Dislocation structure

Figure 6. Dislocation structures observed in (a) T1-S4 at.%Al and (b) T1-S6 at.%Al crystals with the [021] orientation deformed at room temperature.

Yield Stress The effect of hydrogen on the yield stress of iron and steels is unpredictable. For very pure iron single crystals and polycrystals the yield stress is frequently found to be decreased by hydrogen, but it may increase or stay the same, depending on the dislocation structure, crystal orientation and purity of the iron . Little information is available for steels. 

It is partly because of the variable effect of hydrogen (giving both softening and hardening, according to the nature of the slip) that the extrapolation of model experiments on very pure iron to predict the behaviour of commercial materials is so difficult. It is further hindered by the ability of dissolved hydrogen to modify the dislocation structure of a straining material. 

Thin films of metals, alloys and compounds of a few micrometres thickness, which play an important part in microelectronics, can be prepared by the condensation of atomic species on an inert substrate from a gaseous phase. The source of the atoms is, in the simplest circumstances, a sample of the collision-free evaporated beam originating from an elementary substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect the crystallographic form of the condensate. If this surface is at room temperature, a polycrystalline film is usually formed. As the temperature of the surface is increased the deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while other properties such as surface morphology and dislocation structure can be established by electron microscopy. 

The sodium chloride structure is adopted by a large number of compounds, from the ionic alkali halides NaCl and KC1, to covalent sulfides such as PbS, or metallic oxides such as titanium oxide, TiO. Slip and dislocation structures in these materials will vary according to the type chemical bonding that prevails. Thus, slip on 100 may be preferred when ionic character is suppressed, as it is in the more metallic materials. 

The Kirkendall effect alters the structure of the diffusion zone in crystalline materials. In many cases, the small supersaturation of vacancies on the side losing mass by fast diffusion causes the excess vacancies to precipitate out in the form of small voids, and the region becomes porous [11], Also, the plastic flow maintains a constant cross section in the diffusion zone because of compatibility stresses. These stresses induce dislocation multiplication and the formation of cellular dislocation structures in the diffusion zone. Similar dislocation structures are associated with high-temperature plastic deformation in the absence of diffusion [12-14]. 

In crystalline solids, only coherent spinodal decomposition is observed. The process of forming incoherent interfaces involves the generation of anticoherency dislocation structures and is incompatible with the continuous evolution of the phase-separated microstructure characteristic of spinodal decomposition. Systems with elastic misfit may first transform by coherent spinodal decomposition and then, during the later stages of the process, lose coherency through the nucleation and capture of anticoherency interfacial dislocations [18]. 

Consider next the case where the martensite forms in the parent phase as an inclusion. The procedure for obtaining the dislocation structure of the interface is the same as previously. However, the martensite slab is now no longer free to shear... 

Fig. 6.2.2. Dislocation structures formed as the result of pressing a steel ball (0 0.8 mm P = 45.7 mN) into (001) face of NaCl monocrystals after heating to ...

G.P. Potimiche et al Atomistic modelling of fatigue crack growth and dislocation structuring in FCC crystals. Proc. Roy. Soc. London 462, 3707-3731 (2006)... 

Durham, W. B., Goetze, C., Blake, B. (1977). Plastic flow of oriented single crystals of olivine. 2 Observations and interpretation of the dislocation structures. J. Geophys. Res., 82, 5755-70. 

McLaren, A. C., Turner, R. G., Boland, J. N., Hobbs, B. E. (1970). Dislocation structure of the deformation lamellae in synthetic quartz a study by electron and optical microscopy. Contrib. Mineral. Petrol., 29, 104-15. 

Figure 10 STEM bright-field images of CaFj showing inherent dislocation structure after (a) 10 s with little radiation damage, but increasing radiation damage after (6) 30 and (c) 100 s ...

A great variety of structures are formed after deposition of one (or several) metals on the surface of another [1]. The deposited metals may form alloys with each other or they may form islands with some microstructure [7,8] with the substrate in the first or deeper layers [1-6]. Alloy formation at the surface may be observed even in those cases where there is phase separation in the bulk [9-11]. If the size mismatch between the deposited and substrate atoms is large, misfit dislocation structures may be formed [12-14]. 

The nucleation step in the decomposition of CaCOj appears to be determined more by stereochemistry [9] than by energetics [7,8]. Problems encountered in correlating the dispositions of growth nuclei with reactant dislocation structure [7] could arise from the rapid and comprehensive surface modifications accompanying the establishment of reaction conditions. Initial changes that influence surface reactivities have been established as a feature of many dehydrations [84] and similar behaviour might be expected to occur in other solid state decompositions. 

This equation is the main governing equation of continuum mechanics and presides over problems ranging from the patterns formed by clouds to the evolution of dislocation structures within metals. Note that as yet no constitutive assumptions have been advanced rendering this equation of general validity. We will find later that for many problems of interest in the modeling of materials it will suffice to consider the static equilibrium version of this equation in which it is assumed that the body remains at rest. In that case, eqn (2.32) reduces to... 

The subsurface dislocation structures described above can be revealed by electron microscopy as shown in fig. 12.31. Such experiments are suggestive of the physical mechanisms involved in the onset of plasticity and provide the hope of quantifying the crystallography and size of the initial dislocation loops. 

A structure model must be based on a noncontradictory, closed and complete definition. A definition is closed if it does not contain indefinite elements and notions, and it is complete if it includes the description of all structure elements. Thus, for instance, the model in which the amorphous structure is considered as a dislocationally disordered crystal [6.21, 22] becomes not closed if the dislocation structure (in particular, the one of dislocation core) is not defined. At high density of dislocations when their cores may overlap and their structure becomes very indefinite, the model is not closed. The free-volume model [6.23 25], in which the question about geometry and topology of atomic configurations is put aside, is not complete. 

Fig. 6.7. Dislocation structure in LRC The disordered slip layer and core are shown...

Figure 8. Schemes illustrating the dislocation structure of grain boundaries in CP Ti as-processed by HPT (a) and after low temperature annealing (b).

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