Advanced Thermo-mechanical Characterization of Materials with Special Emphasis on In Situ Techniques: In Situ Techniques I
Sponsored by: TMS Structural Materials Division, TMS: Advanced Characterization, Testing, and Simulation Committee, TMS: Nanomechanical Materials Behavior Committee, TMS: Thin Films and Interfaces Committee
Program Organizers: Amit Pandey, LG Fuel Cell Systems Inc.; Sanjit Bhowmick, Hysitron; Jeff Wheeler, ETH Zurich; María Teresa Pérez Prado, IMDEA Materials Institute; Robert Wheeler, MicroTesting Solutions LLC; Josh Kacher, Georgia Tech

Monday 8:30 AM
February 27, 2017
Room: 32A
Location: San Diego Convention Ctr

Session Chair: Sanjit Bhowmick, Hysitron, Inc.; Josh Kacher, Gatech

8:30 AM  Keynote
Local Strains and Crack Initiation in Lamellar Gamma-TiAl: Thomas Edwards1; Fabio Di Gioacchino1; Rocio Munoz-Moreno1; Mark Dixon2; Nigel Martin2; William Clegg1; 1University of Cambridge; 2Rolls-Royce plc
    gamma-TiAl has insufficient slip systems to undergo a general shape change at a lamellar boundary. Deformation is therefore associated with elastic misfit strains, causing cracks to form, which may then grow by fatigue. Minimising the formation of such flaws requires a quantitative understanding of local deformation. This has been studied using in-situ compression of micropillars of a Ti-45Al-2Nb-2Mn-0.8vol% TiB2 alloy. Local strains at a sub-micron rscale were determined by digital image correlation using a Pt speckle pattern. It is shown such materials deform by twinning parallel to the lamellar boundary at a resolved shear stress of approximately 100 MPa. However the twins formed in the alloy here were not the fine twins, 10-200 nm in thickness, typical of polysynthetically twinned crystals. Instead, they nucleated at an interface and then spread across the whole lamella. The reasons for this behavior and the implications for flaw nucleation are described.

9:05 AM  
In Situ TEM Imaging of Defects in Metallic Samples Deforming at High Strain Rates: Thomas Voisin1; Michael Grapes1; Yong Zhang1; Nicholas Lorenzo2; Jonathan Ligda2; Brian Schuster2; Tian Li3; Melissa Santala3; Geoffrey Campbell3; Timothy Weihs1; 1Johns Hopkins University; 2Army Research Laboratory; 3Lawrence Livermore National Laboratory
    To accurately understand and predict the dynamic properties of metals deforming at high strain rate, one can benefit by observing directly defect motion such as dislocations slip and twinning. In this presentation we describe a novel TEM technique that enables one to image the motion, the evolution and the interaction of defects within metallic specimens deforming at high strain rates. The Dynamic TEM at the Lawrence Livermore National Laboratory was used to record 9-frames movies with a delay between frames as short as 50ns while a novel TEM straining stage loads specimens in tension at strain rates as high as 4x10^3/s. The specimens are prepared from bulk samples using femtosecond laser machining and ion milling. Our most recent DTEM observations will be presented and they will be compared with results from in situ straining experiments performed at quasi-static strain rates in a conventional TEM.

9:25 AM  
Investigating Grain Rotations in Ultrafine-grained Aluminum Films Using In Situ TEM Straining with Automated Crystal Orientation Mapping: Ehsan Izadi1; Amith Darbal2; Rohit Sarkar1; Jagannathan Rajagopalan1; 1Arizona State University; 2AppFive LLC.
    Automated crystal orientation mapping in TEM (ACOM-TEM) is highly suitable to obtain statistically meaningful information about microstructural changes in ultrafine-grained and nanocrystalline metals. We used a custom MEMS device to perform in situ ACOM-TEM tensile experiments on a non-textured, ultrafine-grained aluminum film and tracked orientation changes in hundreds of grains to correlate the macroscopic behavior of the film with its microstructural evolution. Our results show extensive orientation changes in a majority of grains during loading. The rotations are partially/fully reversible in a significant fraction of grains during unloading, leading to notable inelastic strain recovery. Surprisingly, a small fraction of grains continue to rotate in the same direction during unloading, even when the applied stress has been reduced significantly. The ACOM-TEM measurements also provide evidence of reversible/irreversible grain boundary migration. The microstructural observations point to a highly inhomogeneous and constantly evolving stress distribution in the film during both loading and unloading.

9:45 AM  Invited
A Greater Understanding of Deformation in BCC Nanocrystalline Metals Using Quantitative In Situ TEM Techniques: Mitra Taheri1; Gregory Vetterick1; Asher Leff1; M Marshall2; Khalid Hattar2; J. Kevin Baldwin3; Amit Misra4; 1Drexel University; 2Sandia National Laboratories; 3Los Alamos National Laboratory; 4University of Michigan
    Through the use of coupled in-situ TEM tensile testing and quantitative dislocation density analysis via precession electron diffraction, a study of deformation in nanocrystalline iron films was performed. Deformation at the crack tip was accommodated by dislocation motion, grain rotation, and grain growth, however twinning was not observed. Interestingly, the concurrent nature of the grain rotation and dislocation motion indicates that grain rotation occurs at fairly large grain sizes and there is no sharp transition from dislocation-mediated to grain boundary sliding mechanisms as grain size is decreased in BCC iron. These results have implications for understanding deformation modes in nanocrystalline materials at the dislocation level, and in particular, give new and additional insight into the deformation of BCC metals.

10:10 AM Break

10:30 AM  
In Situ TEM Study of Atomic Level Phase Transformation in Cerium-based Oxides during Redox Processes: Ruigang Wang1; 1The University of Alabama
    Ceria (CeO2) and cerium-based oxides can release lattice oxygen under oxygen lean condition and storage oxygen under oxygen rich condition. This property, characterized as the oxygen storage capacity (OSC) or redox functionality, has made it a material of considerable interest in applications such as vehicle three-way exhaust clean-up, water-gas shift reactions, gas sensors, carbon dioxide capture, and fuel cell electrodes. In this talk, I will present some recent progress on understanding atomic-level redox behavior of cerium-based oxides using environmental transmission electron microscopy (E-TEM), and on the synthesis and characterization of shape/crystal structure-controlled CeO2 and CeO2-ZrO2 for designing new automotive catalytic converter washcoat and catalyst supporting materials.

10:50 AM  
A New Designed Rig for In Situ Neutron Diffraction Creep Experiments under Different Boundary Conditions: Yiqiang Wang1; Saurabh Kabra2; Shuyan Zhang2; Sayeed Hossain1; David Smith1; 1University of Bristol; 2ISIS, Science and Technology Facilities Council
    The creep often occurs differently along different crystalline orientations in polycrystalline engineering alloys, therefore generated internal stress between different grain orientations. However, it is not well understand how this occurs under different boundary conditions including constant load, constant strain and elastic follow-up control. This work describes a new designed rig for conducting stress relaxation as well as elastic follow-up creep experiments on 316H stainless steel in the ENGIN-X neutron beam line. Three short term measurement results show that the internal stresses between different grain families were found to be constant during creep tests irrespective of different boundary conditions. Elastic follow-up has no effect on the redistribution of lattice strains. The successful of this commissioning give us strong evidence that the long term creep experiments in a neutron diffraction beam line can be achieve in the near future.

11:10 AM  
Progress in In-situ Testing in the Electron Microscope at Cryogenic Temperatures: Jeff Wheeler1; 1ETH Zurich
    High temperature nanomechanical testing is now becoming an established technique. This enables interrogation of the deformation behavior of small volumes of materials under service conditions for many demanding applications. However, for many materials, lower temperatures may be of more interest due to ductile-brittle transitions or rapid grain growth at elevated temperatures. Here, we present the progress in developing a testing system which enables testing at near cryogenic temperatures (-150 °C) in situ in a high resolution scanning electron microscope.

11:30 AM  
Dislocation Drag Coefficient Measurements via In-situ Micropillar Compression Experiments: Tommaso Giovannini1; Finn Giuliani1; Daniel Balint1; Ayan Bhowmik1; 1Imperial College London
    The dislocation drag coefficient is a quantity that provides a quantitative measure of dislocation mobility in crystalline materials. It is a key input to dislocation dynamics models, which are used for making predictions on the microstructural mechanisms that govern deformation. Molecular dynamics simulation is the most common method of estimating the drag coefficient in many materials, as experimental attempts are difficult to perform on individual defects. Moreover, experimental results often show considerable difference with computational estimates. A method is proposed in which the drag coefficient, in an oriented zinc single crystal, is measured during an in-situ micropillar compression experiment. Slip is activated on a single system by orienting the single crystal substrate to provide a favourable Schmid factor configuration. By measuring the slip velocity corresponding to different resolved shear stresses, it is then possible to measure the drag coefficient directly as the gradient of a linear fit to the data.

11:50 AM  
Unusual Brittle to Ductile Transition in Single Crystalline Silicon: In Situ Micro-scale Fracture Studies at Elevated Temperature: Nagamani Jaya Balila1; Jeffrey Wheeler2; Juri Wehrs3; James Best3; Johannes Michler3; Christoph Kirchlechner1; Gerhard Dehm1; 1MPIE GmbH; 2ETH Zurich; 3EMPA-Swiss Federal Laboratories for Materials Science and Technology
    The micro-mechanical fracture behavior of single crystalline silicon was investigated as a function of temperature in situ in the scanning electron microscope equipped with a nanoindenter. A smooth increase in KIC was observed with increasing temperature, in contrast to the sharp transitions observed in previous works. No clear brittle to ductile transition temperature (BDTT) could be identified, but a clear change in cracking mechanism occurs between ~300-400 °C with multiple load drops accompanying the deformation. This demonstrates the influence of small scale plasticity on fracture behavior at temperatures significantly lower than the BDTT of macro-Si. Although the increase in initiation toughness is marginal between 150 and 300 °C, the damage tolerance is improved significantly due to crack branching. This confirms that specimen size affects the BDT in SC silicon.