Part I Introduction and Fundament als

Introduction Modern materials science is based on the fundamental experience that the properties of materials are not unalterably determined by their average chemical composition but they are to a large extent influenced by their microstructure. This applies particularly for their mechanical and electromagnetic properties. Thus, modern "materials research" is often used as a synonym for "microstructure research". While the evolutionary direction of microstructure is prescribed by thermodynamics, its actual evolution path is selected by kinetics. It is this strong influence of thermo-dynamic non-equilibrium mechanisms that entails the large variety and complexity of microstructures typically encountered in engineering materials. It is an essential observation that it is not those microstructures that are close to equilibrium, but often those that are in a highly non-equilibrium state that provide particularly advantageous material property profiles. Microstructure can be defined as the totality of all thermodynamic non-equilibrium lattice defects on a space scale that ranges from Angstr0ms (e.g. non-equilibrium foreign atoms) 1 to meters (e.g. sample surface) (Haasen 1984). Its temporal evolution ranges from picoseconds (dynamics of atoms) to years (corrosion, creep, fatigue). It is one major aim of materials science to quantitatively relate macroscopic sample behavior to micro-structure. This goal imposes the task of identifying and describing those lattice defects, including their collective static and dynamic behavior, that are responsible for specific macroscopic properties. Figures 1.1 and 1.2 show that the characteristic scales which are associated with the various lattice defects establish a certain hierarchy of microstructure. This sequence, however, merely reflects a spatial rather than a crisp physical classification 2. Following this length scale hierarchy, the various levels of microstructure modeling can be roughly grouped into the nanoscopic, microscopic, mesoscopic, and macroscopic regimes. In this context the term nanoscopic refers to the atomic level, microscopic to lattice defects ensembles below the grain scale, mesoscopic to lattice defect ensembles at the grain scale, and macroscopic to the sample geometry. Of course, this subdivision is to a certain extent arbitrary. As will be discussed later, various alternative subdivisions are conceivable. 1 In pure metals, only vacancies or certain foreign atoms provide sufficient entropy to compensate their enthalpy of formation or solution, respectively c va .c(T < T 7n ^t) ~ 10~ 4. 2 For instance, small defects, such as dopants, can have a larger influence on strength or conductivity than large defects such as preciptates. 14 ΠΠΊ μτη mrn fs ps ns μ8 ms s …