Role of Interface in Ion Mixing or Thermal Annealing Induced Amorphization in Multilayers in Some Immiscible Systems
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('CAST (World Lab.), Beijing 100080, China
ABSTRACT Six binary metal systems were selected to study the possibility of forming amorphous alloys by ion mixing or thermal annealing in multilayered films, i.e. the
Ta-Cu, Zr-Nb, Zr-Ta, Y'-Zr, Y-Mo and Y-Ta systems, featuring positive heats of formation (Allf) ranging from +3 to +40 kJ/mol. Firstly, the interfacial free energy consisting of a chemical and an elastic terms was calculated and added to the energetic state of the multilayers. It was found that the excess interfacial free energy increased with increasing the fraction of the interfacial atoms in the multilayers, and could raise the multilayers to an energy level intersecting with or being higher than that of the amorphous phase possessing a typical convex shape. It is therefore possible to produce amorphous alloys in such systems, if the multilayered films included enough fraction of the interfacial atoms. The multilayered samples were then designed and prepared accordingly and both ion mixing and thermal annealing under appropriate conditions resulted in the formation of a number of new amorphous alloys, confirming the above prediction based on the interfacial free energy concern. It is noted that the success of synthesising amorphous alloys by solid-state reaction in the immiscible systems develops a new glass forming technique, namely interfacegenerated spontaneous amorphization, which has a great potential to produce new and relatively thick amorphous films, e.g. a Ta7 zCu28 amorphous film of 800 nm thick was obtained. I, Introduction Since the first amorphous alloy was obtained by liquid melt quenching in early 60s [1, various techniques have been developed to produce amorphous alloys, or metallic glasses, as well as to study the glass forming ability (GFA) of the binary metal systems 121. From the 80s, ion mixing (IM) 131 and solid-state reaction 14], both using alternately deposited metal-metal multilayers to induce amorphization have attracted much attention. It has been proved that IM can greatly extend the glass forming range (GFR) usually being taken as a measure of the GFA, as an effective cooling speed available in IM can be as high as 1012-" k/sec 151. Solid-state reaction,
71 Mat. Res. Soc. Symp. Proc. Vol. 396 © 1996 Materials Research Society
which is also named spontaneous vitrification (SV), is a unique technique requiring a single step only of thermal annealing at relatively low temperatures for producing amorphous alloys. Although the amorphization mechanism has not been understood from an atomic scale, several empirical models have been proposed to predict the GFA upon IM based on the previously obtained experimental results [3,6,7,8]. One intrinsic parameter of the system, i.e. the heat of formation (AHK) calculated by Miedema's theory 191, has been considered by many authors. However, its effect on GFA has been debated for a long time for the positive A1If systems. For instance, J.A.Alonso [71 predicted that amorphous alloy could not be formed by IM in a system with a positive AHf
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