Plastic deformation of crystals

Cross-gliding of screw dislocations has an important effect on the overall plastic deformations of crystals because it is the primary cause of both multiplication, and strain-hardening as discussed above. 

G. I. Taylor, Mechanism of Plastic Deformation of Crystals. I. Theoretical, Proc. Roy. 

Polanyi, My time with x-rays and crystals, in Fifty Years of X-Ray Diffraction, 636. G.I. Taylor, The mechanism of plastic deformation of crystals, Proceedings of the Royal Society A145 (1934) 362-415 E. Orowan, "Zur Kristallplastizitat, Zeitschriftfur Physik 89 (1934) 605-659. On recent studies of dislocation, see R. F. Service, Materials scientists view hot wires and bends by the bay, Science 272 (1996) 484-485. 

G. I. Taylor, The Mechanism of Plastic Deformation of Crystals. Part I. Theoretical, Proceedings of the Royal Society of London Series A145 vol. 855 (1934), pp. 362-387. ... 

An experimental relationship between the microhardness and elastic modulus (E) has been found for various PE materials with different crystallinity values (Flores et al.., 2000). It is important to realize that microhardness - the plastic deformation of crystals at high strains - primarily depends on the average thickness and perfection of the nanocrystals, whereas in the case of the modulus, the elastic response at low strains is dictated by the cooperative effects of both microphases, the crystalline lamellae and the amorphous layer reinforced by tie molecules. The... 

Fig. 4.11 (b)). It is believed that this is because Young s model was developed for the plastic deformation of crystals in a direction parallel to the molecules (Santa Cmz etal, 1993). 

In summarizing, it can be concluded that the microhardness of elongational flow injection moulded PE is influenced by a local double mechanical contribution (a) a plastic deformation of crystal lamellae under the indenter, and (b) an elastic recovery of shish-fibrils parallel to the injection direction after load removal. Further, the Shish-crystals are preferentially formed when high orientation occurs, i.e. at zones near the centre of the mould and at an optimum processing temperature Tp around 145-150 °C. Below this temperature overall orientation decreases due to a wall-sliding mechanism of the mbber-like molten polymer. 

Although the theory of dislocations was initially developed to explain mechanical properties and plastic deformation of crystals, various studies clearly showed that... 

Frank, F. C., Report of the Pittsburgh Conference on Plastic Deformation of Crystals, p. 100. Carnegie Inst. Tochnol. and Office Naval Res., Washington, D.C., 1950. 

The word trap also expresses the fact that these point defects can frequently capture electronic excitation energy. Other well-studied X traps are those of pyrene in anthracene, with a trapping depth of AE= 59 cm b Naturally, there are also triplet X traps, e.g. in 1,2,4,5-tetrachloro-benzene, with AE = 21.3 cm . Host molecules can also act as X traps when they are perturbed not by foreign molecules but by a specific structural defect. Occasionally, in the literature a distinction is made between X and Y traps, depending on whether the lattice perturbation is caused by a structural defect in the crystal (Y trap) or by a foreign molecule (X trap). Plastic deformation of crystals can also produce discrete trapping states, for example in... 

Taylor GI (1934a) The mechanism of plastic deformation of crystals. Part 1, theoretical. Proc R Soc Lond A 145 362-387... 

It follows that in drawing there is a competition between cavitation and activation of crystal plasticity easier phenomena occur first, cavitation m polymers with crystals of higher plastic resistance, and plastic deformation of crystals in polymers with crystals of lower plastic resistance. 

Dislocations occur in lattices other than those of atomic-scale crystals. The best known examples, no doubt, are those in the experiments of Bragg and Nye with rafts of soap bubbles [1]. Their work illustrated the geometry and kinematics of the edge dislocations in these two-dimensional hexagonal lattices, and vividly revealed how macroscopic plastic deformation of crystals is effected by the motion of large numbers of such dislocations. Ahead of the definitive identification of moving dislocations in atomic crystals by electron microscopy [2], Bragg and Nye s experiments had convinced many of the reality and the potential of what had initially been a purely theoretical concept [3-5]. 

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