Dislocations, observation

Screw dislocations observed in single crystals of polymers may have an extraordinarily large Burgers vector, up to 10 nm or more, being equal to the total thickness of lamellar crystals [134]. Such dislocations are easily detectable by microscopes, yet they do not have hollow cores as would be required of such... 

In this study, the appearance and evolution sequence of planar slip bands, in addition to a dislocation cell structure with increasing e,, is identical to that observed in quasi-static studies of the effects of stress path changes on dislocation substructure development [27]. The substructure evolution in copper deformed quasi-statically is known to be influenced by changes in stress path [27]. Deforming a sample in tension at 90° orthogonal to the... 

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. 

Champion and Rohde [42] investigate the effects of shock-wave amplitude and duration on the Rockwell C hardness [41] and microstructure of Hadfield steel over the pressure range of 0.4-48 GPa (pulse duration of 0.065 s, 0.230 ls, and 2.2 ps). The results are shown in Fig. 7.8. In addition to the very pronounced effeet of pulse duration on hardness shown in Fig. 7.8, postshoek electron microscope observations indicate that it is the final dislocation density and not the specific microstructure that is important in determining the hardness. 

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 ... 

After 1934, research on dislocations moved very slowly, and little had been done by the time the War came. After the War, again, research at first moved slowly. In my view, it was not coincidence that theoretical work on dislocations accelerated at about the same time that the first experimental demonstrations of the actual existence of dislocations were published and turned invention into discovery . In accord with my remarks in Section 3.1.3, it was a case of seeing is believing all the numerous experimental demonstrations involved the use of a microscope. The first demonstration was my own observation, first published in 1947, of the process of polygonization, stimulated and christened by Orowan (my thesis adviser). When a metal crystal is plastically bent, it is geometrically necessary that it contains an excess of positive over negative dislocations when the crystal is then heated, most of the dislocations of... 

Amelinckx, S. (1964) The Direct Observation of Dislocations (Academic Press, New York). 

Charles Frank and his recognition, in 1949, that the observation of ready crystal growth at small supersaturations required the participation of screw dislocations emerging from the crystal surface (Section 3.2.3.3) in this way the severe mismatch with theoretical estimates of the required supersaturation could be resolved. 

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