Advanced Characterization of Martensite - 3D & High Resolution: Session 1
Program Organizers: David Rowenhorst, U.S. Naval Research Laboratory; Michael Mills, The Ohio State University

Monday 4:00 PM
July 10, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Michael Mills, The Ohio State University

4:00 PM  Invited
Dictionary-based Indexing of EBSD Patterns for Martensitic Systems: Marc De Graef1; 1Carnegie Mellon University
    Dictionary-based indexing is a new indexing technique in which complete electron backscatter diffraction patterns (EBSPs) are matched against a list of computed patterns for orientations that uniformly sample 3D orientation space. The main difference between this technique and the more traditional Hough transform approach is the fact the complete pattern is used, instead of features within the pattern (i.e., Kikuchi bands); this leads to an improved robustness against noise and allows for smaller patterns to be used without loss of angular/orientational accuracy. In this presentation, we will begin with a brief overview of the physical models underlying the indexing approach, including a forward model for EBSPs. Then we apply the approach to a number of experimental datasets on martensitic systems, in particular Ni-Mn-Ga ferromagnetic shape memory alloys and steel. We will illustrate how this new approach can provide accurate indexing on systems for which the traditional methods fail.

4:40 PM  
Strain-induced Martensitic Transformation in a Co-Cr-W-Mo Alloy Probed by in- and ex-situ Methods: Irmgard Weissensteiner1; Petra Erdely1; Andreas Stark2; Helmut Clemens1; Verena Maier-Kiener1; 1Montanuniversitšt Leoben; 2Helmholtz-Zentrum Geesthacht
    Co-Cr-W-Mo alloys are widely-used materials for dental implants. Thermomechanical processing in combination with heat treatments leads to a significant improvement of the materials properties regarding application and manufacturing. The change in microstructure and hence the chemical composition of the constituent phases influences the susceptibility to strain-induced martensitic transformation (SIMT) of the metastable fcc Co-matrix. The purpose of this study was to correlate the onset of SIMT of a Co-24Cr-8W-3Mo alloy to both, the microstructure and the strain. The samples were compressed at different temperatures and deformation rates in a deformation dilatometer, partly during in-situ synchrotron diffraction. Nanoindentation experiments revealed the local mechanical behavior. Microstructural changes and also influences of production parameters were analyzed by EBSD and atom probe tomography. The studies showed a slight strain rate sensitivity and a distinct orientation anisotropy of the mechanical properties and SIMT. Furthermore, the experiments enabled the evaluation of the temperature-dependent onset of SIMT.

4:55 PM  
Variant Coupling in Ferrous Martensite: Goro Miyamoto1; Tadashi Furuhara1; 1Tohoku University
    According to K-S orientation relationship, 24 variants can be formed in a single austenite grain. Although it is well known that specific K-S variants are coupled or grouped in ferrous martensite, variations of variant coupling with morphology of martensite are not clear so far. Therefore, variant coupling in lath, lenticular, butterfly martensite were characterized in this study. It is found that variants sharing the same close-packed planes parallel relationship tend to form as a group in lath martensite while variants having nearly the same habit plane are coupled preferentially in lenticular and butterfly martensite. Such variant coupling can be explained by strain compatibility at junction planes between two variants.

5:15 PM  
Composition-Dependent Crystallography and Mechanical Properties of Ni-Mn-Ga Alloys: Le Zhou1; Anit Giri2; Kyu Cho3; Yongho Sohn1; 1UCF; 2TKC Global; 3US Army Research Laboratory
    Composition-dependent microstructural and crystallographic development, and mechanical properties of NiMnGa alloys were investigated through a combinatorial approach. Solid-to-solid diffusion couples were prepared to generate compositional gradients and form martensite/austenite interphase boundary upon cooling. Compositions having martensitic transformation temperature near room temperature were extracted at the interphase boundary. Crystallographic variations of the martensitic phase were identified at different compositions, including non-modulated (NM) martensite and modulated (5M or 7M) martensite. Additionally, the pre-martensitic state was observed in the austenitic phase, with distinctive tweed microstructure originating from the local lattice distortions. A decrease of reduced elastic modulus (Er) was observed with Mn or Ni substituting Ga, and Ni substituting Mn for the austenitic phase. However, an opposite trend was found for the martensitic phase. The softening of the elastic constants near the vicinity of martensitic transformation contributed to the sharp decreases of Er near the interphase boundary.