Browsing Professor Frank Reginald Nunes Nabarro, FRS by Issue Date
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ItemThe strains produced by precipitation in alloys(Royal Society of London, 1940) Nabarro, F.R.N.If the process in metals results simply in the interchange of pairs of atoms large strains must be set up when a new phase prcipitates in an alloy. The strain energy involved is calculated for particles of precipitate of various shapes, and is found to be least if the precipitate forms thin plates. The shape of the particle actually formed is influenced by this energy, by the degree of supersaturation of the solid solution, and by surface effects; the calculated shape is shown to agree reasonably well with that observed experimentally. ItemThe influence of elastic strain on the shape of particles segregating in an alloy(Physical Society, 1940) Nabarro, F.R.N.An estimate is given of the way in which the strain energy associated with a particle of precipitate depends upon the shape of the particle. It is shown that the energy can only be reduced if the precipitate forms in flat plates, and then only in the case of particles which have broken away from the lattice of the matrix. this dependence of strain energy on shape causes the precipitate to consist of plates of a shape similar to that observed in Widmanstatten structures. ItemOrientation of nuclear spins in metals(Royal Society of London, 1940) Frohlich, H.; Nabarro, F.R.N.In connexion with the possible use of nuclear magnetism for the magnetic cooling method, the behaviour of the nuclear spins of monovalent metals at very low temperatures is investigated theoretically. ItemAn attempt to estimate the degree of precipitation hardening, with a simple model(Physical Society, 1940) Mott, N.F.; Nabarro, F.R.N.The process of diffusion being assumed to result simply in an interchange of atoms, an estimate is made of the internal strains produced by precipitation. According to the dislocation theory these strains are responsible for the hardness of the material, and so the degree of hardening can be predicted. ItemThe mechanical properties of metallic solid solutions(Physical Society, 1946) Nabarro, F.R.N.The theoretical relation between the lattice strains produced by precipitation in a mineral and the corresponding increase in hardness is extended to the case of lattice strains in metallic solid solutions. The elastic limit of a single crystal of a solid solution is calculated on the assumption that the crystal will slip when the applied external stress is equal to the mean value of the internal stress. This mean is taken over the length of a dislocation, assumed to be in numbers 1000 atoms. The estimate agrees in order of magnitude with the experimental observations. Similar considerations are applied to the hardness of polycrystalline solid solutions. The theory is extended to cases in which the increase of hardness produced by alloying is not large in comparison with the hardness of the pure solvent. ItemDislocations in a simple cubic lattice(Physical Society, 1947) Nabarro, F.R.N.The properties of dislocations are calculated by an approximate method due to Peierls. The width of a dislocation is small, displacements comparable with the interatomic distance being confined to a few atoms. The shear stress required to move a dislocation in an otherwise perfect latttice is of the order of a thousandth of the "theoretical" shear strength. The energy and effective mass of a single dislocation increase logarithmically with the size of the specimen. ItemDiffusion and precipitation in alloys(Institute of Metals, 1947) Nabarro, F.R.N.A general review is given of recent work on diffusion and precipitation in alloys, with special emphasis on the theoretical mechanism of diffusion, and on the factors governing the shape and size of the precipitate. The effects of internal stresses on diffusion and precipitation, and of precipitation on the mechanical properties, are discussed. ItemNuclear ferromagnetism(Physical Society, 1947) Frohlich, H; Nabarro, F.R.N.It is shown that at sufficiently low temperatures metals become ferromagnetic owing to an orientation of the nuclear spins. The domain structure of such ferromagnetics is analogous to that of ordinary ferromagnetics. ItemLaszlo's papers on tessellated stresses: a review(Institute of Metals, 1947-10) Nabarro, F.R.N.Four recent papers by Laszlo on "Tessellated Stresses, Parts I-IV" published in the Journal of the Iron and Steel Institute, are reviewed in detail. The present review contains no new results. ItemDislocation theory and transient creep(Physical Society, 1948) Mott, N.F.; Nabarro, F.R.N.The purpose of this paper is to put forward certain advances in the theory of dislocations, and in particular to discuss their application to the theory of transient creep, in the sense in which the term is used by Andrade (1911, 1914, 1932) and by Orowan (1947). ItemMechanical effects of carbon in iron(Physical Society, 1948) Nabarro, F.R.N.Snoek has shown that when carbon atoms move from one possible set of interstitial sites in the lattice of a-iron to another set they cause shear strains. Cottrell has shown that the stress around a dislocation may be relieved by the migration of solute atoms in its neighbourhood, and that the dislocation is then bound to its present position. By combining these theories with the usual theory of age-hardening it is possible to explain the existence of a yeield point, quench aging, strain aging, delayed yield and blue brittleness as consequences of the presence of carbon in iron. A rough quantitative theory of the time-aging effects is given, and shown to agree with experiment. The need for further experiment is emphasized.