Interplay Between Microstructural Size and Heterogeneity and its Effect on the Mechanical Behavior of Nanocrystalline Metals
Dr. Jagannathan Rajagopalan
Department of Mechanical Science and Engineering
University of Illinois at Urbana-Champaign
Wednesday, March 2
In recent years, there has been tremendous research interest in understanding the size dependent mechanical properties of materials. This interest has been motivated in part by the widespread use of thin films and nanowires in integrated circuits and MEMS/NEMS and in part by the potential of bulk nanocrystalline materials for structural applications. While research on micro/nanoscale materials behavior have provided many critical insights, a majority of the studies have focused primarily on the effect of microstructural size or specimen dimensions on mechanical properties.
In this talk, Dr. Rajagopalan will show how the interplay between the microstructural size and the heterogenity (for example, distribution of grain size and orientation) leads to unusual mechanical behavior that cannot be explained solely by the size dependence of mechanical properties. Specifically, he will demonstrate that a) nanocrystalline metals, unlike their coarse grained counterparts, recover a substantial fraction (50 to 100%) of plastic strain after unloading, and b) ultrafine grained metal films show a pronounced early Bauschinger effect even at high tensile stresses during unloading. He will then present evidence from the situ transmission electron microscopy and x-ray diffraction experiments that strongly indicate that the strain recovery and early Bauschinger effect are a direct consequence of microstructural heterogeneity. Finally, he will show that when the microstructure of nanocrystalline metals is made more homogeneous these unusual phenomena are substantially diminished. These experiments suggest that a size-centric description of the mechanics of nanocrystalline metals is inadequate, and emphasize the need to take into account both the microstructural size and heterogeneity.
Jagannathan Rajagopalan received his B. Tech. and M. Tech. degrees in Mechanical Engineering from the Indian Institute of Technology (IIT) Madras and his Ph.D. in Mechanical Engineering from the University of Illinois at Urbana-Champaign (UIUC). In his doctoral thesis, he studied how the interplay between the size and heterogeneity of the microstructure influences the mechanical behavior of nanocrystalline metals using MEMS based micro testing devices. Currently, he is a post doctoral research at UIUC, where he is exploring the mechanics of living cells (neurons) using novel bioMEMS force sensors. Jagannathan was a recipient of the Mavis memorial fellowship at UIUC in 2007. His research has resulted in several publications, including those in Science, Biophysical Journal and Acta Materialia.