Advanced Characterization Techniques for Quantifying and Modeling Deformation: Session II
Sponsored by: TMS Extraction and Processing Division, TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Advanced Characterization, Testing, and Simulation Committee, TMS: Materials Characterization Committee
Program Organizers: Rodney McCabe, Los Alamos National Laboratory; 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; Olivia Underwood Jackson, Sandia National Laboratories
Monday 2:00 PM
March 15, 2021
Room: RM 13
Location: TMS2021 Virtual
2:00 PM
3D Maps of Geometrically Necessary Dislocations in Shock-loaded Polycrystalline Tantalum: Wyatt Witzen1; Toby Francis1; Tresa Pollock1; Irene Beyerlein1; 1University of California Santa Barbara
TriBeam Tomography has the unique capability of employing Three-Dimensional Electron Backscatter Diffraction (3D EBSD) to map the orientation of grains in 3D space. This intragranular resolution of orientation permits the ability to calculate Geometrically Necessary Dislocations (GNDs) and map these as dislocation densities in 3D space. A sample of polycrystalline BCC Tantalum was shock-loaded in a flyer plate impact test and subjected to this analysis to map dislocation networks present in the material after dynamic deformation. The calculated GND densities revealed dislocation networks associated with multiple subgranular features, including voids and subboundaries. Densities on the order of 1 × 10 15 m -2 were associated with some of these features, including voids lying at the grain boundaries and within the interior of the grains themselves. GND densities associated with the {110}, {112}, and {123} slip families were compared when calculating the total GND densities mapped in the microstructure.
2:20 PM
Dislocation Imaging by Precession Electron Diffraction: Dexin Zhao1; Kelvin Xie1; 1Texas A&M University
Dislocation imaging is usually done by regular bright-field imaging in the transmission electron microscope (TEM) under the two-beam or low-index zone axis (multiple-beam) conditions. However, both techniques have drawbacks. Two-beam condition limits the holistic illumination of dislocations. Regarding the low-index zone axis imaging, more dislocations could be illustrated, but the dynamical effect usually overwhelms the contrast from the dislocation lines. In this work, we employed precession electron diffraction (PED) as a tool for dislocation imaging using deformed AZ31 as a model material. The beam precession could also be utilized to potentially generate high-quality micrographs with enhanced dislocation contrast. One major result of beam precession is to average out the dynamical effect in TEM micrographs and to show high-quality kinematical information. We demonstrated PED is a powerful characterization technique that could remove the dynamical effect and better reveal dislocations under the multiple-beam conditions.
2:40 PM
On the Mechanistic Origins of Maximum Strength in Nanocrystalline Materials: Ankit Gupta1; Gregory Thompson2; Garritt Tucker1; 1Colorado School of Mines; 2University of Alabama
Maximum strength in nanocrystalline (NC) metals/alloys has been suggested to occur at grain sizes in the NC regime, where the governing deformation mechanism transitions from dislocation plasticity to GB mediated deformation. However, this notion has never been confirmed, owing to the difficulty in asserting the contribution of different mechanisms to microstructural strain accommodation. In this study, the contribution of individual nanoscale mechanisms to the overall deformation of NC metals/alloys is calculated from atomistic simulations leveraging continuum-based kinematic metrics to compute mechanistic contributions to microstructural strain. By employing such a quantitative approach, it is shown that the realization of maximum strength in NC metals corresponds to a grain size regime where the operative nanoscale mechanisms transition between intergranular and intragranular mediated mechanisms. The results are discussed in terms of systems’ crystal structure (BCC/FCC), alloy type (binary/medium entropy), where twinning is abundant, and potential artifacts of atomistic simulations as high strain rate.
3:00 PM
Grain Boundary Slip Transfer Classification and Metric Selection with Artificial Neural Networks: Zhuowen Zhao1; Thomas Bieler1; Javier LLorca2; Philip Eisenlohr1; 1Michigan State University; 2IMDEA Materials Institute
The accurate prediction of the interaction between dislocation slip and grain boundaries is a long-standing challenge in the field of crystal plasticity. An artificial neural network (ANN) is used to evaluate the effectiveness of six metrics and their combinations to assess whether instances of slip transfer happen across grain boundaries in coarse-grained oligocrystalline Al foils. This approach extends the one- or two-dimensional projections that were formerly applied to analyze slip transfer data. We observe that the maximum attainable classification accuracy is limited to about 90% and does not substantially increase between low-dimensional projections and the full six-dimensional picture. The most effective metrics reflect the geometric relationship between grains sharing a boundary.
3:20 PM
High Resolution Characterization of Dislocations Using Weak Beam Dark Field Scanning Transmission Electron Microscopy: Jiashi Miao1; 1Ohio State University
Plastic deformation and mechanical behavior of alloys are mainly controlled by dislocation slip and interactions between dislocations. Dislocation characterization using electron microscopy can greatly help the understanding of deformation mechanisms. Recently, a new high resolution defect characterization technique: weak beam dark field scanning transmission electron microscopy (WB DF STEM) was developed. Experimental study showed that WB DF STEM imaging can provide the same resolution in the characterization of dislocations as that offered by the conventional weak beam dark field transmission electron microscopy (WB DF TEM). In this work, WB DF STEM images under different imaging conditions were simulated. Simulation results prove that high resolution imaging of dislocation structures can be achieved in WB DF STEM under proper imaging conditions. The examples of the application of WB DF STEM imaging in studying deformation substructures in different engineering alloys will be given.
3:40 PM
Revisiting the Origin of Indentation Size Effect at Sub-micrometer Scales: Xiaolong Ma1; Wesley Higgins2; Zhiyuan Liang2; Dexin Zhao2; George Pharr2; Kelvin Xie2; 1Pacific Northwest National Laboratory; 2Texas A&M University
Understanding the size effect in indentation testing is crucial to interpreting the mechanical behavior of materials in the small world. The Nix-Gao model and its derivatives, based on the strain gradient assumption, have been widely used to explain the depth-dependent indentation hardness and agree quite well with results from micro-indentation experiments. However, experimental behavior below 1 um starts to deviate from its predictions as depth becomes small. Here, using precession electron diffraction techniques, we characterize the microstructure evolution underneath indents from 100nm to 800nm. We show that dislocations appear relatively more mobile during very low-depth indentation and thus less constrained in the conventionally perceived deformation volume. A mechanism transition from source-limited hardening to dislocation Taylor hardening at the nanometer scale is hypothesized to explain the deviation from the Nix-Gao model. Further indentation experiments on samples with high and low initial dislocation densities lend further support to the hypothesis.
4:00 PM
Critical Resolved Shear Stresses (CRSS) of Hexagonal Titanium from Nanoindentation Optimization: Zhuowen Zhao1; Mario Ruiz2; Jiawei Lu1; Miguel Monclus2; Jon Molina-Aldareguia2; Thomas Bieler1; Philip Eisenlohr1; 1Michigan State University; 2IMDEA Materials Institute
CRSS values of slip systems in hexagonal titanium form the basis of the description of its crystal plasticity. Experimental quantification of individual CRSS value by exclusive activation (conditions of single slip) is very difficult. An inverse methodology to establish CRSS values is through fitting simulated results to experimental counterparts. However, disagreement among CRSS ratios for pure titanium by fitting against different experimental observations at room temperature has been reported. It is also known that temperature and composition could alter the CRSS values or their relative order. This study aims at identifying parameter sets at ambient and higher temperatures for pure titanium, Ti-3Al-2.5V, and Ti-6Al-4V using an efficient approach that is based on surface topography and load-displacement curve from nanoindentation. Quantitative assessment of possible sources that contribute to the deviation between simulated (using optimized parameters) and measured response from nanoindentation is carried out, such that sensitivity of the approach is established.
4:20 PM
Spatial Localization of Dislocation Avalanches in Microplasticity of a High-entropy Alloy: Quentin Rizzardi1; Robert Maass1; 1University of Illinois at Urbana-Champaign
While intermittent plasticity in microplasticity is a well-studied phenomenon, the precise nature of the underlying collective dislocation mechanisms is still being discussed; most research so far has been limited to the statistical aspect of discrete slip events (collective avalanches). Here, we aimed to reconcile the statistical picture with a more pointed look at the actual spatial distribution of slip within the crystal; to that end, we observed a high-entropy alloy microcrystal undergoing uniaxial compression and associated the measured avalanches with their resulting slip morphologies on the microcrystal surface. The categorization of slip and avalanche behavior allows us to shed some light onto what types of slip underlie specific parts of the overall slip-size distribution. We find that certain types of avalanches, as categorized by their respective slip morphology, favor different parts of the full slip-size distribution and exhibit statistical behavior different from the expected power-law form.