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What most people think when they hear the word “dislocation” is very different from what a materials scientist understands from it. The meanings are similar, but the materials scientist’s definition has nothing to do with physiology or bones. It has more to do with crystals.

What, then, is a dislocation from the materials scientist’s viewpoint? A dislocation is, simply put, a mismatch in the order in which atoms are stacked within a crystal. A crystal, by the classical definition is a periodic arrangement of atoms, which continues indefinitely. Thus, in the real world, true crystals don’t exist. It is difficult enough to create a lump of material which has only a single crystal. Most crystalline materials we see are polycrystals, which are made up of several crystals, with slight mismatches in their orientations, huddled together to form the bulk. Each crystal in this polycrystals is referred to as a grain. Now that most of the jargon has been defined, we may return to the business at hand.

Inside a grain, one may expect to have a flawless arrangement of atoms in their regular periodic fashion, to form the crystal. Nature, however, is as imperfect we humans are, and therefore, there are errors in this stacking of crystals. Wherever we find such an error, it is called a defect. A single atom missing is thus a point defect, and an entire line of missing or extra atoms, is a line defect. A dislocation is such a defect. One may also think of a dislocation as an extra half-plane of atoms, forcibly inserted into the material. The dislocation therefore has a stress field associated with, by virtue of the distortion around itself. Dislocations are typically a few nanometers across, and come in a variety of shapes and sizes, from small curvy lines to loops which generate even more loops under an applied stress.

The way I have written about the dislocation could influence the reader to think that it is something evil, destroying the natural order of things. On the contrary, the dislocation is actually our friend. I dare say that if it were not for the dislocation, the technological development of the human civilization, right from the earliest bronze ages, would have been set back by at least a few hundred years, if not a thousand.

What is the reasoning behind so strong a statement? Well, if we were to calculate the theoretical strength of a metal, assuming a perfect crystal structure within it, we would find it to be at least thirty times greater than its actual observed value. It is precisely here that the dislocation is our friend, and aided our prehistoric ancestors in shaping metal implements easily. Imagine, if bronze were thirty times stronger, how much longer it would have taken our ancestors to be able to shape useful tools out of it. Imagine if, after repeated failures in bending such a strong metal, they had simply given up, you & I would be roaming the jungles of India, instead of sitting in a comfortable chair, in a building with steel reinforcements, reading this article on an advanced computer!!

It is therefore necessary to study the behavior and dynamics of dislocations, so that we may know more accurately how they affect the forming of metals and other materials. Typical applications range from steel industries, to the automobile sector, to the nuclear power plants to microelectronics.

It is not only restricted to that. As we enter the nano-age, we are trying to fabricate chips which are a few billionths of a meter thick. This technology is achieved through the use of thin films. These films are a few nanometers thick, and in depositing them, they sustain huge stresses. If the stress in the film exceeds the force which binds it to its substrate, it could just peel off like old paint peels off walls. Imagine what havoc that could play in your computers microprocessor!! Thus the mechanical properties of thin films cannot be studied without considering the dislocation. The dislocation is not always our friend in this tiny world, and it depends exactly where and in what orientation it is present, to decide whether it is our ally or enemy. At such small sizes, where the dimensions of the conductor or semiconductor are comparable to size of the dislocation, the dislocation may also affect the electrical properties of the device.

It is therefore important to study the dynamics of dislocations, and their effects on various properties of the material that contains them. I hope that after reading this article, the word dislocation will mean much more to the reader than it did before.
Adhish Majumdar,
Mar 10, 2011, 1:29 PM