  Simple classical thermodynamic approach to the general description of
metastable states is presented. It makes possible to calculate the explicit
dependence of the Gibbs free energy on temperature, to calculate the heat
capacity, the thermodynamic barrier, dividing metastable and more stable
states, and the thermal expansion coefficient. Thermodynamic stability under
mechanical loading is considered. The influence of the heating (cooling) rate
on the measured dynamic heat capacity is investigated. A phase shift of the
temperature oscillations of an ac heated sample is shown to be determined by
the relaxation time of the relaxation of the metastable nonequilibrium state
back to the metastable equilibrium one. This dependence allows one to calculate
the relaxation time. A general description of the metastable phase equilibrium
is proposed. Metastable states in AB3 alloys are considered. Reasons for the
change from the diffusional mechanism of the supercritical nucleus growth to
the martensitic one as the heating rate increases are discussed. The Ostwald
stage rule is derived.
