Toward A Sound Understanding of Dislocation Plasticity
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Toward A Sound Understanding of Dislocation Plasticity
L.M. BROWN R.F. Mehl Medalist
A novel theory is presented for work hardening in stage II shown by single crystals of ductile face-centered cubic (fcc) metals. The theory is based on the assumption that slipbands are highly elongated ellipsoidal zones with weak interiors, oriented at small, alternating angles with the crystallographic slip planes. Consideration of the stacking of such ellipsoids, together with a condition for the nucleation of a new slipband at the center of an obstacle nearby, yields both the average angle of inclination and the shear offset in the bands, both of which remain constant throughout stage II. The theory makes several predictions concerning the internal stress distribution and the plastic behavior. Before expounding the theory, the concepts upon which it is based are discussed in the context of two much simpler problems: the work hardening of dispersionhardened metals and the cyclic work hardening which forms the basis of understanding fatigue properties. The picture confirms in a natural way that the value of o'm, the stress which marks the end of stage II hardening, may be taken to be equal to the fatigue endurance limit. 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 men 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. Professor L.M. B R O W N , Canadian by birth, has lived in England since 1957. He took his first degree at the University of Toronto in Engineering Physics and then completed a Ph.D. at the University of Birmingham in Physical Metallurgy under the supervision of Professors P.L. Pratt and J.D. Eshelby. He went to the University of Cambridge to take up a postdoctoral position under Professor P.B. Hirsch and has since followed a career on the staff of the Department of Physics (Cavendish Laboratory). In 1963, Professor Brown was awarded a Research Fellowship at Gonville and Caius College, Cambridge and, in 1979, moved as a Founding Fellow to Robinson College, Cambridge, where he directed studies in Natural Sciences until 1987. He was a recipient of the Rosenhain Medal of the Institute of Metals and elected a Fellow of the Royal Society in 1982. His research interests have focused on understanding the properties of matter in terms of structure. This has included studies of all types of solids and optical and electrical properties, as well as mechanical properties. A theme which recurs throughout his published work is the progressive improvement in understanding of plastic properties, particularly metal fatigue and dispersion hardening of metals. Another theme is the application of novel techniques in electron microscopy to the characterization of mate
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