Advanced Characterization Techniques for Quantifying and Modeling Deformation Mechanisms: Session VI
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Advanced Characterization, Testing, and Simulation Committee, TMS: Shaping and Forming Committee
Program Organizers: Rodney McCabe, Los Alamos National Laboratory; John Carpenter, Los Alamos National Laboratory; Thomas Beiler, Michigan State University; Khalid Hattar, Sandia National Laboratory; Wolfgang Pantleon, DTU; Irene Beyerlein, Los Alamos National Laboratory

Wednesday 2:00 PM
March 1, 2017
Room: 33C
Location: San Diego Convention Ctr

Session Chair: M. Arul Kumar, Los Alamos National Laboratory; Kelvin Xie, Johns Hopkins University

2:00 PM  Invited
Atomic-scale Characterization of Boron Carbide with Advanced TEM and Atom Probe Techniques: Kelvin Xie1; Paul Rottmann2; Luoning Ma2; Kevin Hemker2; 1 Johns Hopkins University; 2Johns Hopkins University
    Boron carbide is an attractive engineering material due to its low density and superior hardness, making it a suitable material for many engineering applications. However, many fundamental aspects of boron carbide still remain elusive (e.g. atomic occupancy, relative building block stability, quasi-plasticity, etc.). In this work, we employed the state-of-art characterization techniques (i.e. atom probe tomography, precession assisted crystal orientation mapping (PACOM) and nano-scale strain mapping) to address the aforementioned challenges. In addition, the roles of planar defects and Si-alloying on boron carbide structural stability and quasi-plasticity will be discussed. The new insights we have gained here can guide the design of tougher boron carbide via atomic-level understanding and engineering.

2:20 PM  
Characterization of the Mechanistic Responses of Three Silicon Carbide Variants to Knoop Indentation by TEM: Scott Walck1; Samuel Hirsch1; Kristopher Behler1; Jerry LaSalvia1; 1U.S. Army Research Laboratory
    Understanding the deformation mechanisms in ceramic materials is crucial for developing improved ceramic materials for lightweight body and vehicle armor systems. To gain insight into the mechanistic response of polycrystalline armor ceramics to large contact stresses, transmission electron microscopy (TEM) was used to examine the cross-sections of the inelastically deformed regions beneath Knoop indents. Because of extensive cracking, a multi-step sample preparation technique was developed to preserve the cross sections intact. The steps included the use of a masked ion milling system, SEM, epoxy vacuum infiltration, and the FIB. Results for three commercially available silicon carbide materials at 1 kgf will be shown. In general, TEM characterization of the inelastically deformed regions showed stacking faults, dislocations, micro-cracking, and macro-cracking. The results depended strongly on the initial microstructure and processing of the material. Experimental procedures, results, and analyses will be presented.

2:40 PM  
Measuring Residual Stresses in Boron Carbide in TEM: Luoning Ma1; Paul Rottmann1; Kelvin Xie1; Kevin Hemker1; 1Johns Hopkins University
    Ceramics are generally brittle and usually exhibit low fracture toughness. One effective way to improve ceramic toughness is by introducing compressive residual stresses. However, to measure residual stresses using conventional techniques such neutron diffraction is time consuming and costly. In this work, we carried out nanoscale strain mapping in TEM by acquiring and comparing nano-beam precession electron diffraction patterns. The effect of grain boundary misorientation, grain boundary morphology, inter-granular second phases and inter-granular second phases on the residual stresses that form upon cooling in a commercial boron carbide was investigated. The new insights we have gained here can be useful for guiding fracture modeling and predicting failure of boron carbide.

3:00 PM  
Investigating the On-set of Amorphization in Single Crystal Boron Carbide: Jonathan Ligda1; Jeffrey Lloyd1; Brian Schuster1; 1Army Research Laboratory
    Amorphization of boron carbide under contact loading has been observed and reported on in multiple studies that subjected the ceramic to extreme stress states. Previously, nanoindentation with sharp indenter tips was used and the degree of amorphization was identified afterward. These sharp tips created complex stress fields in the ceramic, resulting in amorphization at all contact loads. In this work, the amorphization in single crystal boron carbide is impeded by performing nanoindentation with a spherical tip. This tip geometry creates a well-defined transition from elastic to in-elastic deformation in the ceramic, and by performing tests at different maximum loads the initiation of amorphization is controlled. The degree and location of amorphous bands under the indents are characterized by Raman spectroscopy and transmission electron microscopy. These results, combined with finite element modeling of an indent are used to determine the stress state and magnitude needed to initiate amorphization.

3:20 PM Break

3:40 PM  
3D Dislocation Structure Evolution Underneath Indentations in Single Crystalline: Karsten Durst1; 1Technical University Darmstadt
    In the present work, the dislocation structure evolution around and underneath spherical indentations in (001) oriented single crystalline strontium titanate (STO) was studied by using an sequential polishing, etching and imaging technique. Thereby we can quantify the dislocation spacing in the pile-ups and relate this moreover to the frictional stress in the material. Furthermore, we find that in the early stage of plastic deformation, the dislocation pile-ups are all aligned in <100> directions, lying on {110}45 planes, inclined at 45 to the (001) surface. At higher mean contact pressure and larger indentation depth, however, dislocation pile-ups are nucleated along <110> directions appear, lying on {110}90 planes, perpendicular to the (100) surface. Using high resolution EBSD techniques, also the geometrically necessary dislocation density is locally resolved and compared to the dislocation densities as analyzed by etch pit technique.

4:00 PM  
Effect of Indentation Load on Deformation Mechanisms in Boron Carbide: Jerry LaSalvia1; Scott Walck1; Kristopher Behler1; 1U.S. Army Research Laboratory
    To gain insight into the mechanistic response of polycrystalline boron carbide subjected to large contact stresses, transmission electron microscopy (TEM) methods were used to characterize the inelastically deformed regions beneath 0.3, 1, and 2 kgf Knoop indents. TEM specimen preparation involved several steps including masked-ion milling (MIM), epoxy infiltration, and focused-ion beam (FIB) milling. In general, TEM characterization showed extensive stress-induced amorphization, microcracking, and macrocracking. At 2 kgf, comminution was also observed. Stress-induced amorphization occurred predominately in discrete slip bands that were nanoscale in width and microscale in length. Slip band trajectories appeared similar to slip lines for blunt-wedge indentation, indicating the importance of shear stress on their formation. Microcrack initiation was observed at slip band intersections. Comminution appeared to be the consequence of the coalescence of both slip band intersections and microcracks. Experimental procedures and results will be presented.

4:20 PM  
From Micro-Cantilever Testing to Deformation Patterning in Hexagonal Polycrystals: Jicheng Gong1; Rajesh Korla2; Mitchell Cuddihy3; T Ben Britton3; Fionn Dunne3; Angus Wilkinson1; 1University of Oxford; 2Indian Institute of Technology - Hyderabad; 3Imperial College London
    We have been trying to better understand and model the heterogeneous patterns of stress, strain and dislocation density that develop during deformation of HCP polycrystals. Micro-scale cantilever bend tests are used to probe the considerable anisotropy of elastic and plastic deformation behaviour in these HCP metals. It is necessary to understand and account for the considerable size effect on strength in such tests so as to extract valid critical resolved shear stress (CRSS) values for each of the important slip systems which are required inputs for constitutive laws in crystal plasticity finite element analysis (CP-FEA). Using data from Zr it will be demonstrated that this approach generates CRSS values which allow the bulk flow stresses of macroscopic polycrystal aggregates to be captured by CP-FEA simulation with no additional fitting parameters. At the microscale deformation patterning in the CP-FEA simulations is benchmarked using HR-EBSD analysis of deformed polycrystals.

4:40 PM  
Influence of Elastic Anisotropy and Local Texture on the Onset of Plastic Slip in Ti-6Al-4V: Samuel Hemery1; Patrick Villechaise2; Loc Signor1; 1ENSMA; 2CNRS
    The low cycle fatigue life of titanium alloys highly depends upon the crack initiation stage which is generally controlled by slip activity in the alpha phase. Prior works highlighted a preferential fatigue crack initiation in [0001] oriented regions. In order to characterize the onset of slip activity at the local scale, in-situ SEM tensile testing was applied to polycrystalline specimens. Several regions of interest with different crystallographic textures assessed using the EBSD technique have been studied. Load partitioning is evidenced at the macrozone scale, thus leading to a heterogeneous deformation. A comparison with single colony experiments is presented to highlight the influence of the elastic anisotropy of the alpha phase on stress heterogeneity according to grain neighborhood. Estimation of critical resolved shear stress from data obtained with polycrystalline specimens is finally discussed and a correction procedure is proposed.

5:00 PM  
Modeling the Evolution of Slip System Strength in α–Phase Ti-7Al Using High-Energy Diffraction Microscopy Data: Darren Pagan1; Nathan Barton1; Paul Shade2; Joel Bernier1; 1Lawrence Livermore National Laboratory; 2Air Force Research Laboratory
    New high-energy X-ray diffraction techniques capable of measuring the elastic strain in hundreds of grains simultaneously have promised to change how micromechanical models are calibrated and advanced. In practice, using such large data sets for model development has proven to be difficult. Establishing new methods to link high-energy diffraction microscopy (HEDM) data and micromechanical models is crucial for optimal use of these new data. A method to extract the evolution of slip system strengths from different families of slip systems from HEDM data is presented. The method is used to quantify the evolution of slip system strengths in α–Phase Ti-7Al. The slip system strength data is then used to inform a micromechanical strength model. Finite element crystal plasticity results show the importance of accurate slip system strength values when attempting to capture individual grain stress behavior in Ti-7Al.