellow, Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, P.R. China. Manuscript submitted April 27, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
613 K (340 C) for thermal annealing, it transforms into a B-type specimen. Since the microstructures of B-type and C-type specimens were different, Lan et al.[25] studied their crystallization behavior. In the experiment, the two types of Pd41.25Ni41.25P17.5 BMGs were thermally annealed at 613 K (340 C) for different periods of time. The longest annealing time period was 8 hours. The crystallization in B-type specimens is characterized by the appearance of spherical crystalline precipitates. In addition, the number density of the spherical crystalline precipitates increases more or less linearly with annealing time. The crystallization behavior in C-type specimens is different. There are two kinds of crystalline precipitates, which are called spherical and cubic crystalline precipitates. The former is different from that in B-type Pd41.25Ni41.25P17.5 specimens in composition. Its number density is substantially less than that in B-type Pd41.25Ni41.2
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