Fracture Properties and Residual Stresses in Small Dimensions: In Situ Fracture Testing Methodologies
Sponsored by: TMS Structural Materials Division, TMS Materials Processing and Manufacturing Division, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Daniel Kiener, University of Leoben; Marco Sebastiani, Roma TRE university; Nagamani Jaya Balila, Max Planck Institut fuer Eisenforschung GmbH; William Gerberich, University of Minnesota; Siddhartha (Sid) Pathak, University of Nevada, Reno

Wednesday 2:00 PM
March 1, 2017
Room: 21
Location: San Diego Convention Ctr

Session Chair: Andrew Minor, UC Berkeley; Jeffrey Wheeler, ETH Zurich

2:00 PM Introductory Comments

2:05 PM  Invited
Nanoscale Strain Mapping of Individual Defects during In Situ Deformation: Thomas Pekin1; Colin Ophus2; Christoph Gammer3; Jim Ciston2; Andrew Minor1; 1UC Berkeley & LBNL; 2LBNL; 3Erich Schmid Institute
    Recent advances in local strain mapping using nanobeam electron diffraction (NBED) has demonstrated the ability to observe single defects and the strain fields around them at a resolution of single nanometers. In addition to measuring the strength of small-volumes, measuring the evolution of strain during plastic deformation is of great importance for correlating the defect structure with material properties. This work will highlight our latest results from in situ strain mapping in an Al-Mg alloy and stainless steel using both contact loading methods (such as in situ nanopillar compression and nanoindentation) as well as non-contact methods such as tensile straining. Our method of local strain mapping consists of recording large multidimensional data sets of nanodiffraction patterns during the test. The resulting dataset contains diffraction data for every point of the STEM image, from which strain maps can be extrapolated on a scale not previously possible during in situ deformation.

2:35 PM  
Studying Plasticity during Fracture at the Micron Scale by Means of Cantilever Experiments in Single-crystalline NiAl and W – HR-EBSD Analyses and Elevated Temperature Measurements: Johannes Ast1; Juri Wehrs1; Johann Michler1; Xavier Maeder1; 1EMPA
    In order to understand the elastic-plastic fracture process at the microscale in the temperature range from room temperature up to 400 C, we perform micro-cantilever experiments inside a scanning electron microscope. The materials we focus on, single-crystalline B2-NiAl and tungsten, have brittle to ductile transition temperatures in the investigated temperature range. In that way the fracture behavior and the fracture toughness can be studied in dependence of temperature. Additionally in-situ high resolution electron backscatter diffraction measurements of the area around the crack tip are performed at various loading states at room temperature. This enables the observation of stresses leading to crack growth in the samples. By analyzing the relative misorientations in the crystal, the plastic deformation in terms of geometrically necessary dislocations can be mapped, quantified and used to describe the effect of dislocations on the fracture behavior.

2:55 PM  
In Situ Stable Crack Growth at the Micron Scale: Giorgio Sernicola1; Tommaso Giovannini2; Punit Patel3; James Kermode3; Daniel Balint2; T Ben Britton1; Finn Giuliani1; 1Department of Materials, Imperial College; 2Department of Mechanical Engineering, Imperial College London; 3Warwick Centre for Predictive Modelling, University of Warwick
    Grain boundaries typically dominate the mechanical properties of ceramics. Therefore, measuring the properties of individual grain boundaries is useful if we want to promote mechanistically informed grain boundary engineering in ceramic processing. Extracting properties of individual boundaries is very difficult in conventional testing, as a large scale fracture experiment tests many grain boundaries and convolute microstructure, crack path and performance. Therefore, we have developed an in situ technique using micro double cantilever beam and direct measurements of stable crack growth to measure the surface energy of SiC and the fracture energy of a glassy interface in a SiC bicrystal. Our measurements of the surface energy are correlated to new DFT calculations of the same interface and match well.

3:15 PM  Invited
In Situ Micron Scale Fracture Toughness Testing and Modeling of a Chevron Notched Bowtie-shaped Beam: Fiona Yuwei Cui1; Richard Vinci1; 1Lehigh University
    The chevron Single Edge Notched three point bending test has many advantages for measuring fracture toughness in bulk system. However employing this technique at the micron scale is challenging. In this study, we are developing a micron-scale bowtie-shaped specimen with a chevron notch to create stable crack growth. In-situ fracture testing was carried out using a Hysitron PI 85 Pico-indenter inside FEI Scios DualBeam FIB. A multi-step loading function was utilized to generate information about compliance change with respective to crack length in each loading cycle. A 3D finite element analysis model was built in Altair Hypermesh and ANSYS to evaluate the compliance change with crack length. Finally, a fracture toughness value was calculated by combining both experimental and analytical results. In order to verify the FEA model, a TEM specimen was fabricated at the notch area after early cycles in a fracture test to measure a real crack length.

3:45 PM Break

4:05 PM  Invited
Liquid Metal Embrittlement at the Micro-scale: Gallium FIB vs. Xenon FIB: Yuan Xiao1; Jeff Wheeler1; 1ETH Zurich
    Micromechanical testing of structures fabricated using focused ion beam (FIB) has allowed significant progress to be made in understand the deformation and properties of small volumes of materials. However, the vast majority of FIB structures are machined using Gallium, which is known to embrittle many metals (e.g. Al, Cu and Fe) by weakening their grain boundaries. This has recently been shown to have a significant negative effect on the strength of micropillars of polycrystalline aluminum. Here we extend upon that work to investigate the effect of Ga FIB on the deformation and fracture properties of grain boundaries of several materials by comparing structures made using both Xe FIB and Ga FIB.

4:35 PM  
Micro-Compression Testing of Mg-Nb Multilayered Nano-Composites for Ultra-High Strength, Formability and Ductility: Manish Jain1; Marko Knezevic2; Nathan Mara3; Irene Beyerlein3; Siddhartha Pathak1; 1University of Nevada Reno; 2University of New Hampshire; 3Los Alamos National Laboratory
    In this work we explore the mechanical response of Mg-Nb multilayered nanocomposites as a function of decreasing layer thickness. Layers of Mg and Nb, with individual layer thicknesses varying from 5 to 50 nm, were synthesized using physical vapor deposition. Transmission electron microscopy (TEM) and X-Ray diffraction (XRD) studies reveal that at lower thicknesses of ~5 nm Mg undergoes an interface strain induced phase transition from hexagonal close-packed (HCP) to a metastable body centered cubic (BCC) structure. Using a combination of indentation testing and micro-pillar compression experiments – where the multilayers were tested in 3 different orientations: with interfaces oriented normal, parallel and inclined 450 to the loading axis – we analyze the elastic and plastic anisotropy of this hitherto unstudied BCC Mg structure. These studies reveal BCC Mg to be super-strong and more ductile than its HCP counterpart, suggesting significant implications for potential lightweight aerospace and automotive applications.

4:55 PM  
High Temperature Mechanical Properties of Materials Synthesized from Graphene and Carbon Nanotubes: Sanjit Bhowmick1; Chandra Tiwarya2; Syed Asif1; Pulickel Ajayan2; 1Hysitron Inc.; 2Rice University
    Nanoengineered 3D materials manufactured from 2D graphene and 1D carbon nanotubes gained visibility in the recent past due to numerous novel applications, primarily in the two most challenges before mankind today, namely, environment and energy. This study focuses on understanding the variation in mechanical properties and deformation mechanism of such 3D materials as a function of temperature. An in-situ nanomechanical instrument, PI 87xR SEM PicoIndenter was used to conduct uniaxial compression of pillar samples that were FIB milled from 3D materials. Materials synthesized from graphene sheets showed completely elastic behavior and brittle fracture at RT. A considerable amount of plasticity was observed at higher temperature. In contrast, carbon nanotube-based bulk materials showed strain softening in loading and significant recovery during unloading both at RT and elevated temperatures.