The early stages of solute distribution below a transition temperature
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The Early Stages of Solute Distribution Below a Transition Temperature
J.B. COHEN R.F. Mehl Medalist
We now recognize that there are many possibilities for decomposition: ordering in the matrix followed by precipitation, decomposition followed by ordering, transition phases, etc. Both energetic and kinetic factors can cause such sequences. The formation of coherent phases in some A1, Cu, and Au systems is reviewed. A simpler understanding of Guinier-Preston (GP) zone development in these alloys has emerged. Also, it is not clear that homogeneous nucleation occurs at all in most systems, either in precipitation or on ordering, because fluctuating regions, like the low-temperature phase, are often present in the solid solution about the solvus. Furthermore, coherency strain may play an important role in the beginnings of these phase transformations, controlling whether precipitation or ordering or even displacive transformation occurs, and may strongly affect the coherent phase diagram.
Professor Cohen is an outstanding teacher and researcher on applying X-ray diffraction to the study of atomic arrangements in imperfectly crystalline solids. This research uses diffuse X-ray scattering, line broadening, and peak shifts to study the defects that control many properties. He has performed the definitive research on the structure of Guinier-Preston zones, which are atom clusters responsible for the strength of many important metallic alloys. Although Guinier-Preston zones were J.B. COHEN, Dean, Robert R. McCormick School of Engineering, and Engelhart Professor of Materials Science, is with Northwestern University, Evanston, IL 60208. The Institute of Metals Lecture was established in 1921, at which time the Institute of Metals Division was the only professional division within the American Institute of Mining and Metallurgical Engineers. It has been given annually since 1922 by distinguished people from this country and abroad. Beginning in 1973 and thereafter, the person selected to deliver the lecture will be known as the "Institute of Metals Division Lecturer and R.F. Mehl Medalist" for that year. METALLURGICAL TRANSACTIONS A
discovered in the late 1930s in A1-Cu alloys, the correct structure was not determined until recently and is much different than previously thought. Professor Cohen determined the morphology of supported catalysts in collaborative research which related the observed morphology to kinetics of reactions. His research on residual stresses now allows measurements of triaxial stresses and stress gradients, including twophase mixtures. He has developed a portable X-ray device for measuring residual stress which is used in the field. He developed diffraction techniques for measuring the size and volume fraction of cavities which develop in metals under stress at high temperatures and which are present in portland cement after curing. His extensive studies of nonstoichiometric compounds include discovery of the clustering of point defects in nonstoichiometric oxides, the basis for understanding their
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