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.