A brief history of dislocation theory
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I.
HARBINGERS AND ANTECEDENTS
W E are all now familiar with some of the pedagogical dislocation analogs, such as the crawling caterpillar and the puckered carpet, Figure I. As documented in two valuable sources 1'2 for historical information, scientists in the 19th century had thought of a grain-type model of the aether containing localized defects, analogous to dislocations, that would enable the aether to deform.3'4 Burton 3 called these defects strain-figures, and Larmors described the creation of a strain-figure, that we would now recognize as a disc of twist boundary terminating within a material: he envisioned the creation of a lens-shaped cavity and the twisting of one surface in its plane, followed by the cementing together of the two sides. Somewhat related, a mosaic-block model of subgrains within a crystal was developed by Darwin 6 to explain the intensity of X-ray diffraction; the walls of these grains now being recognized as small-angle dislocation boundaries. The elastic fields of dislocations in isotropic continua were derived beginning at the turn of the century. Weingarten 7 considered defects formed by the displacement of cut surfaces in bodies and showed that rigid displacements of the surfaces, creating dislocations, were required if strains in the body were to remain bounded. The stress fields of these defects were determined by Timpe 8 and the elastic properties were elaborated by Volterra, 9 who classified the general types of the defects into the six forms shown in Figure 2. Volterra called the defects distorsioni, but later Love, 1~who also contributed to the elastic theory, "ventured to call them dislocations." In the continuum mechanics literature there is still a tendency to follow this nomenclature and call all of these defects dislocations, but in the crystal plasticity area the defects in Figures 2(e), (f), and (g) are almost exclusively called disclinations while those in Figures (b), (c), and (d) are called dislocations. The line of thought leading to the events of 1934 might be considered to have originated in the observations 11'12in the 19th century that metals plastically sheared by forming slip bands or slip packets and in the work in X-rays in the early 20th century by M. VonLaue, P.P. Ewald, W.H. Bragg, and W.L. Bragg culminating in the concept of crystallinity. 13 A series of localized or extended defects was envisioned ~4-19 in an effort to explain slip or fracture of crystals, some in view of the large discrepancy between theoretically predicted strengths for perfect c r y s t a l s 20'21 and experimental results. One of t h e s e 16 had some elements of
J. P. HIRTH is Professor, Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210. This paper is based on a presentation made at the symposium "50th Anniversary of the Introduction of Dislocations" held at the fall meeting of the TMS-AIME in Detroit, Michigan in October 1984 under the TMSAIME Mechanical Metallurgy and Physical Metallurgy Committees. METALLURGICAL TRANSACTIONS A
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