Advanced Characterization of Martensite - 3D & High Resolution: Session 2
Program Organizers: David Rowenhorst, U.S. Naval Research Laboratory; Michael Mills, The Ohio State University

Tuesday 10:20 AM
July 11, 2017
Room: Comiskey
Location: Hyatt Regency Chicago

Session Chair: Mary Scott, University of California, Berkeley

10:20 AM  
3D-XRD Study of Martensite Band Front in Superelastic NiTi Wire: Petr Sittner1; Pavel Sedmák2; Jan Pilch1; Luděk Heller1; Petr Sedlák3; Miroslav Frost3; Jonathan Wright2; 1Institute of Physics of the CAS; 2ESRF; 3Institute of Thermomechanics of the CAS
    Macroscopic fronts of localized deformation in tensioned NiTi shape memory alloy wire were investigated by 3D synchrotron X-ray diffraction method combined with digital image correlation of surface strains and FE modelling of the localized deformation. The 3D-XRD method enables to image elastic strains and stresses in austenite grains within the martensite band front with micrometer resolution. Elastic strain and stress tensors were determined in ~15000 austenitic grains. We have found that the martensite band front, where the martensitic transformation begins, has a form of a nose cone shaped buried interface. From the local stresses in individual grains we reconstructed macroscopic internal stress field in the wire and compared it with the results of FE simulations. Based on combination of experiment and simulation, we reconstructed the martensite band front in tensioned NiTi wire and explained the mesomechanics of its propagation through the wire at constant external stress.

10:40 AM  
High Pressure Torsion Induced Amorphization of NiTi Studied by In-situ Synchrotron Experiments: Michael Kerber1; Erhard Schafler1; Torben Fischer2; Thomas Waitz1; 1University of Vienna; 2Deutsches Elektronen-Synchrotron DESY
    A nanocrystalline structure of shape memory alloys strongly affects their martensitic phase transformations. Strong grain refinement can be achieved by severe plastic deformation such as high pressure torsion (HPT). HPT of NiTi can lead to almost complete amorphization. For the first time, structural changes and amorphization of NiTi induced by HPT were observed in-situ, using synchrotron experiments at the DESY (Hamburg, Germany). A dedicated HPT device allows quasi-constrained deformation of 2.5 mm discs at various pressures, rotation speeds and temperatures. NiTi samples containing B19´ martensite were compressed in the HPT device to build up a hydrostatic pressure of 6 GPa. Concomitant broadening of the B19´ reflections indicates large lattice strains. During torsional deformation at 6 GPa, amorphization is visible by diffusely diffracted intensity superimposing the B19´ reflections already at rather low values of strain γ~2. At a strain of about 30, complete disappearance of the reflections indicates almost full amorphization.

11:00 AM  
Application of Advanced XRD Experimental Methods in SMA Constitutive Modeling: Inspiration and Validation : Miroslav Frost1; Petr Sedlak1; Petr Sittner2; 1Institute of Thermomechanics; 2Institute of Physics
    Current progress in development of advanced X-ray diffraction methods (XRD) allows for more complex and detailed in-situ observations of microstructure of polycrystalline materials under various external conditions. This is particularly attractive for investigation of shape memory alloys (SMA), whose microstructure substantially evolves under changes of temperature or applied stress. On two examples combing experiments with numerical simulations, we show that such observations can also be utilized in development and validation of macroscopic constitutive models. In the first study, unique 3D-XRD data on localization of martensitic transformation in a NiTi wire allowed to quantify strain softening due to phase transformation. In the other one, distribution of phases within the cross-section of a stretched NiTi helical spring was revealed by XRD tomography and utilized to assess capabilities of the model in nonproportional loading modes.

11:50 AM  
Strain Path Changes Performed on Superelastic NiTi during Synchrotron XRD and High Resolution Digital Image Correlation in SEM: Efthymios Polatidis1; Wei-Neng Hsu2; Miroslav Smid3; Jan Capek4; Steven Van Petegem3; Helena Van Swygenhoven2; 1Swiss Light Source, Paul Scherrer Institute; 2Swiss light source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland/Neutrons and X-rays for Mechanics of Materials, IMX, Ecole Polytechnique Federale de Lausanne, CH-1012 Lausanne, Switzerland; 3Swiss light source, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland; 4Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University, 12116 Prague, Czech Republic/ Nuclear Physics Institute of the Academy of Sciences of the Czech Republic, 250 68 Řež, Czech Republic
    Superelastic NiTi alloys can undergo multiaxial loading paths or complex strain path changes during manufacturing processes and operation. It is thus important to study the transformation behavior of these materials under biaxial loading or during stain-path changes. Using a newly developed miniaturized biaxial tensile device that can be implemented for in-situ measurements at the materials beamline (MS) of the Swiss Light Source and in a scanning electron microscope (SEM), biaxial cruciform specimens from superelastic-NiTi alloys (Ni-rich) were developed and deformed. Conventional Digital Image Correlation (DIC) was used during X-ray diffraction allowing to correlate the localization of bands with the diffraction patterns, and High-Resolution DIC in SEM to follow the phase transformation to reveal spatially resolved variant selection). The localization/homogenization of the phase transformation, the martensite variant selection and the microstructural evolution are discussed with respect to the applied strain path and the strain path changes.

11:20 AM  
Three-Dimensional Characterization of Microstructure Evolution in Martensitic NiTi Using High Energy Diffraction Microscopy: Ashley Bucsek1; Darren Pagan2; Darren Dale2; J.Y. Peter Ko2; Margaret Koker2; Aaron Stebner1; 1Colorado School of Mines; 2Cornell High Energy Synchrotron Source
    In this presentation, we utilize High- Energy Diffraction Microscopy (HEDM), an in-situ, non-destructive x-ray diffraction technique, to study microstructure evolution in martensitic NiTi oligocrystalline samples. This technique is perfectly suited for the study of microstructure evolution in SMAs, because it provides 3D micromechanical information on the grain and even subgrain scale, including strain, crystallographic orientation, volume, and topology. These measurements are essential to the understanding of SMA micromechanics, because they are not confined to selected area surface measurements or average measurements over bulk polycrystals. Using HEDM, we can connect subgrain-scale micromechanics to the macroscopic stress-strain behavior. The results include microstructure evolution during detwinning/reorientation, subgrain scale misorientation spread and estimates of lattice strain within each martensite domain, and 3D EBSD-type reconstructions, all of which are in-situ and related back to the macroscopic response.

12:10 PM Break