Advanced Characterization of Martensite - 3D & High Resolution: Session 4
Program Organizers: David Rowenhorst, U.S. Naval Research Laboratory; Michael Mills, The Ohio State University

Thursday 8:00 AM
July 13, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Ashley Bucsek, Colorado School of Mines

8:00 AM  
Advanced Multi-probe Experimental Approaches to Study Complex Lath Martensite in Low-carbon Steels: Lutz Morsdorf1; Jiali Zhang2; Baptiste Gault1; Dirk Ponge1; Cem Tasan2; Dierk Raabe1; 1Max-Planck-Institut fuer Eisenforschung GmbH; 2Massachusetts Institute of Technology
     Martensitic transformations in low-carbon steels induce highly complex microstructures. While the crystallographic hierarchy ranging from prior austenite grains, via packets, blocks, sub-blocks down to laths is well established, another hierarchy arises from the stepwise transformation sequence. For example, early transformed martensite, i.e. just below martensite start, is rather coarse in 3D, most autotempered after quenching and least dislocated. In other words, as-quenched lath martensitic microstructures are highly heterogeneous in terms of lath size variations, defect densities, autotempering states and corresponding local mechanics. Thus, it becomes challenging to comparatively study nano-scale phenomena such as plasticity mechanisms and low-temperature tempering reactions. To improve, we elaborate on practical in-situ and quasi-in-situ experimental methodologies that allow tracking the microstructural evolution at one specific position during, e.g. deformation and heat treatments. Especially high resolution probes, such as electron channeling contrast imaging, transmission electron microscopy, atom probe tomography and nanoindentation benefit from these approaches.

8:20 AM  
Localized Strain Fields in Metastable Austenitic Steels: Anja Weidner1; Robert Lehnert1; Horst Biermann1; 1TU Bergakademie Freiberg
     Since several years, modern CrMnNi steels are in the focus of interest due to their outstanding mechanical properties known as transformation or twinning induced plasticity. However, alongside with the kinetics of the ongoing deformation mechanisms the knowledge of contributions of indivudal microstructural components (martensite, twins) to the global strain behaviour is necessary for deeper understanding of the strain hardening behaviour. The aim of the present paper is the determination of the magnitude of shear strain of indivudal microstructural components based on in situ tests in the scanning electron microscope in combination with digital image correlation and electron backscattered diffraction measurements. The investigations revealed different strain values for individual martensitic grains depending on their crystallographic orientation. The influence of elevated temperature on the appaerence of local strain fields is demonstrated. Martensite variant selection during tensile and compressive half cycle during cyclic loading was observed in correlation with different local strain fields.

8:40 AM  
High Resolution EBSD to Detect Retained Austenite in HSLA Steels: David Rowenhorst1; 1U.S. Naval Research Laboratory
    Naval hull steels have a unique set of requirements, specifically good weldability, low ductile-to-brittle transition temperature and a high strength. Martensitic High-Strength-Low-Alloy (HSLA) steels achieve many of these goals, but have demonstrated lower than desired ballistic performance. However, new alloy heat treatment development has shown that by preserving a small amount of austenite within an HSLA steel, the ballistic performance can be significantly improved, while not compromising the other mechanical properties. In this presentation, we will show how advanced EBSD characterization was able to detect the small islands (<50nm) of austenite dispersed throughout the martensite matrix. Only by using EBSD pattern processing ex-situ to the data collection, were we able to make an accurate measurements of the volume fraction and spatial distribution of the retained austenite within the structure.