Deformation and Transitions at Interfaces : Defects/Grain Boundary Interactions
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

Monday 2:00 PM
February 27, 2017
Room: 23B
Location: San Diego Convention Ctr

Session Chair: Remi Dingreville, PO box 5800

2:00 PM  Invited
A Concurrent Atomistic-continuum Study of Sequential Slip Transfer of Curved Dislocations across Grain Boundaries: Shuozhi Xu1; David McDowell1; Liming Xiong2; Youping Chen3; 1Georgia Institute of Technology; 2Iowa State University; 3University of Florida
    Sequential slip transfer across grain boundaries (GB) plays an important role in grain size- and grain boundary character-dependent plastic deformation in polycrystalline metals. Slip transfer is inherently multiscale since both the atomic scale structure of the interface and the long range fields of dislocation pile-ups come into play. In this work, large scale concurrent atomistic-continuum (CAC) simulations are performed to address the slip transfer of dislocation pile-ups across symmetric and asymmetric tilt GBs (CSL and more general) in fcc Ni. The CAC method is able to describe the interface reactions with fully-resolved atomistics while preserving the net Burgers vector and associated long range stress fields of curved dislocations. Emphasis is placed in exploring the role of specific GB structures and dislocation character in interface absorption-desorption reactions, including evolution of the structure of the interface through multiple dislocation reactions.

2:20 PM  Invited
Investigation of Slip Transfer across Grain Boundaries with Application to Cold Dwell Facet Fatigue: Zebang Zheng1; Daniel Balint1; Fionn Dunne1; 1Imperial College
    Atomistic simulation is utilised to calculate GB energies with respect to misorientation angles which are then utilised within thermally-activated discrete dislocation plasticity to address slip transfer across GBs in Ti 6242 alloy. It is shown that the stress distribution within the classical dwell fatigue soft-hard grain combination, which is argued to be crucial in facet crack nucleation, changes markedly during the load dwell, leading to much higher basal plane stresses because of stress redistribution, or load shedding. The phenomenon is shown not to diminish when slip penetration is permitted, but in fact, to the contrary, pushes up further the GB stresses within the hard grain. The mechanistic basis of load shedding in titanium alloys is argued to be the time constant associated with thermally activated dislocation escape; the dislocation-grain boundary penetration plays a significant role, but is shown to be less important than the thermal activation processes.

2:40 PM  
Atomistic Simulation Algorithm for Studying Dislocation Glide Loop – Grain Boundary Interactions in Aluminum: Khanh Dang1; Laurent Capolungo2; Douglas Spearot1; 1University of Florida; 2Los Alamos National Laboratory
    To study the fundamental nature of dislocation glide loop – grain boundary interactions, atomistic simulations are commonly employed; however, there are limitations to current approaches concerning the generation and propagation of dislocation glide loops. The goal of this work is to develop an atomistic simulation framework for studying dislocation – grain boundary interactions that allows specific dislocation glide loops to be driven into grain boundaries at realistic velocities. First, a bisection algorithm combined with energy minimization methods is used to determine the stabilizing stress of dislocation glide loops as a function of loop radius. Second, the bisection algorithm is extended to molecular dynamics and used to determine the minimum required shear stress to expand dislocation loops at different temperatures. Third, dislocation glide loops are driven into different symmetric tilt grain boundaries. The results from this analysis provide insights on the mechanisms of dislocation glide loop – grain boundary interactions.

3:00 PM  
A Micro-Compression Test Study of Grain Boundary Sliding: Jicheng Gong1; Angus Wilkinson1; 1University of Oxford
    Grain boundary sliding is an important deformation mechanism that contributes to creep and superplastic forming. In tin-based lead-free solders grain boundary sliding can make significant contributions to in service performance. Single crystal and bi-crystal micro-compression pillars were machined from a large grained commercially pure Sn polycrystal using a focused ion beam. For the bi-crystals the grain boundaries were oriented at 45° to the compression axis. Multiple pillars of different widths from 5 µm to 0.5 µm wide were tested. The boundary sliding deformation was more obvious for smaller sample cross-sections and made a larger contribution to the overall deformation. There was a significant size effect in which the resistance to grain boundary sliding increased as the sample size was reduced, however, the size effect for dislocation mediated plasticity in the single crystals showed are more marked strength increase encouraging the transition to grain boundary sliding at smaller length scales.

3:20 PM  Invited
Criteria for Grain Boundary Dislocation Nucleation on Different Slip Systems Obtained by Atomistic Simulations: Eric Homer1; Ricky Wyman1; 1Brigham Young University
    While it is well known that grain boundaries (GBs) can respond in a variety of ways to imposed deformation, the criteria for each response is not well known. This work models a range of triaxial stress states in atomistic simulations to observe dislocation nucleation on different slip systems. The results examine the local, resolved stresses to develop criteria for GB dislocation nucleation on each slip system. The criterion for each slip system are unique and include non-Schmid effects. The implications of the results are discussed, and potential yield surfaces are developed to aid in the interpretation of the criteria.

3:40 PM Break

4:00 PM  Invited
Interface-Mediated Twinning in Small-Scaled BCC Bi-crystals: Jiangwei Wang1; Scott Mao1; 1University of Pittsburgh
    This talk will be based on recent publication on In situ atomic-scale observation of twinning dominated deformation in nanoscale body-centered cubic tungsten, Nature Material by Jiangwei Wang, Scott X. Mao et al. Twinning is a fundamental deformation mode that competes against dislocation slip in crystalline solids. Deformation twinning has been well documented in FCC nanoscale crystals. Here, by using in situ high-resolution transmission electron microscopy, we report that twinning is the dominant deformation mechanism in nanoscale bi-crystals of BCC tungsten. Such deformation twinning is found to be pseudoelastic, manifested through reversible detwinning during unloading. We find that the competition between twinning and dislocation slip can be mediated by loading orientation, which is attributed to the competing nucleation mechanism of defects in nanoscale BCC bi-crystals. Our work provides direct observations of deformation twinning under cyclic loading as well as new insights into the deformation mechanism in BCC nanostructures.

4:20 PM  Invited
Intrinsic Scale Effects in Metal Deformation: Christopher Woodward1; Satish Rao2; Ahmed Hussein1; Brahim Akdim1; Edwin Antillon1; Triplicane Parthasarathy1; 1Air Force Research Laboratory; 2École Polytechnique Fédérale
    Changes in deformation behavior for macro-nano scale samples can be used to inform size dependent plasticity methods. Such scale effects are quantified and validated using lower scale, physics-based models. Experiments show strong size effects in metal micro-pillars with dimensions below ~50 micron. This size dependent behavior is consistent with deformation occurring below a characteristic dislocation correlation length. Micro-scale dislocation evolution simulations exhibit the same behavior and reveal the mechanistic source of strengthening at these scales. In this work, large scale atomistic and dislocation dynamics simulations are used to assess the aspects of ensemble hardening in simple metals. The work hardening rates of micro pillars, uniaxial loaded along <100>, <110>, and <111>, are calculated using dislocation dynamics simulations. Simulations include dislocation intersection cross slip which enhances the rapid increase in dislocation density. Analyses of the evolving dislocation ensembles, including the formation of strong dislocation heterogeneities are reviewed.

4:40 PM  Invited
Quantifying the Dislocation Emission Process from Grain Boundaries with Traction Fields: Huck Beng Chew1; Ruizhi Li1; 1University of Illinois at Urbana-Champaign
    Grain boundaries are potential sites for dislocation emission in nanocrystalline materials. Understanding how the atomic structure of a grain boundary controls its ability to emit dislocations is a significant step towards engineering the grain boundaries of these metals. Here, we propose the notion of continuum-equivalent traction-fields as local quantitative descriptors of the grain boundary interface. From continuum-equivalent traction-fields along <110> symmetrical-tilt nickel grain boundaries, we successfully predict the critical stresses to trigger dislocation emissions. We show that Shockley partials are emitted when the grain boundary tractions, in combination with external tensile loading, generate a resolved shear stress to cause dislocation slip in the specific slip systems. Finally, we will discuss the relationship between the local grain boundary tractions and the grain boundary energy.

5:00 PM  Invited
Stresses in Reverse-deformed Single Crystal Cu: Quantitative Tests of the Composite Model: Lyle Levine1; Thien Phan1; I-Fang Lee2; Ruqing Xu3; Yaakov Idell1; Michael Kassner2; 1National Institute of Standards and Technology; 2University of Southern California; 3Argonne National Laboratory
    In 2011, we used depth-resolved, sub-micrometer synchrotron X-ray diffraction to measure stresses within numerous individual dislocation cell interiors and cell walls in heavily-deformed Cu. Separated, asymmetric stress distributions were found, demonstrating that the evolving dislocation wall structures develop strong dipolar stress fields. These findings proved that the fundamental mechanisms of the Mughrabi composite model for work hardening were correct, but couldn’t address the more important question of whether the composite model could explain mechanical behaviors such as the Bauschinger effect. We present depth-resolved, sub-micrometer diffraction measurements from monotonic and slightly reverse-loaded deformed Cu. The measurements show a substantial decrease in the long-range stresses with only a slight reversal in the plastic strain. These results demonstrate that the composite model is not a viable explanation for the Bauschinger effect in deformed Cu, and that substantial dislocation-wall “unraveling,” as considered in the Orowan–Sleeswyk model of work hardening, occurs during reverse loading.

5:20 PM  Invited
The Development of Physically Based Atomistic Microstructure: The Effect on the Mechanical Response of Polycrystals: Jacob Gruber1; Fadi Abdeljawad2; Hojun Lim2; Stephen Foiles2; Garritt Tucker1; 1Drexel University; 2Sandia National Laboratories
    Molecular dynamics methods are commonly used to study the properties of nanocrystalline systems, usually constructed using Voronoi tessellation methods. In this work, a new method for the generation of atomistic realizations of polycrystalline aggregates from a phase field is presented. The topological properties of generated microstructures are computed and directly compared to structures constructed using Poisson Voronoi tessellation. While there is little difference in the macroscale mechanical response between the two microstructures, substantial differences in the operative deformation mechanisms under uniaxial loading are observed. The results demonstrate that microstructures generated from phase field exhibit a propensity toward dislocation plasticity while those generated from Poisson Voronoi favor grain-boundary mediated mechanisms. These differences suggest that grain topology and grain boundary character significantly affect local responses of polycrystals and that careful consideration of these interface effects is required when attempting to quantify the transition from dislocation-mediated plasticity to interfacial-mediated plasticity in nanocrystalline systems.