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Stretching the range of the glassy state

Glasses are formed on cooling liquids when the atomic dynamics slow down without crystallization. Because the transition from the liquid to the glassy state is kinetic, and not thermodynamically determined, there is a range of glassy states, depending on the cooling rate. All properties of the glass are affected, but fundamentally the states are most often characterized in terms of energy (enthalpy) and volume. A glass that is formed on relatively rapid cooling is less relaxed, and has higher enthalpy and higher volume. Annealing a glass induces structural relaxation (or ageing) that takes the glass to states of lower enthalpy and volume. The reverse process, taking the glass to less-relaxed states of higher enthalpy and volume, has been termed rejuvenation. Rejuvenation can be achieved through such processes as irradiation and mechanical deformation. Effects of ageing and rejuvenation have been widely studied in polymeric glasses; this presentation explores the extension of these ideas to metallic glasses. There is interest in exploring the range of glassy states because of the properties that can be achieved. For metallic glasses, there is a particular current interest in rejuvenation as a route to improved plasticity. Studies will be reviewed, showing that the range of accessible glassy states is very wide, that there are novel developments (particularly in methods for rejuvenation) and that useful property improvements can be achieved.