Crystals dislocation and

In his early survey of computer experiments in materials science , Beeler (1970), in the book chapter already cited, divides such experiments into four categories. One is the Monte Carlo approach. The second is the dynamic approach (today usually named molecular dynamics), in which a finite system of N particles (usually atoms) is treated by setting up 3A equations of motion which are coupled through an assumed two-body potential, and the set of 3A differential equations is then solved numerically on a computer to give the space trajectories and velocities of all particles as function of successive time steps. The third is what Beeler called the variational approach, used to establish equilibrium configurations of atoms in (for instance) a crystal dislocation and also to establish what happens to the atoms when the defect moves each atom is moved in turn, one at a time, in a self-consistent iterative process, until the total energy of the system is minimised. The fourth category of computer experiment is what Beeler called a pattern development... 

The overwhelming evidence accumulated over several decades is that the crystalline lamellae of PE, much like all crystalline ductile metals, deform plastically by the generation and motion of crystal dislocations and that, while twinning and martensitic shears have also been suspected, these do not make... 

Let us look at the growth sites more closely and consider factors which affect their concentration on the metal electrode the composition of the solution and the potential. Like growth sites of any other crystal, those on electrodes are found at the grain boundaries and at crystal dislocations and imperfections. These depend on the history of the electrode, whether it was stretched, hammered or annealed and thus will also depend on the history of the electrode. If the solution contains species that adsorb on the electrode at the crystal growth sites, will be smaller than if the solution was free from such species. This alone demands the exercise of great care in handling experiments and experimental data on electrocrystallization. 

Issues associated with order occupy a large area of study for crystalline matter [1, 7, 8]. For nearly perfect crystals, one can have systems with defects such as point defects and extended defects such as dislocations and grain... 

The physical properties of tellurium are generally anistropic. This is so for compressibility, thermal expansion, reflectivity, infrared absorption, and electronic transport. Owing to its weak lateral atomic bonds, crystal imperfections readily occur in single crystals as dislocations and point defects. 

An example of research in the micromechanics of shock compression of solids is the study of rate-dependent plasticity and its relationship to crystal structure, crystal orientation, and the fundamental unit of plasticity, the dislocation. The majority of data on high-rate plastic flow in shock-compressed solids is in the form of ... 

Calculations of this type are carried out for fee, bcc, rock salt, and hep crystal structures and applied to precursor decay in single-crystal copper, tungsten, NaCl, and LiF [17]. The calculations show that the initial mobile dislocation densities necessary to obtain the measured rapid precursor decay in all cases are two or three orders of magnitude greater than initially present in the crystals. Herrmann et al. [18] show how dislocation multiplication combined with nonlinear elastic response can give some explanation for this effect. 

To answer questions regarding dislocation multiplication in Mg-doped LiF single crystals, Vorthman and Duvall [19] describe soft-recovery experiments on <100)-oriented crystals shock loaded above the critical shear stress necessary for rapid precursor decay. Postshock analysis of the samples indicate that the dislocation density in recovered samples is not significantly greater than the preshock value. The predicted dislocation density (using precursor-decay analysis) is not observed. It is found, however, that the critical shear stress, above which the precursor amplitude decays rapidly, corresponds to the shear stress required to disturb grown-in dislocations which make up subgrain boundaries. 

Explain briefly what is meant by a dislocation. Show with diagrams how the motion of (a) an edge dislocation and (b) a screw dislocation can lead to the plastic deformation of a crystal under an applied shear stress. Show how dislocations can account for the following observations ... 

Figure 3.22. Screw dislocation and crystal growth, after W.T. Read.

Cottrell, A.H. (1953) Dislocations and Plastic Flow in Crystals (Clarendon Press, Oxford). 

Frank, F.C. (1985) Some personal reminiscences of the early days of crystal dislocations, in Dislocations and Properties of Rea Materials (The Institute of Metals, London) p. 9. 

Verma, A.R. (1953) Crystal Growth and Dislocations (Butterworths Seientific Publications, London). 

Ristic, R.I., Sherwood, J.N. and Shripathi, T., 1991. The role of dislocations and mechanical defonuation in growth rate dispersion in potash alum crystals. In Advances in Industrial Crystallization. Eds. J. Garside, R.J. Davey, and A.G. Jones. Oxford Butterworth-Heinemann, pp. 77-91. 

A pecuhar sohd phase, which has been discovered not too long ago [172], is the quasi-crystalline phase. Quasi-crystals are characterized by a fivefold or icosahedral symmetry which is not of crystallographic type and therefore was assumed to be forbidden. In addition to dislocations which also exist in normal crystals, quasi-crystals show new types of defects called phasons. Computer simulations of the growth of quasicrystals [173] are still somewhat scarce, but an increasing number of quasi-crystalline details are studied by simulations, including dislocations and phasons, anomalous self-diffusion, and crack propagation [174,175]. 

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