Continuum physics of phase and defect microstructures: bridging the gap between physical metallurgy and plasticity of aluminum alloys

The problem of how alloy composition and stress, phases and defects interact is of key importance for understanding and successful development of new alloys and tempers Richmond, 1986; Staley, J.T., 1992. Metallurgical aspects affecting strength of heat-treatable alloy products used in the aerospace industry. In: Proceedings IIIrd Int. Conf. on Aluminum Alloys, pp. 107–143]. In the area of phase transformations and microstructures, considerable progress has been achieved using lattice and continuum models with sharp or diffuse interfaces. In the area of defect microstructures and plastic instabilities, there is an improved understanding of material plasticity, different aspects of dislocation patterning, which define the performance of aluminum alloys. While the techniques to describe phase and defect transformations are very different, both types of changes can occur simultaneously when processing real-life aluminum alloys. The issue of concentration-sensitive plasticity models and phase microstructure models incorporating defects (such as dislocations) has been a topic of the materials community interest during the last several years. One should mention such events as the Workshop on “Concentration-Sensitive Plasticity Models” (Rockport, MA, Fall 2000), Materials Research Society (MRS) symposia “Interaction of Phase and Defect Microstructures in Metallic Alloys”; “Influences of Interface and Dislocation Behavior on Microstructure Evolution” (Boston, 1998, 2000). In this paper similar conceptual problems arising both in plasticity theory and in classical thermodynamics are analyzed, which can be traced to the absence of characteristic length scales in the corresponding phenomenological formalisms. It is further demonstrated that the concepts behind strain-gradient plasticity and thermodynamics of non-uniform equilibrium systems have common grounds in the same gradient representation of the corresponding free energy functionals. This similarity gives hope that a unified theory of plasticity and microstructure is indeed possible and can be developed. A survey of recent developments in this direction is presented.