Orowan

A direct measurement of surface tension is sometimes possible from the work of cleaving a crystal. Mica, in particular, has such a well-defined cleavage plane that it can be split into large sheets of fractional millimeter thickness. Orowan... 

Once the precipitates grow beyond a critical size they lose coherency and then, in order for deformation to continue, dislocations must avoid the particles by a process known as Orowan bowing(23). This mechanism appHes also to alloys strengthened by inert dispersoids. In this case a dislocation bends between adjacent particles until the loop becomes unstable, at which point it is released for further plastic deformation, leaving a portion behind, looped around the particles. The smaller the interparticle spacing, the greater the strengthening. 

E. Orowan, ia Symposium on Internal Stresses in Metals, Institute of Metals, London, 1948, pp. 451—453. 

Orowan [7] has shown that when disloeations move with average veloeity V under the influenee of a shear stress r, the rate at whieh plastie shear strain is aeeumulated is given by... 

This process of backward dislocation motion produces reverse plastic flow immediately upon reduction of longitudinal stress from the shocked state. A modification of the Orowan equation, (7.1), to the current situation is... 

E. Orowan, Zur Kristallplastizitat. III. Uberden Mechanisms des Gleitvorganges, Z. Phys. 89, Nenntes und Zehntes Heft, 634-659 (1934). 

Orowan (1949) suggested a method for estimating the theoretical tensile fracture strength based on a simple model for the intermolecular potential of a solid. These calculations indicate that the theoretical tensile strength of solids is an appreciable fraction of the elastic modulus of the material. Following these ideas, a theoretical spall strength of Bq/ti, where Bq is the bulk modulus of the material, is derived through an application of the Orowan approach based on a sinusoidal representation of the cohesive force (Lawn and Wilshaw, 1975). 

With a three-parameter model of the intermolecular potential, the theoretical spall strength is not simply a constant times the bulk modulus. Although the slightly greater accuracy obtained is not critical to the present investigation, an energy balance is revealed in the analysis which is not immediately transparent in the Orowan approach. 

The papers which introduced the concept of a dislocation all appeared in 1934 (Polanyi 1934, Taylor 1934, Orowan 1934). Figure 3.20 shows Orowan s original sketch of an edge dislocation and Taylor s schematic picture of a dislocation moving. It was known to all three of the co-inventors that plastic deformation took place by slip on lattice planes subjected to a higher shear stress than any of the other symmetrically equivalent planes (see Chapter 4, Section 4.2.1). Taylor and his collaborator Quinney had also undertaken some quite remarkably precise calorimetric research to determine how much of the work done to deform a piece of metal... 

Figure, ,20, An edge dislocation, as delincalcd by Orowan (a) and Taylor (b).

Only Orowan remained with the topic and contributed a number of seminal ideas to the theory of the interaction between moving dislocations and other dislocations... 

After 1934, research on dislocations moved very slowly, and little had been done by the time the War came. After the War, again, research at first moved slowly. In my view, it was not coincidence that theoretical work on dislocations accelerated at about the same time that the first experimental demonstrations of the actual existence of dislocations were published and turned invention into discovery . In accord with my remarks in Section 3.1.3, it was a case of seeing is believing all the numerous experimental demonstrations involved the use of a microscope. The first demonstration was my own observation, first published in 1947, of the process of polygonization, stimulated and christened by Orowan (my thesis adviser). When a metal crystal is plastically bent, it is geometrically necessary that it contains an excess of positive over negative dislocations when the crystal is then heated, most of the dislocations of... 

Mott played a major part, with his collaborator Frank Nabarro (b. 1917) and in consultation with Orowan, in working out the dynamics of dislocations in stressed crystals. A particularly important early paper was by Mott and Nabarro (1941), on the flow stress of a crystal hardened by solid solution or a coherent precipitate, followed by other key papers by Koehler (1941) and by Seitz and Read (1941). Nabarro has published a lively sequential account of their collaboration in the early days (Nabarro 1980). Nabarro originated many of the important concepts in dislocation theory, such as the idea that the contribution of grain boundaries to the flow stress is inversely proportional to the square root of the grain diameter, which was later experimentally confirmed by Norman Fetch and Eric Hall. 

Koehler, J.S, Seitz. F., Read Jr., W.T., Shockley, W. and Orowan, E. (eds.) (1954) Dislocations in Metals (The Institute of Metals Division, American Institute of Mining and Metallurgical Engineers, New York). 

Nabarro, F.R.N. and Argon, A.S. (1995) Memoir of Egon Orowan, in Biographical Memoirs of Fellows of the Royal Society, vol. 41 (The Royal Soeiety, London) p. 315. 

Orowan, E. (1952) Fundamentals of brittle behavior in metals, in Fatigue and Fracture of Metals, Symposium at MIT, ed. Murray, W.M. (MIT and Wiley, New York). 

In general terms, as has already been mentioned, plastic deformation is a transport process analogous with electrical and thermal conductivity. These involve an entity to be transported, a carrier that does the transporting, and a rate of transport. In the case of electrical conductivity, charge is the transport entity, electrons (or holes) are the carriers, and the electron net velocities determine the rate. In the case of plastic deformation, displacement, b (cm) is the transport entity, dislocations are the carriers, N ( /cm2), and their velocities, v (cm/sec) determine the shear deformation rate, d8/dt. In two dimensions, the latter is given by the Orowan Equation ... 

Crystal dislocations were invented (circa. 1930) by Orowan, Prandtl, and Taylor to explain why pure metal crystals are soft compared with homogeneous shear strengths calculated from atomic theory. They do this very well. However, roughly 15 years later (circa 1945) it was found that pure semiconductor crystals (e.g., Ge and Si) have hardnesses at room temperature comparable with calculated homogeneous shear strengths. Furthermore, it was known... 

E. Orowan, The Sorby Centennial Symposium on the History of Metallurgy, Gordon and Breach, Science Publ., New York, USA (1965). 

Figure 6.4 Orowan model of precipitation hardening. From Guy (1962).

Orowan bowing mechanisms, 13 502 Orphan Drug Act, 18 686 Orpiment, 3 263t Orris, in perfumes, 18 369 Orr-Sherby-Dorn parameter, 13 478 Ortacrone, molecular formula and structure, 5 95t... 

E being the modulus of elasticity. Hence, 7 = 0.5 aa7mjE. Similar expressions have been obtained by Orowan as early as 1934, and recently in Ref.8. However, the values thus deduced for NaCl crystals were different from those predicted from interatomic potentials. 

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