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I am bit confused. The slip direction is the direction of movement of a dislocation which is denoted by the burgers vector.

This makes sense in an edge dislocation, because the stress is applied perpendicular to the dislocation line and the movement of the line is in the direction of the applied stress. The burger vector is also perpendicular to the dislocation line for edge dislocation. So it makes sense that the direction of motion corresponds to the direction denoted by the burgers vector.

But for screw dislocation, the motion of the dislocation is perpendicular to the applied stress and since the burger vector points in the direction of the dislocation line; the burger vector cannot point in same direction as the direction of movement?

The professor even said that the direction of motion for a screw dislocation is perpendicular to the burgers vector. So how can a burger vector then correspond to the direction of slip when this direction of slip is the direction of movement?

So the points that confuse me:

  • slip direction: specific direction along which dislocation motion occurs

  • burger vector: direction corresponds to a dislocation's slip direction

  • screw dislocation: direction of motion is perpendicular to applied stress and the motion is perpendicular to the burger vector

screw dislocation motion

So these 3 things conflict with each other: the motion of the dislocation is here perpendicular to the burger vector (and i have heard is always perpendicular for screw dislocations), so how can a burger vector denote the slip direction if the slip direction is the direction of the motion of the dislocation? For it to denote the motion of the dislocation shouldn't it align/be parrallel? Or is that the mistake I am making, that a burger vector tells about direction of the dislocation motion, but that it doesn't mean that the motion is in the same direction? That there is just always a fixed relation between the two depending on the type of dislocation, but that the relation isn't always parrallel. It is 90 degrees for screw and 0 degrees for edge related to the dislocation motion. So that indeed the burger vector says something about the direction of the motion but just that it doesn't say that they always in same direction?

Because again clearly here the dislocation motion is not in the same direction as the direction of the burger vector; The motion is from front to back, while the burger vector points in direction of the shear stress in this picture

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  • $\begingroup$ Direction of slip does NOT always coincide with direction of dislocation motion. $\endgroup$ – suzanne Jan 30 '19 at 21:17
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Summary: For an edge dislocation, the Burgers vector is parallel to dislocation motion. For a screw dislocation, the Burgers vector is parallel to the dislocation. The Burgers vector is always parallel to slip.

The diagram below shows both edge and screw dislocations in an indealized cubic lattice. The edge dislocation is on the front face and the screw dislocation is on the right face. Burgers vectors are denoted with white-tipped arrows, and point in the same direction for the two dislocations shown. I have added red arrows that indicate applied shear. Slip occurs along the resolved shear direction, which in this case happens to also be the applied shear direction. Note that slip is the same for both dislocations, and the Burgers vector points in the same direction as slip in both cases. Note that the edge dislocation itself is perpendicular to slip while the screw dislocation is aligned with slip.

If slip continued, the edge dislocation would proceed in the same direction as slip. The screw dislocation would recede away from the front plane, toward the back plane. It must move that direction in this case because otherwise the Burgers vector would have to get longer as strain proceeds and new edge dislocations are introduced, which is impossible. An analogy would be tearing a piece of paper. As you pull the ends of the tear further apart, the tip of the tear recedes along the length of the paper.

Dislocation diagram showing shear in red.

The image is a modified version of an image found at www.geology.um.maine.edu. Original credit: Passchier and Trouw, pg 33 (2005).

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  • $\begingroup$ 'If slip continued, the edge dislocation would proceed in the same direction as slip. The screw dislocation would recede away from the front plane, toward the back plane.' That is my point, that for screw dislocation the dislocation line moves in different direction than the direction of the burger vector. Even though they say that a burger vector points in the direction of dislocation movement $\endgroup$ – strateeg32 Mar 10 '16 at 21:14
  • $\begingroup$ So what confuses me is: slip direction is the direction of dislocation movement which is in the direction of the burger vector, but with screw dislocation the dislocation line does not move in the direction of the burger vector. For example in your picture you see that the burger vector points from right to left while the dislocation moves from front to back. $\endgroup$ – strateeg32 Mar 10 '16 at 21:23
  • $\begingroup$ Burgers vector only points in the direction of dislocation motion if it is an edge dislocation. If it is a screw dislocation it points in the direction of the dislocation. $\endgroup$ – wwarriner Mar 10 '16 at 22:58
  • $\begingroup$ Ah okey so my book is wrong for saying: burger vector points in the direction of slip? Because direction of slip does coincide with dislocation motion right? $\endgroup$ – strateeg32 Mar 10 '16 at 23:02
  • $\begingroup$ Your book is incorrect if it states that the Burgers vector is parallel to the direction of dislocation motion (unless it says only edge dislocations). It is correct to state that Burgers vector is parallel to slip. $\endgroup$ – wwarriner Mar 10 '16 at 23:36

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