Solid State Reactions in the ZR-AL-CU-NI Bulk Metallic Glass Forming Alloy System
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ABSTRACT Diffusion couples combining a h.c.p. Zr 90 A1 0 supersaturated solid solution with a fc.c. Cu 64Ni 36 solid solution were annealed at 410'C for different times. The reaction at the interface was investigated by transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDAX). Cross sectional TEM images and electron diffraction patterns reveal the formation of a non-crystalline layer at the interface between the two solid solutions which grows to a maximum thickness of more than 0.3 jim. Using EDAX, the concentration profiles across the layers were determined. The glassy layer consists of a Zr-Ni-Al alloy with a small amount of Cu. The concentration profiles reveal that two non-crystalline phases coexist in the diffusion couple. One phase is Ni-rich and the other is Zr-rich. A small gradient of the Ni concentration in the Ni-rich amorphous phase and a steeper gradient in the Zr-rich non crystalline phase indicate that the Ni diffusion constant in the Ni-rich phase is larger than the Ni diffusion constant in the Zr-rich phase. In the late stage of the reaction, the growth of a nanocrystalline layer with an average concentration of Cu 90 Nij 0 is observed on the Cu-Ni side of the diffusion couple. Crystallization starts at the Zr-Al side of the diffusion couple.
INTRODUCTION Amorphous phase formation in diffusion couples, which combine an early transition metal like Zr with a late transition metal like Ni, Co or Fe is a well known phenomena, and has been extensively studied in the last years [1-5]. Recently, new families of novel multicomponent glass formers have been discovered which exhibit a much higher glass forming ability than known alloys. Cooling rates of less than 100 K/s are usually sufficient to prevent crystallization and form a glassy state in these alloys, which include La-Ni-Al [6], Zr-Ni-Al [7], Zr-Cu-Ni-Al [8] and Zr-Ti-Cu-Ni-Be [9]. The low cooling rates required for glass formation make it possible to produce samples of a thickness of several millimeters up to centimeters in the smallest dimension. Upon reheating above the glass transition, these bulk metallic glass (BMG) formers show a high thermal stability of the supercooled liquid with respect to crystallization. The Zr65 Al7. 5 Cu 17 .5 Nil 0 alloy, for example, can be heated about 130 K above the onset of the glass transition with a rate of 40 K/min into the supercooled liquid before crystallization occurs. In contrast, a binary amorphous Zr 75Ni25 alloy crystallizes immediately after reaching the glass transition. In a solid state amorphization reaction (SSAR) at a Zr/Ni interface, the low thermal stability of the Zr-rich part of the amorphous layer, limits the growth of the amorphous phase. In this study, we extend the concept of solid state amorphization by an interdiffusion reaction to multicomponent systems. It will be shown that non-crystalline layers involving more than two components can grow to thicknesses which were not attained so far. By analyzing the interdiffusion profiles, several conclusion can b
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