Advanced Characterization Techniques for Quantifying and Modeling Deformation Mechanisms: Session V
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Advanced Characterization, Testing, and Simulation Committee, TMS: Shaping and Forming Committee
Program Organizers: Rodney McCabe, Los Alamos National Laboratory; John Carpenter, Los Alamos National Laboratory; Thomas Beiler, Michigan State University; Khalid Hattar, Sandia National Laboratory; Wolfgang Pantleon, DTU; Irene Beyerlein, Los Alamos National Laboratory

Wednesday 8:30 AM
March 1, 2017
Room: 33C
Location: San Diego Convention Ctr

Session Chair: Yue Liu, Shanghai Jiao Tong University; Marc Legros, CEMES-CNRS

8:30 AM  Invited
Tracking Shear-migration Coupling of Grain Boundaries Using In Situ TEM: Marc Legros1; Nicolas Combe1; Frédéric Mompiou1; 1CEMES-CNRS
     The large proportion of grain boundaries in nanocrystals may compensate the absence of dislocations by influencing the plastic deformation mechanisms. The so-called shear-migration coupling has proved to significantly contribute to plasticity despite being poorly characterized in polycrystals experimentally or theoretically. We have conducted both in-situ TEM experiments and molecular dynamic simulations using the NEB technique (Nudge Elastic Band) and shown that shear-migration coupling involves the displacement of linear defects called disconnections that are specific to grain boundaries. As dislocations in the crystal, the properties of these disconnections seem to guide the coupling mechanism of migrating grain boundaries. They are also involved in GB sliding and grain rotation.

8:50 AM  
Characterization of Dislocation Pile-ups at Special Angle Tilt Boundaries in Pure Nickel by Electron Channeling Contrast Imaging (ECCI) and Molecular Dynamics Simulations: Shanoob Balachandran1; James Seal1; Jialin Liu1; Yue Qi1; Martin Crimp1; 1Michigan State University
    Using the novel SEM based Electron Channeling Contrast Imaging (ECCI) technique, dislocations in polycrystalline pure Nickel, subjected to annealing at 815 °C and four-point bend testing, were characterized. Dislocation pile-ups have been characterized at incoherent Σ3 9R asymmetric tilt boundaries, which typically exist at the ends of coherent Σ3 boundaries in annealing twins in fcc crystals. The Burgers vectors and slip plane of the dislocations in the pile-ups were determined by standard channeling contrast and trace analysis. This analysis reveals that the pile-up dislocations have common Burgers vectors and slip planes across the boundaries, indicating full slip compatibility across the boundaries and that the slip resistance can be fully attributed to the disordered nature at the boundaries. Molecular dynamics simulations have been carried out in an effort to better understand the slip resistance that develops at these common boundaries.

9:10 AM  
Dislocation Characterization in a Scanning Electron Microscope Equipped with an Annular STEM Detector: Patrick Callahan1; Jean-Charles Stinville1; McLean Echlin1; Eric Yao1; Mike Titus1; Dan Gianola1; Samantha Daly1; Tresa Pollock1; 1University of California Santa Barbara
    TEM characterization of dislocation substructure in significant volumes of material in either conventional or STEM modes is typically time consuming and requires a specific configuration to image defects. In this study, an SEM equipped with a STEM detector has been used to study dislocations in various materials, including a polycrystalline Nickel-base superalloy, a single crystal Cobalt-base superalloy, and strontium titanate. Interference effects associated with conventional TEM like thickness fringes and bending contours are significantly reduced and intrinsic defects are imaged clearly even in regions with high density of dislocations. Simulations have been used to determine the energy distributions of transmitted electrons at an accelerating voltage of 30kV, typical of SEM imaging. The experimental and simulated results are compared to standard analyses using TEM. Defect images have been simulated for the SEM geometry, and diffraction contrast in the SEM is discussed.

9:30 AM  
Detection of the Onset of Plasticity in Micro-crystals: In-situ Deformation of InSb Micro-pillars under Synchrotron Coherent X-ray Nanobeam: Ludovic Thilly1; Vincent Jacques2; Christoph Kirchlechner3; 1Pprime Institute - University of Poitiers; 2LPS; 3Max-Planck-Institut für Eisenforschung
    Coherent x-ray micro-diffraction was used to detect and count phase defects (stacking faults, SFs, left in the crystal after glide of partial dislocations) preliminarily introduced by deformation of InSb single-crystalline micro-pillars. Diffraction patterns were recorded by scanning the coherent nanobeam along the pillars axis: peak splitting is observed in the diffraction pattern associated to the top region, in agreement with the presence of few SFs located in the upper part of the deformed pillars. Simulations of coherent diffraction patterns were performed considering SFs randomly distributed in the illuminated volume: they show that not only the number of defects but also the size of the defected volume influences the maximum intensity of the pattern, allowing for a precise counting of defects [Physical Review Letters, 111 (2013), 065503]. Similar diffraction measurements were performed in-situ, during compression, to detect the first lattice defects, i.e. the onset of plasticity appearing in InSb micro-pillars.

9:50 AM Break

10:10 AM  
Comparison of Dislocation Characterization in Tantalum using Electron Channeling Contrast Imaging and Cross-Correlation Electron Backscattered Diffraction: Bret Dunlap1; David Fullwood2; Timothy Ruggles3; Brian Jackson2; Martin Crimp1; 1Michigan State University; 2Brigham Young University; 3National Institute of Aerospace
    Cross-correlation electron backscattered diffraction (CC-EBSD) mapping has been developed over the past few years to rapidly map elastic strain distributions and geometrically necessary dislocations (GNDs) in crystalline materials. Electron channeling contrast imaging (ECCI) also allows direct imaging and characterization of dislocations in similar volumes. The capabilities of these two approaches were directly compared by examining dislocation arrays developed near spherical nanoindentations in body centered cubic tantalum. ECCI contrast and trace analysis were used to characterize the Burgers vectors and line directions of the dislocations. EBSD mapping was carried out over the same areas where ECCI was performed. The EBSD patterns were saved and cross-correlated using the OpenXY software package to determine the elastic strain tensors across the area of interest, and GND maps were subsequently generated. The GNDs were resolved onto specific slip systems and compared to the ECCI results. The advantages and limits of ECCI and CC-EBSD were evaluated.

10:30 AM  
Analysis of Dislocation Structures in Ferritic and Dual Phase Steels Regarding Continuous and Discontinuous Loading Paths: Gregory Gerstein1; Till Clausmeyer2; Florian Gutknecht2; A. Erman Tekkaya2; Florian Nürnberger1; 1Leibniz Universität Hannover; 2TU Dortmund University
    In bulk sheet forming processes the hardening behavior of the material is determined by the choice of the sequence of deformation steps and type of deformation. Loading path changes induce transient hardening phenomena. These phenomena are linked to the formation and interaction of oriented dislocation structures. The aim of this study is to investigate the effect of continuous and discontinuous loading path changes on the dislocation microstructure in a ferritic and a ferritic-martensitic dual-phase steel. A biaxial tester test stand is used, which permits to continuously change the load from tension to shear. Transmission-electron microscopy shows less pronounced evolution of oriented dislocation structures in the ferrite single-phase steel for continuous loading path changes compared to discontinuous loading path changes. This tendency is smaller in dual-phase steel compared to the single phase material. Microstructural results for the ferritic steel are accompanied by simulation results with a transient hardening model.

10:50 AM  
Modeling Dislocation Arrays in Orientation Gradient Microstructures in Ta Thin Films: Elizabeth Ellis1; Ari Kestenbaum1; Shefford Baker1; 1Cornell University
    Tantalum films are widely used in industry, in such applications as thin film capacitors and resistors. When Ta is deposited in the metastable beta phase and then annealed to transform to the stable alpha phase, an unusual microstructure arises: EBSD results show that the orientation of these films rotates continuously through large angles across the film. In order to understand this structure, we have tried to predict the types of dislocations which may give rise to this microstructure. While a Nye tensor approach was not successful for calculating geometrically necessary dislocations directly from EBSD data, a simple model showed that our microstructure can be replicated using a simple rotation scheme. Expanding on this, we developed a genetic algorithm to analyze the dislocations associated with our orientation gradient microstructure and found several recurring features. Based on these findings, we comment on the origin and development of this unique microstructure.

11:10 AM  
Quantifying Strain-path Dependent Dislocation Densities Using Time of Flight Neutron Diffraction and High Resolution Electron Backscatter Diffraction Techniques: David Collins1; Richard Todd1; Angus Wilkinson1; 1University of Oxford
    Certain metallic systems, including steels, are known to provide a ductility gain when subjected to non-proportional over proportional deformation. However, differences in the micromechanics between such strain-paths as plasticity is accumulated are yet to be quantified in a meaningful manner. In this study, the dislocation densities of a ferritic steel were quantified in samples having been subjected to different strain-paths but with the same final macroscopic strain. Strain broadened diffraction line profiles were used to quantify grain-averaged dislocation densities in time-of-flight diffraction data using a Warren-Averbach analysis adapted for neutron data. Values deduced here were compared to spatially resolved, orientation dependent geometrically necessary dislocation densities obtained from high resolution electron backscatter diffraction data. Together, the results provide evidence that the dislocation densities for grain families with common orientations have a strong strain-path dependency.