Aug 7, 2009
Nanowire simulations reveal shape memory effect
Researchers in India have demonstrated a stress-induced martenistic phase transformation in a NiAl nanowire leading to pseudo-elastic and pseudo-plastic recovery of very large strain in the nanostructure. This phenomenon could be used as a building block in nanoscale thermo-mechanical machinery.
First-principle and molecular dynamics (MD) simulations have been used extensively to obtain the properties of nanowires and to identify ways of optimizing their displacing, locomotive, electronic and photonic responses. One approach is to control the phase-transformation behaviour at the nanoscale and realize displacing and locomotive phenomena.
The systematic theoretical study was recently published in Nanotechnology, and these tools and concepts have been applied to investigate the structural and mechanical properties of nanowires. Results show a stress-induced martenistic phase transformation from an initial B2 phase to a BCT (body-centred-tetragonal) phase in a NiAl nanowire at various temperatures (see image above).
A novel phenomenon of temperature and cross-section-dependent pseudo-elastic and pseudo-plastic recovery of strain in such a BCT phase with a recoverable strain of ~30% as compared with 5–8% in polycrystalline materials has been observed. The scientists report that at higher temperatures, with a smaller ratio of surface area to volume (As/V), the nanowires show larger plastic strain (pseudo-plastic behaviour). Such plastic strain reduces with an increase in the As/V ratio at temperatures above 200 K.
These temperature and cross-section-dependent pseudo-elastic/pseudo-plastic strain recovery effects will be useful in the application of shape memory and strain sensing to nanoscale devices, biomedical implants and tissue replacement.
The simulations also reveal that the yield stress of the nanowire decreases with decreasing cross-sectional dimensions and increases with a decrease in temperature. A constant elastic modulus of ~80 GPa of the B2 phase is observed over the temperature range 200–500 K for nanowires with cross-sectional dimensions in the range of 17.22–22–28.712 A°, whereas the elastic modulus of the BCT phase shows a decreasing trend with an increase in temperature.
Further work on metallic as well as intermetallic nanowires is in progress. Such an effort is likely to help in the design of various nanoscale devices for use in harsh environments as the work will provide more information on how nanowires can be used to tune the thermomechanical properties of a structure.
About the author
Vijay Kumar Sutrakar is a scientist working within the Mechanical Engineering Design Division of the Aeronautical Development Establishment, Defence Research and Development Organization, Bangalore, India. He is also pursuing a PhD in the Department of Aerospace Engineering, Indian Institute of Science, Bangalore, India. His areas of interest include finite element modelling and analysis of aircraft structures, fatigue and fracture mechanics, molecular dynamics simulations of nano-materials, and the design of nano/advanced-materials for high-temperature and high-strength applications. D Roy Mahapatra is currently an assistant professor in the Department of Aerospace Engineering, Indian Institute of Science, Bangalore. His research interests are in the area of integrative systems with sensors and actuators, multiscale and multifunctional materials, micro and nano-engineering, mechanics of materials, phase transformation with application to smart materials, dynamics and control of distributed parameter systems, wave propagation and health monitoring.