Advanced Characterization Techniques for Quantifying and Modeling Deformation Mechanisms: Session I
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

Monday 8:30 AM
February 27, 2017
Room: 33C
Location: San Diego Convention Ctr

Session Chair: David Collins, University of Oxford; Ricardo Lebensohn, Los Alamos National Laboratory

8:30 AM  Invited
Recent Applications of Micromechanical Modeling Directly Coupled with Advanced Characterization Techniques of Polycrystalline Materials: Ricardo Lebensohn1; Reeju Pokharel1; 1Los Alamos National Laboratory
    We report our most recent advances in micromechanical modelling of heterogeneous materials, using direct input from voxelized microstructural images of polycrystalline aggregates. New Fast Fourier Transform (FFT)-based algorithms for complex constitutive behaviors of plastically deforming materials, including dilatational plasticity, non-local plasticity, and dynamic plasticity with micro-inertial effects, combined with EBSD, X-Ray Computed Tomography (CT) and High Energy Diffraction Microscopy (HEDM) will be presented and discussed.

8:50 AM  
Effects of Crystallographic Structure on Damage Evolution Using Diffraction-amalgamated Grain-boundary Tracking Technique: Kyosuke Hirayama1; Hiroyuki Toda1; Teruyuki Shimoji1; Yasuto Tanabe1; Kentaro Uesugi2; Akihisa Takeuchi2; 1Kyushu University; 2Japan Synchrotron Radiation Research Institute
    The behavior of individual grain during plastic deformation is affected by shape and size of grains and the crystal orientations for the grains in polycrystalline. We proposed new diffraction-amalgamated grain-boundary tracking (DAGT) technique, which was developed by combining the grain boundary tracking (GBT) technique and a pencil beam XRD. The method provides a description of the crystallographic orientations of individual grains in polycrystalline material during deformation by 4D (3D + time). In addition, the crystal orientation of each grain in plastic deformed material with high strain rate over 20% is determined by means of this technique. Therefore we conducted the assessment of the void growth during deformation in polycrystalline aluminum alloy based on determination of crystal orientation before and after deformation for each grain by using DAGT technique.

9:10 AM  
A Correlation between Digital Image Correlation and Grain Misorientation Distribution Mapping to Capture the Localized Plastic Deformation in a Polycrystalline Titanium Alloy: Vahid Khademi1; Carl Boehlert1; Thomas Bieler1; Masahiko Ikeda2; Samantha Daly3; Zhe Chen3; 1Michigan State University; 2Kansai University; 3University of California Santa Barbara
    It is well known that the elastic and plastic response of polycrystalline titanium alloys to applied load is heterogeneous at the microscale. Understanding the details of heterogeneous plastic deformation is essential for identifying the failure mechanisms of a material. In this study, in-situ SEM tensile testing was performed in conjunction with Digital Image Correlation (DIC) strain mapping and electron backscatter diffraction (EBSD) on a solution treated rolled sheet of a beta Ti-13Cr-1Fe-3Al (wt.%) alloy. The sample was loaded up to ~10% plastic strain in nine steps. Four metrics were examined to generate misorientation distribution maps. The results indicated the best match between the DIC map and the misorientation distribution map were obtained when the chosen reference orientation was the average orientation of the undeformed parent grain. In most of the grains, a high local misorientation gradient was correlated with strain concentration in DIC maps.

9:30 AM  
Mapping the Deformation of Brazed Joints in Ti-6Al-4V Specimens Using High Angular Resolution Electron Backscatter Diffraction (HR-EBSD) and High Spatial Resolution Digital Image Correlation (HR-DIC): Jun Jiang1; Yongjuan Jing2; Ben Britton1; 1Imperial College London; 2Beijing Research Institute of Aviation Engineering
    New brazing technique enable aero-engine materials Ti-6Al-4V to be joined, increasing in strength and removing the heat affecting zone. The performance of the brazed zone should be evaluated, particularly as there are strong variations in local microstructure associated with brazing. In this study deformation mechanisms near the interface of joint and matrix were revealed by coupling in-situ HR-DIC and HR-EBSD techniques, enabling the activity of geometrically dislocation density, residual stress and plastic strain to be captured with high fidelity.

9:50 AM Break

10:10 AM  
Effect of Strain and Stress Holds on Deformation in Ti-6Al-4V: Microscale Evidence of Load Shedding: David Collins1; Hamidreza Abdolvand2; Zhen Zhang3; Fionn Dunne3; Angus Wilkinson1; 1University of Oxford; 2Western University; 3Imperial College London
    Many Ti alloys, including Ti-6Al-4V, exhibit a marked decrease in fatigue life at room temperatures when a hold period is introduced at the maximum of the loading cycle in what is termed cold-dwell fatigue. Modelling has indicated that load shedding from soft grains to neighbouring hard grains is a crucial part of cold dwell fatigue. Experimental evidence at the microscale is however lacking and to address this Ti-6al-4V samples deformed in simple load-unload and load-hold-unload tests have been compared. Pre- and post- test characterisation using high-resolution electron backscatter diffraction of the residual intragranular stress variations and geometrically necessary dislocation densities are quantified within individual crystals. Differences in patterns of stress and dislocation density distributions for soft and hard grain pairs in samples deformed with and without a hold period will be presented. These experimental results will be compared to crystal-plasticity finite element simulations that capture the time dependent plasticity.

10:30 AM  
Analysis of Strain Localization During Creep of a Polycrystalline Superalloy Using SEM-DIC: Connor Slone1; Michael Mills1; 1The Ohio State University
    High-temperature deformation of superalloys is a complicated process involving many possible mechanisms that depend on temperature, applied load, and microstructure. Quantifying and modeling deformation is particularly difficult in components with inhomogeneous microstructures such as variations in precipitate size; therefore, measurement of local deformation is essential to developing a physics-based understanding of their mechanical behavior. In this work, scanning electron microscopy digital image correlation (SEM-DIC) is employed with nanoscale hafnia speckles to measure local plastic deformation during creep with spatial resolution of ~1μm. This technique, in conjunction with measurements of local elastic deformation via HR-EBSD, demonstrates local strain accumulation around specific microstructural features like precipitate-coarsened zones. Regions with intense local deformation are removed via focused-ion beam and analyzed with transmission electron microscopy, which reveals the physical nature of the defect content associated with strain accumulation. These experiments directly link quantitative local measurements of strain to underlying physical mechanisms.

10:50 AM  
High Resolution Strain Measurements in a Polycrystalline Superalloy during Deformation at Intermediate Temperature: J.C. Stinville1; M.P. Echlin1; W.C. Lenthe1; F. Bridier2; M. Soare3; S. Ismonov3; P. Bocher4; T.M. Pollock1; 1University of California Santa Barbara; 2DCNS Research; 3GE Global Research; 4Department of Mechanical Engineering, École de Technologie Supérieure, Montréal, Canada
    Damage mechanisms in polycrystalline metallic alloys involve the accumulation of plastic strain at the sub-grain level. Measurements of the heterogeneous strain field that results from the complex grain boundary network present in engineering alloys are combined with microstructural and orientation information to determine the deformation mechanisms present at intermediate temperatures. Digital image correlation (DIC) has been extended to higher resolution to measure in-plane strains at the sub-grain scale and below, where plastic strain localization can be directly correlated with physical slip bands. Strain fields were measured in a René 88DT polycrystalline nickel-base superalloy to assess the deformation processes under monotonic and cyclic loading at room and intermediate temperature. Significant temperature effects were observed in the local strain field and are related to a change in deformation mechanisms. A new comparative approach is presented to correlate microstructure sensitive modeling results from crystal plasticity with high-resolution DIC measurements.

11:10 AM  
Understanding the Role of Competing Slip Systems during Formation of Stress Hotspots in Hexagonal Close Packed (HCP) Materials: Ankita Mangal1; Elizabeth Holm1; 1Carnegie Mellon University
    Stress hotspots are regions of high stresses in a microstructure, and are correlated with nucleation of damage such as voids in ductile fracture. They tend to form near microstructural features, such as grain boundaries, triple and quadruple junctions and usually form in textures corresponding to maxima in the Taylor factor for a given loading condition. However, in HCP materials, due to competition between slip systems, this trend can reverse. We simulate deformation in an uniaxial tensile test in HCP polycrystals with equiaxed grain structures using Viscoplastic Fast Fourier Transform (VPFFT) code. The hotspots locations and relative slip activities are studied to understand correlations between their formations for different textures. This facilitates creation of feature vectors for a supervised machine-learning algorithm, which predicts the location of stress hotspots. The results show the power and limitations of the machine learning approach applied to the polycrystalline grain network.

11:30 AM  
In-situ Neutron Diffraction Studies of Load Path Changes in 316L and AZ31: Tobias Panzner1; Tram Trang1; Karl Sofinowski2; Steven Van Petegem1; Manas Upadhyay1; Helena Van Swygenhoven2; 1Paul Scherrer Institut; 2Paul Scherrer Institute & EPFL
    A multiaxial deformation setup for insitu mechanical deformation studies using neutron diffraction [Acta Mat 105 (2016)404] is used to study load path changes. The biaxial rig implemented in the time of flight neutron diffraction beamline POLDI of the Swiss neutron source uses cruciform samples that can be deformed in-plane in free chosen force combinations up to the limits of 50 kN and 100 kN of the 2 loading axis. The macroscopic strain is measured contact free by a 3D-DIC system. The influence of the cruciform geometries on the evolution of the stresses in the gauge area is discussed and backed up with finite element simulations. The evolution of inter- and intra-granular strains during various load path changes is studied by following the lattice strains and the peak broadening for 316L stainless steel and AZ31. The results are discussed in terms of the microstructural evolution, complementary analyzed by EBSD.

11:50 AM  
A Multi-scale FE-FFT Approach to Study Lattice Strain Evolution during Biaxial Strain Path Changes: Manas Upadhyay1; Anirban Patra2; Wei Wen2; Steven Van Petegem1; Tobias Panzner1; Ricardo Lebensohn2; Carlos Tome2; Helena Van Swygenhoven3; 1Paul Scherrer Institute; 2Los Alamos National Laboratory; 3Paul Scherrer Institute & EPFL
    A multi-scale elastic-viscoplastic finite element (FE) and fast Fourier transform (FFT) approach is proposed to study lattice strain evolution during biaxial strain path changes of cruciform shaped samples of 316L SS. At each material point, the microscale viscoplastic self-consistent model is invoked to solve the local equilibrium problem. The model uses a dislocation density based hardening law designed to appropriately capture the stress state during strain path changes. The FE predicted stresses are used as macroscopic boundary conditions to drive the mesoscale elasto-viscoplastic FFT model. The FFT model uses the same dislocation density based hardening law. Results show that this FE-FFT approach captures the trends in lattice strain evolution for different biaxial strain path changes. A quantitative analysis of 200 grain family is performed to explain the differences in magnitudes of their lattice strain evolution as a function of the strain path.