MMMB Research Seminar
February 25 @ 11:30 am - 12:30 pm
Correlation between Macro-scale Mechanical Behavior and Small-scale Nanoindentation in Steels
Professor Heung Nam, Department of Materials Science and Engineering, Seoul National University
Recently, nanoindentation has been employed to probe the small-scale mechanical behavior of materials for a wide range of academic or engineering applications. The response of a material to the nanoindentation is usually represented as a form of the load-displacement curve. When metallic materials undergo irreversible permanent deformation, the discrete physical events, such as dislocation nucleation, dislocation source activation, phase transformation or mechanical twinning, can be detected as discontinuities of displacement or load during nanoindentation. These results in a sudden displacement excursion during load-controlled-instrumented nanoindentation, so-called pop-in. In this talk, some observations of indentation pop-ins and microstructural studies will be presented to provide micromechanical insight into the strain-induced phase transformation of metastable austenite in steel. The microstructural changes was observed by means of an automotive mapping technique with TEM that the partial volume of prior austenite had transformed into several martensite blocks with different variants. From a finite element calculation combined with phenomenological approach for martensitic transformation, it was confirmed that each variants corresponded to those for which the transformation strain effectively accommodates external stress. As for another topic, the evaluation of single crystal elastic constants of austenite and ferrite phases in high-nitrogen duplex stainless steel will be presented. An elastic self-consistent model combined with optimization process for in-situ neutron diffraction data was utilized. The optimized elastic constants were validated by indentation moduli of each phase obtained by nanoindentation. In addition, the stacking fault energy of austenite was evaluated based on the neutron diffraction profile and the single crystal elastic constants, and correlated with the observed deformation microstructure.
Prof. Heung Nam Han earned his Ph.D. in Metallurgical Engineering at Seoul National University, South Korea in 1995. In 1996, he joined Oxford Center for Advanced Materials and Composites, Department of Materials, University of Oxford, UK, as a postdoctoral researcher, where he worked on consolidation of vacuum plasma spray formed metal matrix monotape. From 1997 to 2002, he has made many contributions on mechanical properties and microstructure prediction of AHSS during hot-rolling process in POSCO Research Laboratory. From 2003 to 2004, he joined a research group for the development of TRIP-aided multiphase steels in Korea Institute of Materials Science (KIMS). Since 2004, he has been a professor in Department of Materials Science and Engineering at Seoul National University. His research interests have been concentrated on transformation-induced plasticity, mechanical behavior for materials with 3-D periodic/non-periodic structure, nano-indentation, and electroplasticity.