Deformation and Transitions at Interfaces : Meso/Microstructural Scale Mechanical Behavior of Polycrystals II
Sponsored by: TMS Functional Materials Division (formerly EMPMD), TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Computational Materials Science and Engineering Committee, TMS: Mechanical Behavior of Materials Committee, TMS: Thin Films and Interfaces Committee
Program Organizers: Saryu Fensin, Los Alamos National Laboratory; Thomas Bieler, Michigan State University; Rozaliya Barabash, OakRidge National Lab; Shen Dillon, Universe of Illinois; Jian Luo, University of California, San Diego; Doug Spearot, University of Florida

Wednesday 8:30 AM
March 1, 2017
Room: 23B
Location: San Diego Convention Ctr

Session Chair: Thomas Bieler, Michigan State University

8:30 AM  Invited
Accounting for the Micromechanical Effect of Grain Boundaries Using a New FFT-based Strain-gradient Polycrystal Plasticity Formulation: Ricardo Lebensohn1; Alan Needleman2; 1Los Alamos National Laboratory; 2Texas A&M University
    In this work we present a novel implementation of Gurtin's strain-gradient polycrystal plasticity theory to study the micromechanical effect of grain boundaries (GBs). Bittencourt et al's (2003) non-local formulation has been incorporated in Lebensohn et al's (2012) EVP-FFT algorithm. Numerical procedures for the accurate estimation of higher-order derivatives of micromechanical fields, required for feed-back into single crystal constitutive relations, are identified and applied. A simple case of a periodic laminate is used to assess the numerical stability of the algorithm and to study the influence of the different model parameters. The new formulation is next applied to the case of a 3-D fcc polycrystal, accounting for non-local effects associated with GBs and illustrating the possibilities offered by the proposed numerical scheme to accurately solve large problems in reasonable computing times.

8:50 AM  
Investigating Deformation at Grain Boundaries by SEM-DIC: Zhe Chen1; Samantha Daly1; 1University of Michigan
    In this study, the microstructure and microscale deformation behavior of polycrystalline materials was characterized using an experimental approach combining several techniques. The evolution of full-field surface deformations was continuously tracked during in-situ mechanical loading, and the displacement and strain fields were obtained by scanning electron microscopy combined with distortion-corrected digital image correlation (SEM-DIC). Electron backscattered diffraction (EBSD) was used to characterize the undeformed microstructure, and combined with femtosecond laser ablation assisted serial sectioning to reconstruct the 3D microstructure from deformed samples. Localized deformation including slip and twinning were identified in the high-resolution strain maps, and subsequently correlated with the active deformation modes through quantitative analysis of the displacement and strain fields. The activity of slip and twinning, and their interaction at grain boundaries will be discussed. The experimental results will be compared with a crystal plasticity finite element model developed in an integrated computational materials engineering (ICME) framework.

9:10 AM  Invited
Residual Stress and Dislocation Density Distributions near Grain Boundaries in Deformed Materials: Angus Wilkinson1; Jun Jiang2; T Ben Britton2; David Wallis1; Lars Hansen1; 1University of Oxford; 2Imperial College London
    Cross-correlation-based analysis of EBSD patterns allows variations in the elastic strain (hence stress) and lattice rotation tensors to be mapped at high spatial resolution in the SEM. Furthermore the lattice rotation gradients can be used to determine lower bound estimates of the geometrically necessary dislocation density. The datasets produced are well suited to exploring the accumulation of high stress and/or dislocation densities within a microstructure and plots showing their correlation with positional metrics such as distance to the nearest grain boundary or triple junction will be presented. Comparison will be made between datasets obtained from cubic metals eg fcc Cu deformed at room temperature and Si deformed at elevated temperatures. The effects of increased anisotropy as a consequence of lower symmetry will be explored in the hcp metals Ti and Zr and the orthorhombic mineral olivine.

9:30 AM  
Role of Grain Boundary Sliding in Deformation of Polycrystalline Materials: Ajey Venkataraman1; Marissa Linne2; Samantha Daly3; Michael Sangid1; 1Purdue University; 2University of Michigan; 3University of California, Santa Barbara
    Grain boundary sliding (GBS) is an important deformation mechanism that is known to be activated at high temperatures, low strain rates and small grain sizes. While crystalline slip has been extensively studied, GBS is relatively less understood, and has been mostly restricted to nanocrystalline materials and high-temperature loading. In this study, crystalline slip and GBS are modeled as coupled deformation mechanisms using crystal plasticity simulations within a finite element framework. Distinct flow and hardening laws are assigned to the grain “core” and “mantle” in FCC materials. The grain core accommodates dislocation glide and the mantle accommodates sliding in addition to glide. The constitutive response for our model is compared to full-field strain response experimentally obtained at the microscale-level. The effects of grain mantle width and grain sizes are studied. These results will help improve the sophistication and accuracy of deformation modeling and significantly advance the fundamental knowledge of material behavior.

9:50 AM  Invited
Crystallographic Rotation, Deformation, and Damage: Jay Carroll1; Hojun Lim1; Brad Boyce1; Corbett Battaile1; Blythe Clark1; 1Sandia National Laboratories
    Validation studies of crystal plasticity finite element models (CP-FEM) typically compare model predictions of strain fields with experimental measurements. Understanding grain rotations and their relationship to local deformation is necessary for trustworthy strain field predictions as well as deformed texture predictions from CP-FEM models. This work presents experimental measurements of grain rotations of single crystal Ta specimens due to applied deformation. These measurements, made by repeated electron backscatter diffraction (EBSD) measurements, are compared to predictions from a BCC crystal plasticity model. Multicrystal specimens are also used to validate CP-FEM predictions of both grain rotations and strain by comparisons to experimental strain measurements from high resolution digital image correlation and EBSD measurements. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

10:10 AM Break

10:30 AM  Invited
Statistical Analysis of Grain Boundary Structure-Property Relationships: Srikanth Patala1; 1North Carolina State University
    Grain boundaries influence a wide array of physical properties in polycrystalline materials and play an important role in governing microstructural evolution under extreme environments. While the importance of interfaces is well documented, their properties are among the least understood of all the defect types present in engineering material systems. This is due to the vast configurational space of interfaces, resulting in a diverse range of structures and properties. In the talk, I will present statistical tools (two-point correlation functions and dimensionality reduction techniques) for constructing reliable reduced-order models for energies as a function of the complete crystallographic degrees of freedom of interfaces. These techniques are expected to play an important role in the analysis of grain boundary structure-property relationships as they may be extended to the quantification of complex properties, such as diffusivity, conductivity, corrosion resistance, and defect-interface interactions.

10:50 AM  Invited
A Non-local Continuum Mechanics Treatment of the Dynamics of Interfaces: Laurent Capolungo1; 1Los Alamos National Laboratory
    While coarse graining techniques have largely improved over the past decades, the question of the treatment of material interfaces remains a critical challenge. The work presented proposes to use field defect mechanics [1] in addition to a non-local treatment of elastic behavior, in the lineage of Eringen, in order to model the dynamic evolution of material interfaces at the nano-scale. In this approach, material interfaces are represented by their interfacial defect content, formally represented with dislocation and/or disclination fields. Near interfacial defect, the local elastic behavior of the material is seen to depart from that of the macroscopic one thereby providing a driving force for both stabilization and migration of interfacial defects. The mechanical framework is applied to both the cases of bicrystals and polycrystals. It is shown that the model proposed can successfully reproduce key features of dislocation/grain boundary interactions, of grain boundary migration etc.

11:10 AM  Invited
Nanoscale Strain Mapping at Interfaces Using Scanning Nanobeam Electron Diffraction: Andrew Minor1; 1UC Berkeley & LBL
    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. Our method of local strain mapping consists of recording large multidimensional data sets of nanodiffraction patterns from which strain maps can be extrapolated with nanometer resolution from complex microstructures. Here we will highlight our latest results from in situ strain mapping around interfaces such as grain boundaries in steel, ferroelectric domains in thin films and nanowires, and precipitates in Ti alloys.

11:30 AM  
Polycrystalline Plasticity Simulations with Anisotropic Discrete Dislocation Dynamics: John Graham1; Anthony Rollett2; Richard LeSar1; 1Iowa State University; 2Carnegie Mellon University
    Polycrystalline materials make up the majority of the materials used in the world, and the ability to determine their mechanical properties is of great interest. Much work has been done in developing methods to determine these properties through computer simulation methods. We present a polycrystalline plasticity simulation approach based on a fast Fourier transform dislocation dynamics (FFTDD) method. In FFTDD, the dislocations are represented as a discontinuity in the plastic distortion tensor, which is a direct measure of the slip in the lattice caused by the dislocations. The stresses arising from the system of dislocations are determined within an FFT formalism. From this, the stresses on each dislocation are calculated and the dislocations are evolved within the polycrystalline system. We will review the method and discuss applications.

11:50 AM  Invited
Quantification of Dislocation Behavior and Deformation Twinning at High Strain Rates: Mitra Taheri1; Shang-Hao Huang1; Evan Kahl1; Asher Leff1; Christopher Barr1; Logan Shanahan1; JP Liu2; Yong Zhang2; Leslie Lamberson1; 1Drexel University; 2University of Science & Technology Beijing,
    The mechanism of and transition between slip and twinning is well-studied at “conventional” strain rates. At high strain rates, however, dislocation nucleation occurs more readily and it is not clear how these increased nucleation phenomena alter the development of twinned microstructures. While a wealth of information is available for high strain rate deformation of FCC, HCP, and some BCC materials, specific details regarding dislocation substructures and their association with slip to twin transitions are not clear. This presentation will highlight experiments, using correlated Kolsky and multiscale microscopy techniques, that attempt to pinpoint the transition between slip and twinning at high strain rates in stainless steels and high entropy alloys. The transition will be discussed as a function of the nucleation, density, and propagation of dislocations (and dislocation substructures), and microstructural features such as orientation, chemistry, and stacking fault energy.

12:10 PM  Invited
The Effect of Microstructure on Strain Localisation in Two-phase Ti-alloys: Michael Preuss1; David Lunt1; Joao Quinta da Fonseca1; 1University of Manchester
     Microstructural effects on mechanical properties are particularly pronounced in Ti-alloys due to the limited number of easy slip systems in the alpha phase. Of fundamental importance here is the effect of the microstructure on strain localisation. For such comparison, high-resolution digital image correlation (HR-DIC) has been applied to monitor 2D deformation patterns. This technique now enables one to record strain maps with spatial resolutions better than 100 nm in combination with high strain resolution and across hundreds of grains enabling statistical analysis of strain localisation.In the present paper we will use this experimental methodology to identify the role of various microstructure constituents in Ti-6Al-4V on strain localisation. In addition, the combination of grain orientation mapping based on EBSD and HR-DIC analysis has enabled us to compare shear strain contribution from the various slip systems during plastic deformation, which will be discussed in the presentation.