Our research is focused on studying the deformation behavior of materials at small length scales. Specifically, we use in-situ micromechanical testing in a scanning electron microscope (SEM) to study the mechanical deformation behavior of materials under extreme conditions: high and cryogenic temperatures, high strain rates, impact, scratch and fatigue. Apart from standard nanoindentation, we use different focused ion beam (FIB) milled test geometries like micropillars for compression and cantilevers for fracture tests at small length scales. These geometries circumvent the difficulties associated with analyzing indentation data that comprises of a complex stress and strain distribution existing below the pyramidal indenter tip. The combined use of nanoindentation and SEM allows us to “visualize” the deformation characteristics (e.g. slip, twin, cracks, etc.) in true in-situ fashion during the micromechanical tests.
Transient time dependent plasticity in nanocrystalline metals
One of the major thrusts of our research has been on extending the micron-scale plasticity techniques to study transient time dependent plasticity in nanocrystalline metals. With unprecedented load and displacement resolution (often in nano-Newtons and sub-nm respectively), instrumented testing methods offer a unique opportunity to study the small strain rates (or stress changes) typically associated with time-dependent transient tests. We have developed new, reliable and robust experimental protocols and techniques for extracting accurate deformation parameters (creep and strain rate sensitivity exponents, apparent activation volumes and energies) from strain rate jump, stress relaxation and creep tests. Our work was among the first to demonstrate that the extracted parameters from micropillar strain rate jump,stress relaxation and creep tests correlate well with bulk tensile tests performed on nanocrystalline metals, conclusively validating the accuracy of our test protocols and our data analysis methods. This provides confidence in applying these test methods at the micron scale to other materials systems to study their transient time dependent properties across temperatures.
G. Mohanty et. al., Elevated temperature, strain rate jump microcompression of nanocrystalline Nickel, Philosophical Magazine, 95 (2015) 1878-1895 Special Issue: Nano-mechanical Testing in Materials Research and Development IV.
J. Wehrs et. al., , Comparison of in-situ micromechanical strain rate sensitivity measurement techniques, JOM 67 (2015) 1684-1693, Special topic collection: In Situ Mechanical Testing in Electron Microscopes
G. Mohanty et.al., Room temperature stress relaxation in nanocrystalline Ni measured by micropillar compression and miniature tension, Journal of Materials Research, 31 (2016) 1085-1095