Title： Correlation between Macro-scale Mechanical Behavior and Small-scale Nanoindentation in Metallic Materials

Time ： Thursday3:00pm-5:00pm, August 22, 2019

Location：Meeting room 1, School of Mechanical Engineering, Xingqing Campus

Lecturer: Professor Heung Nam Han from Seoul National University

Inviter: Professor Qi ZHANG and Professor Yanshan LOU

Abstract：

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 mechanically-induced twinning, can be detected as discontinuities of displacement or load during nanoindentation. These are basically geometrical softening behaviors, and results in a sudden displacement excursion during load-controlled-instrumented nanoindentation, so-called pop-in.

In this talk, some experimental results of nanoindentation and microstructural studies will be presented to provide micromechanical insight into the strain-induced phase transformation of metastable austenite in steel.Themicrostructural changeswas observedby means of an automotive mapping technique with TEMthat the partial volume of prior austenite had transformed into several martensiteblocks 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 study usingnano-indentation,the remarkable ductility difference betweenthermo-mechanically treated and recrystallizedtungstens will be talked.The statistical distribution of the maximum shear stress of the pop-ins corresponding to the onset of plastic yielding were analyzed on the basis of dislocation nucleation or movement of pre-existing dislocation. In addition, we have performed a series of molecular dynamic simulations to obtain the grain boundary fracture strength of pure tungsten. The measured maximum shear stress and calculated fracture stress were compared for three specimens and the effect of pre-deformation on the ductile-brittle transition of tungsten was analyzed.

As for the last topic, the evaluation of single crystal elastic constants of austenite and ferrite phases in high-nitrogen duplex stainless steel will be presented. In this study, 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.