Kinetic States of Order in Highly Nonequilibrium Materials

Two concepts from the kinetic theory of disorder→order transformations are presented. Kinetics provides control over the path taken en route to equilibrium, permitting different types of microstructures to be synthesized. Variations in microstructural evolution can be parameterized by two or more order parameters. "Kinetic paths" through two or more order parameters are convenient for experimental study because they are independent of the vacancy concentration in the material. Saddle points in the free energy surface have little consequence to the thermodynamic state, but if an alloy approaches a saddle point its kinetic evolution tends to stall. This stalled state is an example of a "pseudostable" state.