Tuesday, February 14, 2012

Researchers at IIT-Kanpur give a new insight into micromechanics

When you bend a rod, say made of aluminium, and then straighten it back, why doesn’t it get back to its perfect shape, unlike a metal spring that retains its original frame even after being tampered with? Also, why is it easy to bend a thin rod? The answers to these questions have been around since the 1930s, when researchers in the field of micromechanics of materials first suggested the concept of dislocations, which are defects in the crystals or grains within the material. These dislocations not only weaken crystals, but also cause deformation by moving and leaving the crystal, the effect being permanent, in contrast to the spring which is elastic or reversible in nature.

Now, researchers at the Indian Institute of Technology, Kanpur, (IIT-K), have come up with a new concept about these subtle goings-on inside the crystals of the materials that challenge some of the ideas that have been around over the past six decades. In this, the researchers also attempt to take forward the work on micromechanics of legends from the 1950s, like John Douglas Eshelby.

A dislocation introduces energy, and this energy is released when it reaches the surface, thus making plastic deformation irreversible. The IIT-K researchers have suggested that the slip of these dislocations might be reversible. “We discovered something which is a double oxymoron — reversible plastic deformation due to elasticity. In the process, we discovered a new class,” says Anandh Subramaniam, assistant professor at the Department of Materials Science and Engineering, IIT-K. Two papers that Anandh co-wrote with fellow researchers Prasenjit Khanikar and Arun Kumar were published last year by The Philosophical Magazine, one of the oldest scientific journals in the world.

Research into the micromechanical behaviour of materials, which includes areas such as dislocation dynamics, is crucial to their usage in industrial applications such as fashioning a car door out of metal. “The motion of dislocations is what allows change of shape, and impeding the motion of dislocations is what gives strength to the material. So, these are two opposite sides of the coin,” explains Dipankar Banerjee, former chief controller, R&D, at the Defence Research and Development Organisation, and currently professor with the Department of Materials Engineering at the Indian Institute of Science (IISc).

While dislocations cause deformation by moving, and finally leave the crystal, it doesn’t always require an external force for them to do that. That’s because they are automatically attracted to free surfaces, a phenomenon explained by a concept known as ‘image force’. It is called so because a hypothetical negative dislocation is assumed to exist on the other side of the free surface, causing an attraction towards it. This ‘image construction’ is usually used for the calculation of the force and ‘image forces’ can lead to crystals becoming spontaneously dislocation-free.
“Over the past 60 years, people have known this concept called image force and have developed theories, etc,” says Anandh. “So, the first of the two things we showed is that the image force construction theory actually breaks down when we actually want it to work.” The reason, he explains, is because when the dislocation is less than about 100 atomic spacings from the surface it causes small deformations to the surface, partly relaxing the energy and hence altering the image force. In a nanocrystal, the entire domain can deform. For example, a thin plate would bend in the presence of a dislocation. In such cases, the standard formula cannot be used, he says.

The more interesting thing their computer simulation models showed was that the image force can also be zero in some cases. This means that a dislocation moving inside a crystal can be in neutral equilibrium just like a ball rolling on a plane or an Anglepoise lamp where mechanism of spring and lever allows it to be moved to various configurations without putting in any effort. “Over a range of positions, irrespective of where the dislocation sits, its energy does not change. So, this is the new concept we have found out,” says Anandh.

“We can describe things between a material and a structure, which we call a structure ecause it has some geometry and a material because the defect has a crystallographic origin. So, we found out that actually we have to define a new term called material-structure, and that there can be neutral equilibrium or zero-stiffness material-structures,” says Anandh. He says further research also indicated that an edge dislocation is stable in a finite crystal, a question that’s some 60 years old. However, the research is at a very fundamental level and potential applications are still to be thought on, says Anandh.

Computer simulation and modelling has been a key development over the past decade from the industrial standpoint of metallurgy, which earlier used an entirely experimental approach, says Dipankar Banerjee. “So, you substantially reduce your experimental costs and time in introducing new shapes and materials into service,” he says. While the materials used for structural applications are fairly mature, the key focus is to reduce the costs of applying such materials and also the timescale of engineering them, he says.

Source: The Financial Express, February 14, 2012

No comments:

Post a Comment

Blog Archive