Composition Dependence of Phase Separation and Crystallization in Deeply Undercooled Zr-(Ti-)Cu-Ni-Al Alloys

摘要

Different bulk glass forming alloys in the neighborhood of Zr_(52.5)Cu_(17.9)Ni_(14.6)Al_(10)Ti_5 (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, Zr_(52.5)Cu_(17.9)Ni_(14.6)Al_(10-x)Ti_(5+x) with -5≤x≤+2.5, the glass transition temperature, T_g, 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 T_c. In contrast to T_g, T_c increases with x and thus intercepts with T_g 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 T_c. We conclude that T_c, respectively the relation between T_c and T_g, determines the crystallization behavior and the thermal stability of these bulk metallic glasses.