LIFE-CYCLES OF ENGINEERING MATERIALS

The objectives of materials science are considered in context with com plete cycles of materials. The matter passes through the following sta ges: primary raw material from deposits (alpha), production of enginee ring materials (W), fabrication of systems (F) their use (G), their fa ilure (V), and recycling or redepositing (Omega). The changes of energ y and entropy are discussed which occur during this sequence principal ly due to separation, mixing and ordering of atoms. A characteristic f eature of modern technology is a reduced amount of material required f or a certain useful function (due to improved material properties), wh ile the complexity of technical systems is increasing. Consequently, t he individual cycles of a large number of materials are combined in a system for the duration of its use. This leads to efficient machines, but also to complex mixtures of matter at the end of its use, i.e, to an increase of entropy. After a material has done its duty efficiently , safely, and for a long period of time, it finally will become scrap. There exist several options for its further fate. They depend on the microstructure of the material, on fabrication, design and failure of the machine. Most favourable is the closure of a cycle by regaining se condary raw materials (Omega --> alpha) or secondary engineering mater ials (Omega --> W), - least fortunate is a fine dispersion of a dilute mixture of atoms on the solid surface of the earth, in water or in th e atmosphere. Intermediate is concentrated dumping in secondary deposi ts or regaining of stored energy by combustion. Cycles can be closed r ather easily with established materials based on iron, or even better with light metals (Al, Mg). The same is true for well-classified ceram ic glasses and thermo-plastic polymers. A closure of cycles is also po ssible by acquiring carbon from CO2 via biosynthesis of natural polyme rs (starch, cellulosis). It will be a rewarding task to future materia ls science to provide the fundamentals for aiding the development of ' 'recycling-friendly'' materials, fabrication, and design methods. Mate rials science should also help to create a more rational discussion of the best choices for materials selection and recycling methods.