The nature of the deformation of materials depends on the physical and chemical state of the materials. Thermodynamics provides a rigorous way by which the physical and chemical state of materials can be characterized. A brief account is made of the concepts of thermodynamics of reversible as well as irreversible processes that are needed to understand the plastic deformation of materials and related processes. The principles governing the chemical equilibrium are outlined including the concept of chemical potential, the law of mass action, and the Clapeyron slope (i.e., the slope of a phase boundary in the pressure-temperature space). When a system is out of equilibrium, a flow of materials and/or energy occurs. The principles governing the irreversible processes are outlined. Irreversible processes often occur through thermally activated processes. The basic concepts of thermally activated processes are summarized based on the statistical physics.

Key words entropy, chemical potential, Gibbs free energy, fugacity, activity, Clapeyron slope, phase diagrams, rate theory, generalized force, the Onsager reciprocal relation.