Shown above are two pure screw dislocations, one of which contains a kink and the other a jog. Take the positive direction of the lines as shown by the arrows. Both the kinked segments and the jogged segments are pure edge dislocations. (a) Redraw the above kinked dislocation and show on your drawing the location of the extra plane of atoms for both kinked segments. Be careful to get the sense of both segments correct. (b) Repeat (a) for the jogged dislocation. (c) Suppose the glide plane shown above were a (111) plane in a fcc crystal. Which of the two pairs of segments would you think could be removed by gliding motion of themselves more easily? Explain. (d) Explain how the glide motion of screw dislocations containing jogs could produce vacancies.
The Correct Answer and Explanation is:
Here are the solutions to the questions based on the provided image.
Solution
(a) & (b) Location of the Extra Plane for Kinks and Jogs
The location of the extra half-plane of an edge dislocation segment is determined by its line direction (l) and its Burgers vector (b). The extra material is on the side defined by the vector cross product l × b. For the positive screw dislocations shown, the line direction l is roughly parallel to the Burgers vector b.
- (a) Kinked Dislocation: Kinks are segments that step from one low-energy position to another within the same glide plane. The two kink segments shown are edge dislocations of opposite sign.
- For the upper kink, the line vector l points generally down and to the left. The cross product l × b points below the glide plane.
- For the lower kink, the line vector l points generally up and to the right. The cross product l × b points above the glide plane.
- (b) Jogged Dislocation: Jogs are segments that step the dislocation line out of its primary glide plane. The jog segments are also edge dislocations of opposite sign.
- For the upper jog, the line segment has moved out of the page. Its line vector l is perpendicular to the glide plane. The cross product l × b points to the left of the jog.
- For the lower jog, the line segment has moved into the page. Its line vector l points into the page, and the cross product l × b points to the right of the jog.
The drawing below illustrates the location of the extra half-planes (indicated by the ⊥ symbol) for both cases.
(c) Ease of Motion: Kinks vs. Jogs
The kinked segments can be removed more easily by gliding motion.
Explanation: Dislocation glide is a conservative motion, meaning it occurs without the creation or destruction of atoms and is therefore energetically favorable. For glide to occur, the dislocation line must move within its glide plane, which is the plane containing both the dislocation line vector (l) and the Burgers vector (b).
For the kinked dislocation, both the main screw segments and the kinked edge segments lie entirely within the same (111) glide plane. Therefore, the entire dislocation can move forward by the lateral glide of the kinks along the main dislocation line. This is a low-energy, conservative process.
For the jogged dislocation, the jog segments have a line vector l that is not in the (111) glide plane of the main screw dislocation. The glide plane for the jog itself (defined by its own l and b) is different from, and in this case perpendicular to, the glide plane of the screw dislocation. For the jog to move along with the gliding screw dislocation, it must move non-conservatively by climb, which is a much more difficult, thermally activated process.
(d) Vacancy Production by Jogs
The glide motion of a screw dislocation containing jogs can produce vacancies through a process of non-conservative jog motion.
Explanation: When a shear stress causes the main screw dislocation to glide, the jog is forced to move with it. However, the jog cannot glide in the same direction as the screw portion. Its path of easy glide is on a different plane. To keep up with the screw dislocation, the jog must undergo climb, which is motion perpendicular to its glide plane.
Climb is a non-conservative process that requires the transport of mass. Specifically, if a jog of interstitial character (i.e., it represents a small extra piece of an atomic plane) is dragged through the crystal, it must shed atoms to move. As it moves one atomic spacing, it leaves behind a string of vacant lattice sites. This effectively creates a row of vacancies in its wake. Conversely, if a jog of vacancy character is dragged, it produces a row of interstitials. This process requires significant thermal energy to create the point defects, which is why jogs act as strong obstacles to dislocation motion, particularly at low temperatures.
