Mechanical Behavior of Nanostructured Materials: Mechanical Properties of Thin Films, Low Dimensional Material
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Xinghang Zhang, Purdue University; Yuntian Zhu, North Carolina State University; Joseph Poon, University of Virginia; Suryanarayana Challapalli, University of Central Florida; Enrique Lavernia, University of California, Irvine; Haiyan Wang, Texas A&M University

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

Funding support provided by: AJA International; Hysitron Inc.

Session Chair: Yuntian Zhu, North Carolina State University; Cynthia Volkert, Universität Göttingen; Marc Legros, CEMES-CNRS

8:30 AM  Invited
Experimental Observations of the Mechanical Behavior of Nanocrystalline Thin Films: Kevin Hemker1; Suman Dasgupta1; Paul Rottmann1; 1Johns Hopkins University
    Nanocrystalline metals are well known for their high strength and low ductility, but their response is not purely linear elastic. Dislocation motion is mitigated but not eliminated by the reduced grain size, and other deformation mechanisms can be activated and carry plastic deformation. Microtensile experiments of submicron freestanding nanocrystalline films have been coupled with TEM observations to elucidate the underlying deformation mechanisms. Mechanical and thermal stability, stress-induced grain boundary and twin boundary migration, dislocation activity and the role of impurities will be discussed. In particular, emerging in situ techniques such as TEM-based Precession Assisted Crystal Orientation Mapping (PACOM) now allow for nanoscale observations of individual grains and grain boundaries that provide quantitative mechanistic information about nanoscale deformation mechanisms. Key parameters of interest to this study include the effect of grain size and grain boundary misorientation, character and chemistry on mobility and the attendant mechanical response of nanocrystalline metals.

8:55 AM  
Exploring Nanoindentation Induced Stress Field Propagation in Nanoporous Thin Films: Tyler Vanover1; Nicolas Briot1; Thomas Balk1; 1University of Kentucky
    Nanoindentation is a versatile technique that has been used extensively to probe the surface of bulk materials to obtain mechanical behavior data. Here, this technique is utilized to study the mechanical response associated with nanoporous (np) thin films of Si and Au with a variety of thicknesses. In this study, the authors explore the depth to which the stress field penetrates into the material and around the indenter tip by milling cross-sections of the indents with a focused ion beam and examining the compressed ligaments and densification effects. It has been realized that the np material is compressed and densified not only directly underneath the indenter, but also beyond the diameter of the indent. These results will be discussed in the context of how shallow the indents must be to prevent substrate effects from being incorporated into the resultant mechanical outputs.

9:15 AM  Invited
In-Situ Electron Microscopy of Fracture and Flow: Bahne Kapelle1; Andreas Kelling1; Florian Süß1; Cynthia Volkert1; 1University of Göttingen
    Deformation and fracture are complex and highly non-equilibrium processes which determine the strength of materials. Gaining insights into the mechanisms controlling these dynamic processes requires dynamic methods such as in-situ electron microscopy, which allows observations at length scales down to Angstroms and times scales between 0.1 seconds and several hours. I will present studies on tensile loading of high quality Au nanowires using in-situ testing in the TEM and the SEM, where deformation is controlled by surface nucleation of dislocations. A comparison with classical nucleation theory reveals both agreement as well as some fundamental contradictions which I will discuss. I will also present our in-situ studies on crack propagation in polycrystalline and single crystalline Ti, where we can directly observe the plastic zone and crack path in the TEM. By relating the crack propagation to the strain energy release rate, we gain understanding of the features controlling toughness.

9:40 AM  
Grain Boundaries Shear-migration Coupling and Its Impact on Plastic Deformation in Nanocrystalline Metals: Marc Legros1; Frédéric Mompiou1; Nicolas Combe1; Ehsan Hosseinian2; Olivier Pierron2; 1CEMES-CNRS; 2Georgia Institute of Technology
    The absence of dislocations in nanocrystalline metals precludes an easy plastic deformation. A key alternative mechanism is called the shear-migration coupling of grain boundary. This mechanism is able to produce a significant shear under stress, accompanied by grain growth that may further help relaxing the applied mechanical load. This mechanism is however poorly characterized in polycrystals experimentally or theoretically. We have conducted both in-situ TEM experiments, grain orientation mapping and molecular dynamic simulations that 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. The same disconnections are also involved in GB sliding and grain rotation.

10:00 AM Break

10:20 AM  Invited
Strength and Deformation of Far-from-Equilibrium Metallic Systems at the Nano-scale: High-Entropy Alloys and Metallic Glasses: Julia Greer1; Rachel Liontas1; Adenike Giwa1; H. Diao2; Peter Liaw1; 1California Institute of Technology; 2U Tennessee
     High entropy alloys represent a material class characterized by cubic-like thermally-stable solid solution phases. We fabricated nano-sized pillars with diameters between 400nm and 5microns contained within individual phases of Al0.7CoCrFeNi. We present orientation and phase mapping along with uniaxial compression results for each phase. Results show that samples BCC-like phase (A2+B2) exhibit high strengths of ~3 GPa, smooth stress-strain data, and significant hardening. Samples from FCC-like phase have discrete strain bursts, a lack of global hardening, and relatively high strengths, a signature of single-crystalline nano-plasticity. We also investigated mechanical behavior and atomic structure of sputtered glassy Zr-Ni-Al nanopillars with widths of 75-215 nm. In-situ tensile tests conducted inside SEM reveal extreme ductility in metallic glass nanopillars, reaching >150% true plastic strains, and necking down to a point. Using MD simulations, TEM, and synchrotron XRD, we explain the observed mechanical behavior through changes in free volume and short-range order.

10:45 AM  
Grain Size or Film Thickness? Influence of the Two Main Length Scale Parameters on the Mechanical Reliability of Polymer-supported Metal Films: Oleksandr Glushko1; Megan Cordill1; 1Erich Schmid Institute
    In this presentation the relationship between grain size, film thickness and mechanical damage induced by monotonic and cyclic tensile loading in Cu and Au films will be analyzed. For in-situ monitoring of microstructural and topological changes the electric resistance signal measured during the mechanical tests was utilized. The SEM and EBSD were used for direct characterization of mechanical damage and possible changes in the microstructure. It will be shown that restricted ductility due to small grain size acts as a drawback in monotonic tensile test but provides a significant advantage during cyclic loading. Thicker films with larger grain size can be strained to higher strains without extensive cracking but fail at much lower cyclic strains due to the formation of extrusion/crack couples. The maximum density of cracks induced by cyclic loading is predominantly defined by the film thickness and does not depend on the grain size.

11:05 AM  Invited
The Mechanical Behavior of Highly Oriented, Nano-layered HCP/BCC Composites: Irene Beyerlein1; Milan Ardeljan2; Marko Knezevic2; Nathan Mara1; Daniel Savage2; Sven Vogel1; Rodney McCabe1; John Carpenter1; 1Los Alamos National Laboratory; 2University of New Hampshire
    Over the years, two-phase nanolaminate composites have demonstrated an unusually broad number of desirable properties, such as high strength, high strain to failure, thermal stability, and resistance to light-ion radiation. Recently we have explored bi-phase HCP/BCC nanolaminates with layer thicknesses < 100 nm made either via severe plastic deformation or magnetron sputtering. Experimental testing of these nanolaminates shows exceptionally high strength, ductilty, and thermal stability. Microstructural characterization, via a suite of techniques, indicates that with both processing methods, the nano-sized crystals are highly oriented. This presentation highlights our modeling and experimental efforts to understand the linkages between interface properties, preferred texture and stress-strain response. Specifically we employ a spatially resolved multiscale 3D crystal plasticity based model to study the orientational stablity, slip activity, and strength.

11:30 AM  
Structure Dependent Creep Behavior of CuNb Nanolaminates: Jaclyn Avallone1; Thomas Nizolek1; Irene Beyerlein1; Nathan Mara2; Tresa Pollock1; 1University of California Santa Barbara; 2Los Alamos National Laboratory
    CuNb nanolaminates show extraordinary microstructural stability at high temperatures and stresses when compared to their single-phase components. Metallic multilayers with nanoscale layer thicknesses have higher strength and resistance to damage by shock and irradiation than those on the microscale. Unfortunately, at elevated temperatures, nanolaminates break down by layer pinch-off and coarsening more rapidly than microlaminates. Mechanical behavior of these multilayers is influenced by thermal evolution of the laminate structure, therefore, it is important to understand the relationship between annealing time and creep performance. Accumulatively roll bonded CuNb laminates spanning the micro- and nanoscales were subjected to load-jump creep tests to determine minimum creep rates at various stresses and annealing times. Finer layer thickness laminates with longer thermal exposure times are the most creep resistant. Characterization of the structural evolution and analysis of this novel creep behavior will be discussed with respect to initial and evolved structures.

11:50 AM  
Influence of Severe Plastic Deformation on the Local Deformation Behavior of Nanostructured Metals under Extreme Conditions: Verena Maier-Kiener1; Alexander Leitner2; Reinhard Pippan3; Daniel Kiener2; 1Montanuniversität Leoben - Physical Metallurgy & Materials Testing; 2Montanuniversität Leoben - Materials Physics; 3Austrian Academy of Sciences - Erich-Schmid-Institute for Materials Science
    In this work, the thermally activated deformation behaviour of Au was studied by applying different nanoindentation testing techniques from room temperature up to 400 °C. While for single crystalline Au the indentation properties agree well with literature, the highly deformed nanocrystalline Au sample shows a surprisingly low modulus at temperatures exceeding 200 °C. In order to better understand this phenomenon, different other fcc- and bcc-materials produced by both, bulk as well as powder HPT-fabrication, were additionally tested up to 400 °C. It is shown that for all materials the modulus of the powder-deformed samples exhibits a pronounced reduction at higher temperatures, while for the bulk derived nc-states the modulus behaves according to literature. This novel phenomenon is discussed in the context of a higher concentration of vacancies at the grain boundaries, additional creep effects, or changes in the sink-in formation due to different work hardening behaviours.