Transmission Electron Microscopy of Fe 79.5 B 6.5 C 14 Network Alloys: Part II
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INTRODUCTION
CHEN and Turnbull found that a Pd-Au-Si glassy foil, prepared by a splat quenching and annealed at a temperature slightly above its glass transition temperature Tg, exhibits image contrast when studied by transmission electron microscopy (TEM). The result suggests the occurrence of amorphous phase separation. Later, Chou and Turnbull[2] showed that the amorphous phase separation is consistent with a spinodal mechanism[3,4] by small-angle X-ray scattering. However, the alloy system, Pd-Au-Si, has a negative heat of mixing. Its free energy curve, G, therefore, is concave upward, forbidding amorphous spinodal decomposition. To circumvent the difficulty, Chen and Turnbull[1] proposed that the origin of the amorphous spinodal decomposition is due to unique short-range order (SRO) in the glassy/liquid state of the alloy system. Since the work of Chen and Turnbull[1] and that of Chou and Turnbull,[2] different research groups have investigated this phenomenon, but the results obtained are conflicting. For example, Tanner and Ray[5] were able to demonstrate that the as-quenched Zr36Ti24Be40 glassy specimens exhibit image contrast when studied by TEM. For a long time, the observation is regarded as an evidence of amorphous phase separation. There are other groups[6–9] reporting results that are in agreement with amorphous phase separation. On the other hand, the results obtained by some research groups[10–14] contradict amorphous phase separation. For instance, Nagahama et al.[13] found that the TEM image contrast observed by Tanner and Ray[5] in Zr36Ti24Be40 glassy [1]
Y.L. YIP, C.C. LEUNG, and S.W. MOK, Research Associates, K.H. YIP, Graduate Student, and HIN WING 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] Manuscript submitted June 7, 2013. Article published online November 7, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
specimens might be due to chemical etching effects during specimens preparation. Because of a lack of direct evidence, amorphous phase separation in metallic glasses and bulk metallic glasses (BMGs) of negative heat of mixing has become a controversial issue. Recently, Lan et al.[15] showed that amorphous phase separation occurs in Pd-Ni-P BMGs by using highresolution transmission electron microscopy, high angleannular dark field in scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy. Subsequently, Lau et al.[16] showed that Pd40+0.5xNi40+0.5xP20x BMGs undergo amorphous phase separation for 1 £ x £ 3.5. Besides, the volume fractions of the different amorphous phases in these phase-separated specimens were measured. It was found that the lever rule is obeyed. The observations suggest that there is a metastable liquid/amorphous miscibility gap (MLMG) in undercooled molten Pd-Ni-P. As a homogeneous alloy melt is undercooled into its MLMG, it decomposes into multiple liquid sub-networks (or spinodals). If crystallization is bypassed, it becomes a bulk
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