Physics of nonequilibrium systems – Shape Memory Alloys

Shape-memory alloys are so-called smart systems which “remember” their shape even under significant deformations, and revert back to the original shape upon heating.

These are extremely important system in many technologies –  microactuators for microvalves and micropositioners, microvalves, space technology, and so on.

There several aspects of these systems are extremely interesting from fundamental physics view point as well, for example, they represent a class of system that display…

Physics of Noise in Nonequilibrium Systems – Shape Memory Alloys and Smart Systems

Noise away from Equillibrium

When subjected to external force, physical properties in many complex natural systems respond in bursts that follow very similar laws. This universality is manifestation of a critical state which governs many phenomena, from avalanche of atomic species causing propagation of fracture, to jerky movements of continental plates during earthquakes, or even stock market fluctuations. In solid sate physics, systems that display avalanches are often far from thermal equilibrium, and evolve through bursts when an external force is applied. Ever since Bak-Tang-Wiesenfeld proposed their seminal theory of self-organized criticality in 1987, phase transitions in driven non-equilibrium systems have been scrutinized repeatedly. The central uncertainty in this field, which has now survived more than two decades, concerns whether these systems self-adjust to the critical state irrespective of the driving field, or if the self-similarity occurs only at a critical driving force, or often called a “global instability”.  The binary shape-memory alloys, such as Nickel-Titanium, form an ideal playground for an experimentalist to find an answer to this question.

Our work in this field has two directions:

• Shape memory alloys belong to a class of smart materials that can ‘remember’ its shape even if bent or deformed to great extents, and recovers the original shape when slightly heated. These systems are of great practical importance, with applications ranging from space technology to microactuators to micromechanical designs. This behavior originates  from a very specific  structural transition, known as the martensite transformation (see the schematic below). Martensite transformation is a non-equilibrium first order phase transition, often referred as an ‘athermal’ transition, and we wish to understand such a phenomenon with new experimental techniques. Our technique is based on measuring real time fluctuations in electrical resistance which provides a new and microscopically intuitive estimates of several structural parameters.

• Recently, based on the higher order avalanche statistics, we have experimentally detected, for the first time, a critical driving field, where the system becomes truly self-similar at macroscopic length scales. Conventional techniques, used in many experiments, failed to detect this instability. Our experimental probe is directly sensitive to long-range correlations in the system, and shows singular divergences at specific points during the phase transition (see figure below).

Selected Publication(s):

1. U. Chandni, Arindam Ghosh, H. S. Vijaya, S. Mohan, Physical Review Letters 102, 025701 (2009).