Advanced Characterization Techniques for Quantifying and Modeling Deformation: Dislocations and Planar Faults
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Shaping and Forming Committee, TMS: Materials Characterization Committee
Program Organizers: Rodney McCabe, Los Alamos National Laboratory; Thomas Bieler, Michigan State University; Marko Knezevic, University of New Hampshire; Irene Beyerlein, University of California, Santa Barbara; Wolfgang Pantleon, Technical University of Denmark; C. Tasan, Massachusetts Institute of Technology; Arul Kumar Mariyappan, Los Alamos National Laboratory

Tuesday 8:30 AM
February 25, 2020
Room: Theater A-2
Location: San Diego Convention Ctr

Session Chair: Benoit Merle, University Erlangen-Nuremberg (FAU); Meimei Li, Argonne National Laboratory


8:30 AM  Invited
Deformation Mechanisms in Irradiated Materials Revealed by in situ High Energy Synchrotron X-rays: Meimei Li1; Xuan Zhang1; Jonathan Almer1; Jun-Sang Park1; Peter Kenesei1; 1Argonne National Laboratory
    High-energy synchrotron x-rays have a unique advantage over other characterization techniques in probing deformation and failure mechanisms in structural materials because of their deep penetration, high brilliance, and high flux. This presentation will highlight the applications of wide-angle x-ray scattering and small-angle x-ray scattering for in situ 2D characterization of grain-averaged deformation behavior, and high-energy diffraction microscopy and x-ray tomography for in situ 3D studies of grain-scale heterogeneous deformation in neutron-irradiated ferritic and austenitic alloys. It will address the effects of irradiation-induced defects on deformation and failure mechanisms, and the effect of irradiation on deformation-induced phase transformation in Fe-based alloys. The critical microstructural parameters, e.g. dislocation density, substructure size, internal strain/stress, void population inferred from the high-energy synchrotron x-ray measurements will also be discussed.

9:00 AM  
Simulated STEM-DCI Imaging of Dislocations in High-entropy Alloys: Joseph Tessmer1; Mulaine Shih2; Marc De Graef1; 1Carnegie Mellon University; 2The Ohio State University
    Experimental Scanning Transmission Electron Microscope-Defect Contrast Imaging (STEM-DCI) of dislocations in High-Entropy Alloys (HEAs) has shown that dissociated dislocations can display a strong variance in dissociation distance. Preliminary work with Molecular Dynamics (MD) simulations has shown that this variance could be due to the solid-solution nature of HEAs leading to local changes in the stacking fault energy. These simulated results can be interpreted in multiple ways, such as Nye tensor analysis, but direct comparison with experimental STEM-DCI images can be challenging. Simulation STEM-DCI images from the MD simulation output data provides the opportunity for a one-to-one comparison between experiment and simulation, providing further insight on the mechanisms of defect behavior in these (and potentially other) material systems.

9:20 AM  
Influence of Plastic Deformation on the Disorder → Order Transformation in 18 Carat Red Gold Alloys Studied by In-situ High-energy X-ray Diffraction: Marina Garcia Gonzales1; Steven Van Petegem1; Nadine Baluc2; Fanny Lalire3; Helena Van Swygenhoven1; 1Paul Scherrer Institut; 2Ecole Polytechnique Fédérale de Lausanne; 3Varinor S.A.
    18 carat red gold is an age-hardenable alloy that hardens via chemical ordering. Above a critical temperature the alloy exhibits a chemically disordered microstructure. Below this temperature, chemical ordering takes place, resulting in the formation of harder ordered Au50Cu50 nano-precipitates. This causes distortions in both the disordered matrix and nanoprecipitates, resulting in strong misfit strains. This, in turn, significantly affects the mechanical properties. In this work, we study the influence of plastic deformation and thermal history on the ordering kinetics. Therefore, we have performed in situ high-energy X-ray diffraction experiments during controlled cooling from the high-temperature phase to room temperature, followed by various thermal treatments. The experiments are performed on samples with and without prior plastic deformation, and complemented by post-mortem microscopy observations. We find that dislocations induced by plastic deformation significantly enhance ordering at the early stages of transformation. These findings serve as input for improving thermo-mechanical processing routes.

9:40 AM  
Avalanche Statistics and the Intermittent-to-smooth-transition in Microplasticity: Gregory Sparks1; Yinan Cui2; Giacomo Po2; Quentin Rizzardi1; Jaime Marian2; Robert Maass1; 1University of Illinois at Urbana-Champaign; 2University of California Los Angeles
    Plastic flow at small scales is generally observed to be intermittent, whereas the stress-strain behavior of bulk crystals is mostly smooth. Here we find that when the external deformation rate of small-scale crystals approaches the speed of the crystallographic slip velocity, an intermittent-to-smooth transition of plastic flow is observed. By defining a rate-dependent intermittency-parameter, this phenomenon can be captured with a power-law covering 5.5 orders of magnitude for Au and Nb micron-sized single crystals with experiments, and via simulations for Nb crystals. Our results indicate that the transition to smooth flow is driven by a gradual truncation of the underlying truncated power-law that describes the intermittently evolving system. This is caused by a competition of internal and external rates, which aligns with the well-known transitions from serrated to non-serrated flow in metallic glasses or materials with dynamic strain aging.

10:00 AM Break

10:20 AM  Invited
Influence of Grain Boundary Mediated Deformation on the Ductility of Freestanding Metallic Thin Films: Benoit Merle1; 1University Erlangen (Fau)
     New insights into the mechanisms accommodating large plastic deformations in freestanding metallic films are provided by in situ mechanical testing on 30-200 nm thick PVD gold, copper, and silver films in a Transmission Electron Microscope (TEM) and an Atomic Force Microscope (AFM).From in-situ tensile testing in the TEM, it was found that grain boundary mediated deformation mechanisms significantly increase the tolerance of the samples to small cracks and leads to a ductility well above 10% plastic strain, which can be rationalized by the high strain-rate sensitivity of the films. However, larger samples typically fail around 1% strain, presumably as a result of larger defects introduced during fabrication. Using the bulge test technique inside an AFM, it was found that introducing grain boundaries and increasing the material strength is key to maximizing the fracture toughness and therefore extending the ductility of such large freestanding specimens.

10:50 AM  
Solute Interaction with Dislocation Cores in α-iron: an Atom Scale Experimental Study: Maxime Vallet1; Estelle Meslin1; Michael Walls2; Lisa Ventelon1; 1CEA - Saclay; 2Laboratoire de Physique des Solides
    Recent in-situ TEM experiments carried out in pure Fe have highlighted an unexpected reappearance of a Peierls mechanism in the temperature range of the dynamical strain aging. The formation of an interaction between screw dislocations and solute atoms (C and N) has been proposed to explain this new mechanism. Recent ab initio calculations from our group have highlighted that this interaction would cause the reconstruction of core dislocations from the usual easy core to the hard core configuration. Until now, no direct evidence of the solute atom effect on dislocation cores has been observed due to the very high spatial resolution required. In this work, we studied the segregation of solute atoms on dislocation cores in pure Fe in order to evaluate the dislocation core atomic structure by coupling atomically resolved HAADF-STEM with electron energy-loss spectroscopy (EELS) and atom probe tomography.

11:10 AM  
Size Effects in Dislocation-mediated Pore Growth: Ashley Roach1; Fulin Wang1; Jungho Shin1; Gyuseok Kim2; Irene Beyerlein1; Daniel Gianola1; Darby Luscher3; 1UC Santa Barbara Materials Department; 2University of Pennsylvania; 3Los Alamos National Laboratory
    While pore growth is a crucial mechanism for spallation and ductile failure, continuum-scale models have limited predictive capabilities. Phenomenological plasticity models represent underlying deformation mechanisms for specific pre-determined environments and can’t capture size effects that may dominate at the micron-scale. Atomistic simulations reveal dislocation mechanisms mediating nano-scale pores, but with restrictive size and temporal limitations. We meet this multi-scale modeling challenge, marked by a gap between continuum and atomistic models, by designing a new “meso-scale” experiment, where deformation is sensitive to specific mechanisms and test specimen are large enough for detailed measurements. Thin film tensile specimen containing patterned pores of varying diameters are tested in-situ with transmission-SEM. Discrete dislocations can be readily observed while pore growth is quantified in relation to pore size, crystallographic orientation, proximity to grain boundaries, and applied load. To guide and interpret experiments, a CPFE-based pore growth model is employed to predict size and orientation effects.

11:30 AM  
Revisiting the Wilkens Function: a Discrete Dislocation Dynamics-based Study of Strain Broadening in Diffraction Profiles: Aaron Tallman1; Darshan Bamney2; Douglas Spearot2; Laurent Capolungo1; 1Los Alamos National Laboratory; 2University of Florida
    Accurate diffraction-based estimates of dislocation content provide value to materials modeling and in-service materials evaluation by coupling microstructure characterization with mechanical behavior. Many approaches have been proposed to provide such estimates by analyzing the broadening of diffraction peaks, most of which rely on idealizing dislocation microstructures to make predictions. With the advent of mesoscale modeling of dislocation networks, such strict assumptions may be relaxed. The work here is built using the formulations of line profile analysis as a foundation, while bypassing the idealization of straight dislocations. Discrete dislocation dynamics simulations are used with a virtual diffraction algorithm to generate synthetic diffraction profiles and to establish connections between strain broadening and dislocation content. A data reduction provides a smaller parametric basis to describe microstructure using peak broadening. The reduced data is used to formulate a surrogate model which leverages the physics-derived formulations of line profile analysis to accurately predict dislocation content.