The attached figure is taken from 'Dislocations and their role in strengthening' by James CM Li in 'The Strengthening of Metals' (ed.) Donald Peckner. It shows an edge dislocation moving on (111)[01-1] of an FCC crystal. (For easier interpretation, the slip system is indicated on the left of the scanned image).
To my understanding, the edge dislocation should have the configuration shown on the right side ('Actual dislocation', red colour). However, in the diagram, there appear (inside red dotted circle) to instead be 2 perpendicular edge dislocations (configurations shown on right side in blue).
My question is, has the single dislocation been resolved into two perpendicular dislocations? This is fine for the Burgers' vector, but doesn't the atomic arrangement change when we do this? Or are there actually two dislocations and I have misunderstood the diagram?
A dislocation ( edge, screw, or mixed) can be always dissociated (resolved) in two (more) components,. The problem to solve is the energy of the dislocation. If the energy of a dislocation (related to the square of its Burgers vector) is higher that the sum of the energies of the two dislocations, then the original dislocation is less stable than the new ones. Otherwise, the opposite is true. As a matter of fact, every system tends to a state of lower energy...
Dear Prof. Aquilano,
I agree completely. But I believing you are referring to splitting of a perfect dislocation into partials, which is often energetically favourable. That is not the case here; almost certainly the a [01-1] dislocation will not physically split into [00-1] and [010] (both perfect dislocations).
Rather, the question is about the representation - there appear to be two edge dislocations in the diagram- is there some explanation for this, or have I misunderstood something.?
Dear colleague Aashranth B!
All the variety of dislocations in crystalline metallic materials, except perhaps "sedentary", can be represented as a combination of the fractions of the edge and screw components. Apparently, the one shown in Fig. scheme can be explained in this way. With a certain effect, for example, when the applied stress vector changes, the change in the ratio of the edge and screw components will explain the spread of plastic deformation.
With best wishes, prof. I.Vakulenko.
Dear Prof. Vakulenko, thank you for your answer. However, looking at the given Figure, it seems like there are two edge dislocations, not one edge and one screw; isn't that so?
Dear Aashranth B.!
Judging from the figure, the Burgers vector of a dislocation can have any value, which is not true. If we turn to elementary knowledge of the dislocation mechanism of propagation of plastic deformation, the observed shift with an arbitrary orientation should be replaced by a combination of fragments from edge and screw dislocations of different lengths. Such an approach is due to the achievement of the minimum energy accumulated by the system and is fairly unambiguously described in numerous works by A.H.Cottrell, Bilby, etc.
With best wishes, Prof. I.Vakulenko
Thank you once again. I will go through the suggested work.
As I understand, a mixed dislocation (therefore having some curvature, not a straight line) can be (mathematically) treated as vector sum of edge and screw components. But in a pure edge dislocation (straight line), as the Fig. 7 caption and related text suggests, can there still be two such components?
Dislocations that are able to move in crystals with a fcc lattice, for example, from the action of an applied stresses, are limited to slip systems as a result of a combination of {111} [100].
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Right, I believe you meant {111} dislocations in FCC. But the figure seems to indicate two (edge) dislocations on {001} planes (as drawn in blue) - isn't that so?
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