In-Situ TEM Deformation Studies of Dislocation Generation and Motion in High-Purity Mo Single Crystals
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(The material was received in this condition from Lawrence Livermore National Laboratory.) The thickness of the cut samples was reduced to a final thickness of about 100-micrometers by mechanical grinding. The rectangular straining stage samples had dimensions of 3xlOxO.lmm and were cut so that the tensile axis was near . Electron transparent samples were prepared by jet polishing in a twin jet polisher. The electrolyte was a 20% sulfuric acid in methanol solution, and the polishing conditions were 25 V, 1.6 A, and -10° C. The samples were strained in a single-axis tilt straining stage, which is capable of stage displacement rates as low as a few nim per second. The electron microscopy was performed in the JEOL 4000 EX controlled environment transmission electron microscope [13], which is equipped with a Gatan TV rate camera system. The dynamic events were recorded on Super VHS videotapes for later analysis. Static images were recorded using the conventional plate system, the straining stage being sufficiently stable to permit this. RESULTS Although the Mo samples were given a high temperature anneal under ultra-high vacuum conditions, a significant number of dislocations were retained in the material. Examples of the different dislocation structures are shown in the bright-field images presented in Figure 1. In some regions of the sample only isolated straight dislocations existed, Figure l(a), whereas in others dislocation tangles were observed, Figures l(b) and I (c). The density of the dislocation tangles also varies: in some regions the tangles are isolated from each other, whereas in others the dislocation tangles are extensive and involve many dislocations.
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Figure 1: Initial distribution of dislocations. Images show areas with (a) isolated dislocations, (b) isolated tangles, and (c) complicated tangles. The formation of dislocation tangles is common in materials exhibiting non-planar glide [14], but it is surprising that they exist in such densities after the annealing treatment. g * b analysis of the dislocation tangles shows that they primarily consist of different b = a/2 dislocations, with short b = a dislocations forming at junctions in the tangle. Similar observations have been made in Nb [2], so this is not unexpected. These junction dislocations will contribute to the stability of the tangle [15], until the applied stress is sufficient to unzip the junction. During straining, dislocation tangles have been observed to impede the motion of other dislocations, as well as to act as dislocation sources. On initial straining, and occasionally under the influence of just the electron beam, edge dislocations were observed being emitted from the dislocation tangles. An example of
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Figure 2: Evolution of a tangle with time at low stress. Edge dislocations leaving the tangle trail immobile screw dislocations behind them. this during straining is shown in the sequence of images presented in Figure 2, which shows two edge dislocations leaving a tangle. As these ed
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