Bulk amorphous metallic alloys: Synthesis by fluxing techniques and properties
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INTRODUCTION
UNDERCOOLING phenomena in metallic melts are of significant importance, because novel microstructures and phases can be explored by controlled undercooling and solidification.[1–7] In general, the microstructure developed upon crystallizing the melt depends on the degree of undercooling at which crystallization takes place. Large degrees of undercooling are needed to obtain novel microstructures and amorphous alloys. Early studies on crystal nucleation by Turnbull and Cech[8] and Turnbull[9] demonstrated that the main competition to undercooling resided in oxides and other inclusions in the melt which acted as heterogeneous nucleation centers for crystallization. By using small droplets and emulsion techniques, they were able to isolate the heteronucleants and obtained the relative undercoolings DTLN/TL, where TL is the liquidus temperature, on the order of 0.2 for a number of metallic elements. Other techniques, including containerless electromagnetic levitation[10–13] and fluxing,[5,14,15] have been used to extend the range of undercooling and to study the microstructure development from undercooled liquids, the magnetic properties of undercooled melts,[16] and the liquid-solid interfacial tension.[17] If heteronucleants are removed from the metallic melt, crystal nucleation will be homogeneous. The physics of homogeneous nucleation in undercooled metallic melts were developed by Turnbull and Fisher[18] following the earlier work of Gibbs and others. Rapid solidification techniques can be used to extend the range of undercooling in metallic melts and produce amorphous alloys. To make an amorphous alloy, the metallic YI HE, formerly Postdoctor, Los Alamos National Laboratory, is Senior Packaging Engineer, Intel Corporation, Chandler, AZ 85226. TONGDE SHEN, Postdoctor, and R.B. SCHWARZ, Laboratory Fellow, are with Los Alamos National Laboratory, Los Alamos, NM 87545. This article is based on a presentation made in the ‘‘Structure and Properties of Bulk Amorphous Alloys’’ Symposium as part of the 1997 Annual Meeting of TMS at Orlando, Florida, February 10–11, 1997, under the auspices of the TMS-EMPMD/SMD Alloy Phases and MDMD Solidification Committees, the ASM-MSD Thermodynamics and Phase Equilibria, and Atomic Transport Committees, and sponsorship by the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory. METALLURGICAL AND MATERIALS TRANSACTIONS A
melt must be undercooled from the melting temperature (TL) to the glass transition temperature (Tg). Assuming that the melt has a low density of heteronucleants, the main competition to glass formation will arise from homogeneous nucleation. Turnbull[19] studied homogeneous nucleation in undercooled melts and showed that the most important parameter describing the nucleation rate is the ratio Trg 5 Tg/TL, or reduced glass transition temperature. The solid curves in Figure 1 show homogeneous nucleation rates for various values of Trg calculated using the parameters listed in Reference 20. The nucleation rate becomes negligible
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