13.7 - Metallic binary liquid-glass transition

Summary

Many complications in the metal physics description of binary alloys have been referred to as arising from the concentration dependence of the force fields. Thus a simpler approach seems called for in discussing the metallic binary liquid-glass transition.

Cohen and Turnbull (1959) [see also Turnbull and Cohen (1961) and Cohen and Grest (1979)] proposed a free-volume model in order to examine the thermodynamic and diffusive behaviour in the vicinity of the glass transition. In this model [see Li, Moore, and Wang (1988a, b)]:

1. An atom in the supercooled liquid or glass, for the most part, vibrates in a cage formed by its surrounding atoms, and

2. The atom inside the cage may escape to a void and diffuse from its original position, when it gains sufficient activation energy to overcome the barrier between its cage and the void.

The void referred to is defined as having a free volume greater than an atomic volume and is adjacent to the cage.

Point (1) has been established to be valid from a computer simulation of the static and dynamic properties for a Lennard-Jones (LJ) system (Kimura and Yonezawa, 1983). However, the same computer study implies that point (2) may not be actually applicable to the atomic mean square displacements.

Because of the fundamental differences between a LJ system and a metal, Li, Moore, and Wang (1988a,b) have made similar computer studies of metallic binary systems, which can become metallic glasses by a rapid quench from the melt not only in computer experiments but also in the laboratory (in which LJ systems such as argon never become glassy).