Composition Dependence of Phase Separation and Crystallization in Deeply Undercooled Zr-(Ti-)Cu-Ni-Al Alloys
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Composition Dependence of Phase Separation and Crystallization in Deeply Undercooled Zr-(Ti-)Cu-Ni-Al Alloys Andreas A. Kündig1,2, Jörg F. Löffler1 and William L. Johnson1 W. M. Keck Laboratory, California Institute of Technology, Pasadena, California 91125, U.S.A. 2 Institut für Metallforschung, ETH Zürich, CH-8092 Zürich, Switzerland 1
ABSTRACT Different bulk glass forming alloys in the neighborhood of Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit105) have been investigated by differential scanning calorimetry (DSC), x-ray diffraction (XRD) and small-angle neutron scattering (SANS). Along the Ti/Al line in composition space, Zr52.5Cu17.9Ni14.6Al10-xTi5+x with –5 ≤ x ≤ +2.5, the glass transition temperature, Tg, and the undercooled liquid regime (the difference between the first crystallization temperature and the glass transition temperature) continually decrease with increasing x. SANS measurements of annealed alloys show interference maxima, giving evidence for decomposition on the nanometer scale, up to a critical temperature Tc. In contrast to Tg, Tc increases with x and thus intercepts with Tg in the range –2.5 ≤ x ≤ –1.25, depending on the time scale of the experiment. At this composition, significant changes in DSC traces and XRD patterns are observed. Additional isothermal DSC experiments show that the onset times for crystallization are significantly different for temperatures below and above Tc. We conclude that Tc, respectively the relation between Tc and Tg, determines the crystallization behavior and the thermal stability of these bulk metallic glasses. INTRODUCTION Several bulk glass forming alloys with critical cooling rates of about 10 K/s were recently found in the Zr-(Ti-)Cu-Ni-Al system [1-3]. Alloys in this system can show a substantially different crystallization behavior upon annealing, even for small changes in composition and/or annealing temperature [4, 5]. For glassy Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit105), the formation of nanocrystals in an amorphous matrix has been observed upon annealing above Tg [6], following decomposition of the homogeneous undercooled melt. The critical temperature for this decomposition process is about 810 K for this alloy, significantly above the glass transition temperature of 680 K. In this work, we have applied differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray diffraction (XRD), and small-angle neutron scattering (SANS) on bulk glass forming alloys with compositions in the neighborhood of Vit105. The characteristic values of these measurements, i.e., the glass transition temperature Tg, the crystallization temperature Tx, and the critical temperature Tc, depend significantly on small composition changes following a line in composition space through Vit105. These characteristic values are related to each other and its influence on the crystallization behavior is discussed. EXPERIMENTAL PROCEDURE Alloys with composition changes from the original alloy Vit105 were prepared by arc melting in a titanium gettered argon atmosphere by replacing ± 2.5%
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