chapter  3
96 Pages

Glass-Forming Ability of Alloys

We reviewed in Chapter 2 the principles of glass formation and realized that ametallic melt needs to be signicantly undercooled to atemperature below Tg, the glass transition temperature, for it to transform into the glassy state. It was also brie¡y mentioned that not all metallic alloys can be transformed into the glassy condition. In spite of this, avery large number of metallic glasses have been synthesized in binary, ternary, quaternary, and higherorderalloy systems. Two dated compilations can be referred to for the alloy systems and compositions and other details where the formation of metallic glasses in the form of thin ribbons was reported [1,2]. There were also afew reports of the formation of glassy phases in pure metals rapidly solidied from their molten state (see, e.g., Ref. [3]). But later, careful analysis revealed that the glassy phase in these pure metals was mainly stabilized by the presence of impurity atoms and that a minimum concentration of solute atoms was necessary for glass formation. Pure metals Bi and Sn were, however, produced earlier in the amorphous state by evaporating very thin lms on to asubstrate maintained at alow temperature of 4K[4-6]. Even though some basic empirical rules and some of the thermodynamic conditions that need to be satised to form aglass were known, there was no rigorous scientic basis, at least at the beginning, for choosing the alloy compositions that could be formed as glasses. Many glasses were produced more or less by trial and error. In fact, the veryrst synthesis of a metallic glass in the Au-25 at.% Si alloy by rapid solidication processing (RSP) in 1960 by Pol Duwez and his students [7] was by accident (in more than one way!).