Question

An interstitial in the lattice structure puts the surrounding bonds lattice in tension or compression? How does this stress affect movements of dislocations? Use figures/schematics to explains.

Answer 1

Any deviation from perfectly periodic arrangement of atoms along a line is called line imperfection.

There are two common types of dislocations; Edge dislocation (or Taylor-Orowan dislocation) and Screw dislocation (or Burger’s dislocation).

1. EDGE Dislocation

Fig 1

Figure 1 shows the "Line dislocation" in which deforming force is applied on upper part of a cubic material. The plane ABEF acts as a slip plane. The upper half is now pushed sideways such that is shifts by a amount b, the slip vector as indicated. As a result, the region to the left of EF is slipped while the region to the right of EF remain unslipped. The line EF within the crystal marks the boundary between the slipped and unslipped regions and so is the dislocation line. The upper half of the block will clearly be under compression while the lower half is under tension. The latticce of the front face after the applied force will look like as the other figure.

If there were n vertical atomic planes before the slip in both the upper and lower halves of the crystal, then, after the unit slip, n atomic plane above the slip plane try to join (n-1) atomic planes below. Consequently, one vertical atomic plane in the upper half of the crystal has no counterpart in the lower half. It has to terminate on the slip plane. This dislocation lies at the edge of this extra half plane, and hence the name “edge dislocation”.

Just like the electrical charges, dislocations of the same sign repel, and those of opposite sign attract each other as shown in above figure. Two edge dislocation X, Y of opposite sign on the same slip plane attract and annihilate one another, leaving a perfect crystal with a consequent reduction in the elastic energy of the system.

2. SCREW Dislocation

Fig 2

The geometry of screw dislocation may be understood in terms of slip process of a different kind shown in Fig 2 (a) and Fig 2(b). Fig 2. (a) shows the structure of a crystal containing a screw dislocation. The structure arises as a result of the slip of ABCDEP by vector b relative to rest of the crystal and EF is the dislocation line. Fig 1(b) shows the arrangement of atoms in the crystal structure.