Metadislocation motion

So far, no direct observation of metadislocation motion, for instance by in situ tensile tests in a transmission electron microscope, has been reported. Therefore, there is as yet no direct experimental confirmation of whether metadislocations motion takes place by glide or climb. However, besides the strong evidence in favor of climb motion given above, there is also a potent argument against glide. 

In the previous section, we have identified metadislocations as loops with (001) habit planes and [001] Burgers vectors. They are pure edge loops, which can only expand and contribute to strain, if their segments move by climb. Glide motion of the segments would merely lead to unaltered movement of the loop along its ghde cylinder, which does not contribute to strain. 

Fig. 30(c) shows a climb step (movement along [100] to the left) of a metadislocation by one a-lattice constant. Again, the initial and final tiles are drawn in black and gray lines, respectively. For a climb step, a much smaller number of vertex jumps is necessary. The number of necessary vertex jumps is now limited to the core region and it is now finite, no matter how far the associated phason planes are extended. Since each vertex jump physically represents a number of local atomic movements (Section 3.1.2), climb motion obviously is connected with much less atomic rearrangement than glide motion. 

It is obvious that the core structure of the experimental [Figs 44(a) and 44(b)] and predicted [Fig. 42(d)] metadislocation are represented by the same tile. Hence they both have the same Burgers vector. However, they are connected to different types of planar defects. While the metadislocation in Fig. 42(d) is associated with six phason planes, the metadislocation in Fig. 44(b) is associated with a slab of R-phase. The phason elements on the left-hand side of the metadislocation core change the stacking sequence of the ideal T-phase structure A,B,A,B,A to a sequence A,A,A,B,B. These additional defects are required to accommodate the symmetrical metadislocation core into the structure and have to move along with the latter. In other words, the three phason lines act as escort defects to the metadislocation core, which move ahead and clear the way for the latter. Upon movement, the metadislocation locally transforms the T-phase structure, leaving a slab of modified R-phase in its wake. Different types of metadislocations in T- and R-phase structures and their modes of motion are discussed in Section 6.4. 

In this section, different types of theoretically predicted and experimentally observed metadislocations are compared and their mode of motion is analyzed. Fig. 48 depicts the experimentally observed metadislocations in orthorhombic and monoclinic s-phases, AI13M4- and T-phases. Their cores are represented by... 

The broad variety of constructible and observed metadislocations suggests a definition independent of their mode of motion and the type of associated planar defect A metadislocation is a line defect with a Burgers vector corresponding to a... 

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