Synthesis of Cu 47 Ti 34 Zr 11 Ni 8 Bulk Metallic Glass by Warm Extrusion of Gas Atomized Powders
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E. Rozhkova Ames Laboratory, Iowa State University, Ames, Iowa 50014
P. Huang, P.B. Wheelock, M.F. Besser, and M.J. Kramer Ames Laboratory, Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50014
M. Calvo-Dahlborg and U. Dahlborg LSG2M-UMRT7584, Ecole des Mines, 54042 Nancy Cedex, France (Received 29 May 2001; accepted 31 October 2001)
Cu47Ti34Zr11Ni8 amorphous gas atomized powders were consolidated by warm extrusion. After consolidation near 723 K using an extrusion ratio of 5, the material retains between 88% and 98% of the amorphous structure found in the gas atomized powder. The onsets of the glass transition and crystallization temperatures of this extruded material are observed respectively at slightly higher and lower temperatures than those of the starting powders. These temperature shifts are attributed to a composition change in the remaining amorphous phase during partial devitrification throughout the extrusion process. Powders extruded at the same temperature, but using higher extrusion ratios of 9 and 13, exhibit substantial devitrification during the consolidation process yet still deform homogeneously.
I. INTRODUCTION
A new family of amorphous metallic alloys has been recently developed. Unlike the preceding binary and ternary metallic alloys which form only under very high cooling rates (e.g., 104–107 K s−1), the new alloys require critical cooling rates (i.e., the minimum cooling rate required to avoid the onset of crystallization) of 102 K s−1 or less for glass formation. These alloys are referred to as bulk metallic glasses because their low critical cooling rate allows them to be fabricated with large dimensions ranging from millimeters to centimeters.1 Examples of alloys which form bulk metallic glasses include La–Al– Ni,2 Zr–Al–Cu–Ni,3 Ti–Zr–Cu–Ni–Be,4 Ti–Zr–Cu–Ni,5 and Zr–Ti(Nb)–Al–Cu–Ni.6 Recently, Johnson1 provided a discussion of how certain multiple chemical and topological properties of different atoms in these multicomponent alloys influence glass-forming ability in bulk metallic glasses. In general, alloys which have (i) phases that exhibit low and especially deep eutectic temperatures, (ii) elements with a range of atomic radii, and (iii) a liquid with a strongly negative enthalpy of mixing visa´-vis the crystalline phase are considered to be strong candidates for forming bulk metallic glasses. Note that these criteria are not new to bulk systems but rather 186
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J. Mater. Res., Vol. 17, No. 1, Jan 2002 Downloaded: 06 Apr 2015
have been discussed for over 25 years with regards to compound formation and solid solutions. In fact, reports as far back as 19757 entertained that these factors were correlated with the ease of glass formation in metallic alloys. Not only do bulk metallic glasses resist crystallization during cooling from the liquid but after cooling into an amorphous state they can be heated to a temperature between their glass transition temperature (Tg) and crystallization temperature (Tx) for a period of
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