A continuum thermodynamic approach to the phase-field method: the order parameter as internal state variable

The phase-field method is well established for simulating microstructure evolution in computational materials science, providing a numerically efficient tracking of interfaces and surfaces by means of an order parameter. The derivation of its evolution equation is usually based on a variational approach or a corresponding principle of virtual power. Both approaches consider the order parameter as an additional degree of freedom and assume a diffuse interface region from the outset. This work examines the interpretation of the order parameter as an internal state variable, instead of an additional degree of freedom, since it represents an observable rather than a controllable quantity. Furthermore, the phase-field method is considered as an approximation of the sharp interface theory of a continuum containing a singular surface. A Cauchy continuum with a material singular surface is considered as starting point. The evolution equation of the order parameter is derived consistently in the context of continuum thermodynamics by exploitation of the Clausius–Duhem inequality. In this context, the equation of heat conduction and the thermomechanical coupling is discussed regarding the diffuse interface region and the role of the latent heat due to phase evolution. Based on restrictions of the free energy, special cases of the evolution equation are presented. For a special case, the coincidence of the evolution equation obtained by the presented approach and the classical variational approach is demonstrated. Based on the presented approach, the classical Allen–Cahn/Ginzburg–Landau equation is obtained by assuming a spatially homogeneous temperature distribution.