Dislocation nucleation and vacancy formation during high-speed deformation of fcc metals

Recently, a dislocation free deformation mechanism was proposed by Kiritani et al., based on a series of experiments where thin foils of fcc metals were deformed at very high strain rates. In the experimental study, they observed a large density of stacking fault tetrahedra, but very low dislocation densities in the foils after deformation. This was interpreted as evidence for a new dislocation-free deformation mechanism, resulting in a very high vacancy production rate. In this paper we investigate this proposition using large-scale computer simulations of bulk and thin films of copper. To favour such a dislocation-free deformation mechanism, we have made dislocation nucleation very difficult by not introducing any potential dislocation sources in the initial configuration. Nevertheless, we observe the nucleation of dislocation loops, and the deformation is carried by dislocations. The dislocations are nucleated as single Shockley partials. The large stresses required before dislocations are nucleated result in a very high dislocation density, and therefore in many inelastic interactions between the dislocations. These interactions create vacancies, and a very large vacancy concentration is quickly reached.Comment: LaTeX2e, 8 pages, PostScript figures included. Minor modifications only. Final version, to appear in Philos. Mag. Let

Modelling of dislocation generation and interaction during high-speed deformation of metals

Recent experiments by Kiritani et al. have revealed a surprisingly high rate of vacancy production during high-speed deformation of thin foils of fcc metals. Virtually no dislocations are seen after the deformation. This is interpreted as evidence for a dislocation-free deformation mechanism at very high strain rates. We have used molecular-dynamics simulations to investigate high-speed deformation of copper crystals. Even though no pre-existing dislocation sources are present in the initial system, dislocations are quickly nucleated and a very high dislocation density is reached during the deformation. Due to the high density of dislocations, many inelastic interactions occur between dislocations, resulting in the generation of vacancies. After the deformation, a very high density of vacancies is observed, in agreement with the experimental observations. The processes responsible for the generation of vacancies are investigated. The main process is found to be incomplete annihilation of segments of edge dislocations on adjacent slip planes. The dislocations are also seen to be participating in complicated dislocation reactions, where sessile dislocation segments are constantly formed and destroyed.Comment: 8 pages, LaTeX2e + PS figures. Presented at the Third Workshop on High-speed Plastic Deformation, Hiroshima, August 200

‘So people know I'm a Sikh’: Narratives of Sikh masculinities in contemporary Britain

This article examines British-born Sikh men's identification to Sikhism. In particular, it focuses on the appropriation and use of Sikh symbols amongst men who define themselves as Sikh. This article suggests that whilst there are multiple ways of ‘being’ a Sikh man in contemporary post-colonial Britain, and marking belonging to the Sikh faith, there is also a collectively understood idea of what an ‘ideal’ Sikh man should be. Drawing upon Connell and Messerschmidt's discussion of locally specific hegemonic masculinities (2005. “Hegemonic Masculinity: Rethinking the Concept.” Gender and Society 19 (6): 829–859), it is suggested that an ideal Sikh masculine identity is partly informed by a Khalsa discourse, which informs a particular performance of Sikh male identity, whilst also encouraging the surveillance of young men's activities both by themselves and by others. These Sikh masculinities are complex and multiple, rotating to reaffirm, challenge and redefine contextualised notions of hegemonic masculinity within the Sikh diaspora in post-colonial Britain. Such localised Sikh masculinities may both assert male privilege and reap patriarchal dividends (Connell, W. 1995. Masculinities. Cambridge: Polity Press), resulting in particular British Sikh hegemonic masculinities which seek to shape the performance of masculinity, yet in another context these very same performances of masculinity may also signify a more marginalised masculinity vis-à-vis other dominant hegemonic forms

Quantum mechanics without spacetime II : noncommutative geometry and the free point particle

In a recent paper we have suggested that a formulation of quantum mechanics should exist, which does not require the concept of time, and that the appropriate mathematical language for such a formulation is noncommutative differential geometry. In the present paper we discuss this formulation for the free point particle, by introducing a commutation relation for a set of noncommuting coordinates. The sought for background independent quantum mechanics is derived from this commutation relation for the coordinates. We propose that the basic equations are invariant under automorphisms which map one set of coordinates to another- this is a natural generalization of diffeomorphism invariance when one makes a transition to noncommutative geometry. The background independent description becomes equivalent to standard quantum mechanics if a spacetime manifold exists, because of the proposed automorphism invariance. The suggested basic equations also give a quantum gravitational description of the free particle.Comment: 8 page

Protective Nature of the Surface film Formed on the Weathering Steel

Weathering steel containing Cu, P, etc., has been widely used for various steel structures such as, bridges, buildings, towers, etc., because of its maintenance free characteristics resulting from the formation of a protective and adhesive film during the exposure of the structure to the atmosphere. This feature facilitates the weathering steel to be used without any surface treatment. It is well known that rate of atmospheric corrosion dep-ends upon : i) the length of time, the moisture is in the contact of the surface ii) the extent of pollution in the atmosphere in which the material is exposed and iii ) the chemical composition of the iron and steels Here, i and ii both are of great importance . It is seen that sample of the same material exposed in a similar manner lost hundred times as much weight in the humid industrial atmosphere of England, as in the dry semi-tropical atmo-sphere of Khartown, Egypt.Variation in the composition of iron or steel (i.e., factor iii) may play as important role as atmospheric environment. For example, during exposure in an industrial atmosphere , a structural steel with 0.04% Cu lost 13.8g/sq.dm after 8 years of exposure, whereas a similar specimen of steel containing 11.4% Cr lost only 0. I6g/sq.dm'. Figure 1, is a time-corrosion plot2 of three steels in marine and industrial atmosp heres. It is seen from the figure that the loss in weight of copper steel during the first 0.5 and 1.5 years is considerably less in marine than in industrial atmos-phere , but from the shape of the curves it is probable that the corrosion losses in marine atmosphere will eventually be higher than in industrial atmosphere. Though the metal loss in 3.5% Ni steel is low it behaves in the similar manner as Cu steel.Opposite is true in case of more complex Ni- Cu-Si- P steel i . e., metal loss is less in industrial atmosphere to that of marine atmos-phere . It was suggested that such a reduced loss in weight was due to a very thin and adherent film formation on the latter surfac

Research Notes : United States : Evaluation of soybean germplasm for stress tolerance and biological efficiency : To evaluate soybean germplasm and cultivars for stress tolerance toward - Moisture Stress

Soybean genotypes were screened for water-stress tolerance in a split-plot experimental design with four replications. Irrigated and stress treatments were assigned main plots and the genotypes were assigned subplots. The stress water condition was induced by covering the plots with plastic

Prospect for room temperature tunneling anisotropic magnetoresistance effect: density of states anisotropies in CoPt systems

Tunneling anisotropic magnetoresistance (TAMR) effect, discovered recently in (Ga,Mn)As ferromagnetic semiconductors, arises from spin-orbit coupling and reflects the dependence of the tunneling density of states in a ferromagnetic layer on orientation of the magnetic moment. Based on ab initio relativistic calculations of the anisotropy in the density of states we predict sizable TAMR effects in room-temperature metallic ferromagnets. This opens prospect for new spintronic devices with a simpler geometry as these do not require antiferromagnetically coupled contacts on either side of the tunnel junction. We focus on several model systems ranging from simple hcp-Co to more complex ferromagnetic structures with enhanced spin-orbit coupling, namely bulk and thin film L1$_0$-CoPt ordered alloys and a monatomic-Co chain at a Pt surface step edge. Reliability of the predicted density of states anisotropies is confirmed by comparing quantitatively our ab initio results for the magnetocrystalline anisotropies in these systems with experimental data.Comment: 4 pages, 2 figure