Evolution of local mechanical behavior during high strain rate deformation of tantalum

Shear localization is often a failure mechanism in materials subjected to high strain rate deformation. It is generally accepted that the microstructure evolution during deformation and the resulting heterogeneities strongly influence the development of these shear bands. However, current crystal plasticity models fail to effectively capture the heterogeneous plastic deformation. One critical missing piece in these models is the information on the development of local mechanical heterogeneities during deformation. Consideration of local microstructure evolution through electron back scattered diffraction (EBSD) measurements constitutes only one aspect of the characterization of a deformed material. Another equally important attribute, which is somewhat difficult to capture, is the change in local mechanical properties at the submicron length scale. With the recent advances in spherical nanoindentation data analysis, there is now an unprecedented opportunity to obtain insights into the change in local mechanical properties during deformation in materials at submicron length scales. In this study, we quantify the evolution of microstructure and local mechanical properties in tantalum under dynamic loading conditions, to capture the structure–property correlations at the submicron length scale, with an aim to gain insights into failure mechanisms in these materials. A split Hopkinson pressure bar is used to dynamically (strain rates ~103) impart predetermined levels of strain into high purity tantalum samples. Insights into the structure–property relationships in these samples will be obtained by combining local mechanical property information captured using spherical nanoindentation with complimentary structure information at the indentation site obtained using EBSD. As a first step, the orientation dependence of the indentation yield strength in tantalum in the fully annealed condition (negligible dislocation density) will be mapped. This yield contour has tremendous value in studies on deformed polycrystalline samples. When characterizing deformed samples (appreciable dislocation content, nonuniformly distributed in the sample), the yield contours will be used to reliably de-couple the contributions to the local indentation yield strength from the local crystal lattice orientation and the local dislocation density. Thus, local dislocation density variation within deformed samples, as a function of orientation, grain boundaries, and triple junctions will be established. This constitutes an extremely powerful dataset from which insights into the local mechanical properties of various structural constituents at a submicron length scale will be extracted. This information can in turn be used to determine the contribution of individual features to the local as well as overall (macroscale) performance of the material. Specifically, this study will provide an understanding of the evolution of local structure and mechanical properties during high strain-rate deformation of tantalum, in particular effects of orientation and individual grain boundaries on the development of mechanical heterogeneities within the deformed microstructure. This study will also serve to establish an initial, comparative basis for future studies focused on quantifying the strength of damaged tantalum

Probing nanoscale damage gradients in irradiated materials with spherical nanoindentation

We discuss applications of spherical nanoindentation stress-strain curves in characterizing the local mechanical behavior of materials with modified surfaces. Using ion-irradiation on tungsten as a specific example, we show that a simple variation of the indenter size (radius) can identify the depth of the radiation-induced-damage zone, as well as quantify the behavior of the damaged zone itself. Using corresponding local structure information from electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) we look at (a) the elastic response, elasto-plastic transition, and onset of plasticity in ion-irradiated tungsten under indentation, and compare their relative mechanical behavior to the unirradiated state, (b) correlating these changes to the different grain orientations in tungsten as a function of (c) irradiation from different sources (such as He, W, and He+W)

Controversies in Exit Polling: Implementing a Racially Stratified Homogenous Precinct Approach

In November 2000, exit poll interviews with voters in Florida indicated that Al Gore won the state. As a result, many television networks declared Gore the winner of Florida, a pivotal state to winning the presidency in 2000. Only a few hours later, the first vote tallies from the Florida Secretary of State\u27s office revealed that George W. Bush was in fact leading in Florida. After 45 days of recounts and lawsuits, it was clear that the exit polls were wrong; Bush had won the state by the narrowest of margins. As a result of the flawed exit poll the media and pollsters scoured and reanalyzed the methodology used in 2000 to prepare and correct for the 2004 presidential election. The old system, Voter News Service (VNS) was scrapped entirely, and Edison-Mitofsky Research was chosen to implement a new and more accurate national exit poll in 2004 by a consortium of news organizations retained by the Associated Press called the National Election Pool (NEP). What happened? Exit poll results from Edison-Mitofsky showed John Kerry ahead in Ohio, Florida, and New Mexico—all states which he lost to Bush in 2004.

Nanoindentation strain rate jump test-based prediction of fracture and the brittle to ductile transition in tungsten

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Effects of Helium Implantation on the Tensile Properties and Microstructure of Ni₇₃P₂₇ Metallic Glass Nanostructures

We report fabrication and nanomechanical tension experiments on as-fabricated and helium-implanted 130 nm diameter Ni₇₃P₂₇ metallic glass nanocylinders. The nanocylinders were fabricated by a templated electroplating process and implanted with He+ at energies of 50, 100, 150, and 200 keV to create a uniform helium concentration of 3 atom % throughout the nanocylinders. Transmission electron microscopy imaging and through-focus analysis reveal that the specimens contained 2 nm helium bubbles distributed uniformly throughout the nanocylinder volume. In situ tensile experiments indicate that helium-implanted specimens exhibit enhanced ductility as evidenced by a 2-fold increase in plastic strain over as-fabricated specimens with no sacrifice in yield and ultimate tensile strengths. This improvement in mechanical properties suggests that metallic glasses may actually exhibit a favorable response to high levels of helium implantation

Radiation damage tolerant nanomaterials

Designing a material from the atomic level to achieve a tailored response in extreme conditions is a grand challenge in materials research. Nanostructured metals and composites provide a path to this goal because they contain interfaces that attract, absorb and annihilate point and line defects. These interfaces recover and control defects produced in materials subjected to extremes of displacement damage, impurity implantation, stress and temperature. Controlling radiation-induced-defects via interfaces is shown to be the key factor in reducing the damage and imparting stability in certain nanomaterials under conditions where bulk materials exhibit void swelling and/or embrittlement. We review the recovery of radiation-induced point defects at free surfaces and grain boundaries and stabilization of helium bubbles at interphase boundaries and present an approach for processing bulk nanocomposites containing interfaces that are stable under irradiation.United States. Dept. of Energy. Office of Basic Energy Sciences (Award 2008LANL1026

The influence of 3-D interfacial structure and morphology on the mechanical behavior of nanocomposites

2-dimensional (2D) sharp interfaces with distinct boundaries demarcating an abrupt discontinuity in material properties in nanolayered composites have been studied for almost twenty years and are responsible for enhanced behaviors such as strength, radiation damage tolerance, and deformability. However, 2-D interfaces have their limitations with respect to deformability and toughness. 3D interfaces are defined as heterophase interfaces that extend out of plane into the two crystals on either side and are chemically, crystallographically, and/or topologically divergent, in three dimensions, from both crystals they join. Here, we present the mechanical behavior of two different classes of nanocomposites: 1.) nanolayered Cu/Nb containing interfaces with 3D character and 2.) Tungsten-based 3D ordered mesoporous composites consisting of a porous W scaffolding with silicon carbide or silicon nitride infill. Micropillar compression results show that the strength of Cu/Nb nanocomposites containing 3D interfaces is significantly greater than those containing 2D interfaces. Shear banding in 3D Cu/Nb is observed during pillar compression with retention of continuous layers across the shear band. We will present our recent results on deformation of such 3-D interfaces and structures, and describe this evolution mechanistically through the use of atomistic simulations. Please click Additional Files below to see the full abstract

Origins of size effects in initially dislocation-free single-crystal silver micro- and nanocubes

We report phenomenal yield strengths—up to one-fourth of the theoretical strength of silver—recorded in microcompression testing of initially dislocation-free silver micro- and nanocubes synthesized from a multistep seed-growth process. These high strengths and the massive strain bursts that occur upon yield are results of the initially dislocation-free single-crystal structure of the pristine samples that yield through spontaneous nucleation of dislocations. When the pristine samples are exposed to a focused ion-beam to fabricate pillars and then compressed, the dramatic strain burst does not occur, and they yield at a quarter of the strength compared to the pristine counterparts. Regardless of the defect-state of the samples prior to testing, a size effect is apparent—where the yield strength increases as the sample size decreases. Since dislocation starvation and the single-arm-source mechanisms cannot explain a size effect on yield strength in dislocation-free samples, we investigate the dislocation nucleation mechanisms controlling the size effect through careful experimental observations and molecular statics simulations. We find that intrinsic or extrinsic symmetry breakers such as surface defects, edge roundness, external sample shape, or a high vacancy concentration can influence dislocation nucleation, and thus contribute to the size effect on yield strength in initially dislocation-free samples.This is a manuscript of the article Published as Griesbach, Claire, Seog-Jin Jeon, David Funes Rojas, Mauricio Ponga, Sadegh Yazdi, Siddhartha Pathak, Nathan Mara, Edwin L. Thomas, and Ramathasan Thevamaran. "Origins of size effects in initially dislocation-free single-crystal silver micro-and nanocubes." Acta Materialia 214 (2021): 117020. doi: https://doi.org/10.1016/j.actamat.2021.117020. © 2021 Elsevier. This manuscript is made available under the Elsevier user license (https://www.elsevier.com/open-access/userlicense/1.0/). CC BY-NC-ND