Symposium Organizers
Yang Xiang, Hong Kong University of Science and Technology
Stefan Sandfeld, TU Bergakademie Freiberg
Yao Shen, Shanghai Jiao Tong University
Jian Wang, University of Nebraska–Lincoln
CM6.1: Continuum Dislocation and Plasticity Theories
Session Chairs
Tuesday PM, April 18, 2017
PCC North, 100 Level, Room 126 C
11:30 AM - *CM6.1.01
Macroscopic Plasticity from Discrete Dislocation Dynamics
Sabyasachi Chatterjee 1 , Giacomo Po 2 , Xiaohan Zhang 3 , Amit Acharya 1 , Nasr Ghoniem 2
1 , Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 2 , University of California, Los Angeles, Los Angeles, California, United States, 3 , Stanford University, San Francisco, California, United States
Show AbstractA procedure for developing a macroscopic continuum theory of plasticity from discrete dislocation dynamics involving averaging in space and time will be discussed. Results from a computational implementation of the procedure will be demonstrated.
12:00 PM - *CM6.1.02
Dislocation Patterning—Meso-Scale Interactive Behavior of Dislocations Studied through Dislocation Density–Function Dynamics
Alfonso Ngan 1 , K.W. Siu 1 , H.S. Leung 1 , B.Q. Cheng 1
1 , University of Hong Kong, Hong Kong China
Show AbstractThe formation of dislocation patterns ranging from highly ordered to fractal-like morphologies as metals deform plastically is strong manifestation of the group interactive behavior of dislocations in the meso scale. Past attempts to understanding dislocation patterning include the analytical models of the Walgraef-Aifantis type that consider the diffusive transport of dislocation density in one dimension. From a statistical mechanics point of view, dislocation patterning has also been interpreted as a result of the balance between the interactive energy and configurational entropy of the dislocations, and as a noise factor akin to the thermal temperature increases, the dislocation distribution is predicted to transit from being homogeneous to regular cellular and then to irregular cellular.
The application of discrete dislocation dynamics to predict dislocation patterns has been hampered by the high quantities of dislocations involved. In this talk, a new simulator based on the dynamics of dislocation-density functions is used to predict the formation of dislocation subcells under oscillatory loading conditions. This simulator adopts an “all-dislocation” approach in which the flux, production and annihilation, as well as the Taylor and elastic interactions between dislocation densities are considered without distinguishing between geometrically necessary (GNDs) and statistically stored dislocations (SSDs). Elastic interactions between dislocations in 3D are treated in full accordance with Mura’s formulation for eigen stress. Dislocation generation is considered as a consequence of dislocations to maintain their connectivity. For an FCC model corresponding to Al, softening during vibrational loadings as well as enhanced cell formation are predicted. The simulations reveal that subcells form during oscillatory loadings due to the enhanced elimination of SSDs by the oscillatory stress, leaving behind GNDs with low Schmid factors which then form the subgrain walls. The oscillatory stress helps the depletion of the SSDs, because the chance for them to meet up and annihilate is increased with reversals of dislocation motions.
12:30 PM - CM6.1.03
Homogenisation of Dislocation System and Dislocation Pattern Formation
Yichao Zhu 1 , Yang Xiang 2
1 , Dalian University of Technology, Dalian China, 2 , Hong Kong University of Science and Technology, Hong Kong Hong Kong
Show AbstractProper formulation of multiple-scale dislocation interactions is crucial for continuum model of dislocation to capture the various types of dislocation patterns formed in crystalline materials. In this talk, starting with discussion on homogenising the behaviour of a row of dislocation dipoles, we will show by matched asymptotic expansion that discrete dislocation dynamics (DDD) can be effectively upscaled by a set of evolution equations for dislocation densities along with a set of equilibrium equations for variables characterising the self-locked dislocation structures (SLDSs) which can be treated quasi-steadily on the continuum scale. The stress to unlock the SLDSs, i.e. the flow stress, can be determined by checking the solvability conditions of the local equations that govern the steady state of SLDSs. Based on these findings, a general strategy of summarisng the collective bebaviour of many dislocations will be presented. Under this guideline, a (continuum) flow stress formula for multi-slip systems, which resolves more details from the underlying dynamics than the ubiquitously adopted Taylor-type formulae, is derived. Moreover, the continuum dynamics of the formation, migration and dissociation of SLDSs on parallel slip planes can be successfully formulated in good accordance with the underlying DDD.
12:45 PM - CM6.1.04
A Systematic Approach to Compare the Energy Densities of Discrete and Continuum Dislocations Models
Nina Gunkelmann 1 , Giacomo Po 2 , Stefan Sandfeld 1
1 Institute for Materials Simulation, Friedrich-Alexander University Erlangen-Nürnberg, Fürth Germany, 2 Mechanical and Aerospace Engineering Department, University of California, Los Angeles, California, United States
Show AbstractThe energy density of systems of dislocations is ubiquitous to describe the interaction of the defect microstructure. While discrete dislocation simulations are based on the elastic theory of dislocations, continuum dislocation theories are not readily able to recover a realistic energy density because short-range dislocation interactions cannot be resolved. Recently, kinematically consistent continuum theories of dislocations have been developed which are able to predict the motion of dislocations in a given velocity field. However, despite its importance, up to now there exist no consistent theoretical expression for the free energy density of a microstructure, from which the dislocation velocity could be inferred.
Our approach to obtain the energy density is based on detailed statistical analysis of DDD simulations. We show comparisons between analytical solutions, DDD and continuum data for simple special cases and achieve very good agreement. By analyzing more complex microstructure, for which elaborate continuum variables are required, we show which informations are relevant and which informations are lost in a continuum description. We show that for GND dominated systems the dislocation interactions are adequately described by the elastic strains from the classical linear eigen-distortion theory. For SSD dominated systems short-range contributions between adjacent slip planes cannot be captured by the continuum representation. Using a Taylor-type yield stress the agreement between DDD and continuum data is improved.
Finally, we present first steps towards benchmarking energy expressions which are commonly used in the continuum plasticity community. Our approach can serve as foundation for systematic data mining and evaluation of continuum formulations.
CM6.2: Continuum and Discrete Dislocation Dynamics
Session Chairs
Tuesday PM, April 18, 2017
PCC North, 100 Level, Room 126 C
2:30 PM - *CM6.2.01
A Continuum Dislocation Model of Wedge Micro-Indentation
Giacomo Po 1
1 , University of California, Los Angeles, Los Angeles, California, United States
Show AbstractRecent EBSD experiments have revealed the heterogeneous dislocation microstructure forming under a wedge indenter in fcc crystals, where micro-meter dislocation patterns challenge the predictions of traditional models of plasticity. In order to explain the formation of these features, in this work we present a model of wedge indentation based on the continuum theory of dislocations. The model accounts for large deformation kinematics through the standard multiplicative split of the deformation gradient tensor, where the incompatible plastic component of deformation results from the flux of dislocations on different and interacting slips systems. Constitutive equations for the dislocation fluxes are determined from a dissipative variational principle given two thermodynamic potentials. As a result, each dislocation density satisfies an initial-boundary value problem with convective-diffusive character, which is coupled to the macroscopic stress-displacement problem governing the deformation process. Solution to the self-consistent continuum formulation is found using the finite element method. In particular, computer simulations are performed for cases that mimic the experimental conditions used in wedge micro-indentation experiments of fcc Cu, Al, and Ni. A comparison of overall dislocation density distribution and macroscopic mechanical response is presented.
3:00 PM - CM6.2.02
Enriching DDD Simulations by Plastic Slip Reconstruction
Dominik Steinberger 1 , Riccardo Gatti 2 , Stefan Sandfeld 1
1 , Friedrich-Alexander-Universität Erlangen-Nürnberg, Furth Germany, 2 UMR 104 CNRS-ONERA, LEM, Paris France
Show AbstractThe mechanical behavior of a crystalline materials is determined by its microstructure. As such understanding the complex interactions of each component within the microstructure is vital for understanding microplasticity. Dislocations—one-dimensional lattice defects—represent one component of the microstructure as the boundaries of an area over which slip occured. Their spatial evolution is thus coupled to plastic deformation, which eventually results in a stress-field and hence interactions with other dislocations and defects in the crystal lattice. Both experiments and simulations used to study dislocations offer more and more information on similar lengthscales that allow synergetic effects between these two disciplines. The discrete to continuous (D2C) framework [1] attempts to provide a “language” that, thus far, enables the quantitative comparison of dislocation structures from both, experiments and simulations. To truly provide insight into the deformation behavior of crystals, however, the area swept by dislocations is important. Being able to reconstruct the plastic slip area from dislocation data obtained by simulations and experiments may thus further enhance the understanding of phenomenon like the formation of dislocation patterns. The amount of data thathas to be stored from discrete dislocation dynamics becomes exceedingly large due to the increase of computational power and thus amount of dislocations within a simulation. If possible, the additional information of the plastic slip area within the output should be avoided.
In this presentation we briefly introduce the discrete to continuous (D2C) framework and typical applications within the dislocation structure analysis. We then discuss an algorithm to reconstruct the plastic slip area of “static” discrete dislocation data and which information is necessary to implement it. It will be outlined how the information of the plastic slip area can be embedded into the D2C framework to gain additional insight into the plastic behavior of metals. We then proceed to use the reconstructed plastic slip to compute the internal stresses of single system timesteps and ensemble averages of data obtained by discrete dislocation dynamics for different boundary conditions in an eigenstrain approach. This approach is compared to a fast fourier transform based approach possible for periodic systems. The ensemble averaged quantities of discrete data will subsequently be used in a data mining approach to identify characteristics typical for a dislocation system.
[1] D. Steinberger, R. Gatti, and S. Sandfeld (2016). “A Universal Approach Towards Computational Characterization of Dislocation Microstructure”. In: JOM 68.8, pp. 2065–2072. doi: 10.1007/s11837-016-1967-1.
3:15 PM - CM6.2.03
Plastic Zone Properties at a Crack Tip Investigated with the Discrete-Continuous Model
Riccardo Gatti 1 , Laurent Korzeczek 1 , Benoit Devincre 1 , Arjen Roos 2
1 , LEM UMR 104 CNRS-ONERA, Chatillon France, 2 , Safran Tech, Saclay France
Show AbstractIn this study, the dynamics of dislocations inside the plastic zone formed at the vicinity of trans-granular short cracks is investigated at the mesoscopic scale with the Discrete-Continuous Model (DCM). The results of this model, based on a coupling between 3D Dislocation Dynamics and Finite Elements simulations, are here presented and compared with a conventional Crystal Plasticity Model (CPM) appropriately set up to correctly reproduce slip system interactions in FCC single crystals.
Several crack orientations were studied for single or polycrystal materials, under monotonous or cyclic loading. A detailed analysis of the respective slip system activity around the crack in relation with shielding and blunting mechanisms is presented. To calculate the 3D strain energy release associated to plastic deformation and controlling crack propagation, a G-theta integral method is used and compared with alternative local force solutions. Also, the strong differences observed between the DCM and the CPM simulations are systematically analyzed and discussed. These results are of particular interest for the development of more physically justified dislocation density based model devoted to fracture problems in ductile materials.
3:30 PM - CM6.2.04
Boundary Behaviour and Confinement of Screw Dislocations
Marco Morandotti 1
1 , Technische Universität München, Munich Germany
Show AbstractIn this talk, I will explore the behaviour of screw dislocations near the boundary of the domain and how it is possible to confine them inside it.
In the first part, an analytical proof that dislocations tend to migrate to the boundary is presented, together with an explicit law describing the phenomenon. Examples in simple domains are presented.
In the second part, I will discuss how it is possible to confine a system of screw dislocations inside material by prescribing the strain on the boundary.
These results come from joint works with Thomas Hudson, Ilaria Lucardesi, Riccardo Scala, and Davide Zucco.
3:45 PM - CM6.2.05
Ultrahard Amorphous-Crystalline Hybrid Steel Nanolaminates
Wei Guo 1 , Yifei Meng 2 , Xie Zhang 3 , Vikram Bedekar 4 , Hongbin Bei 5 , Rajiv Shivpuri 6 , Jian-Min Zuo 2 , Jonathan Poplawsky 1
1 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Department of Material Science and Engineering and Materials Research Laboratory, University of Illinois at Urbana–Champaign, Urbana, Illinois, United States, 3 Department of Computational Materials Design, Max-Planck-Institut für Eisenforschung GmbH, Dusseldorf Germany, 4 Material Science Research and Development, Timken World Headquarters, North Canton, Ohio, United States, 5 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 6 Department of Integrated Systems Engineering, The Ohio State University, Columbus, Ohio, United States
Show AbstractAmorphous and ultrafine grain (UFG) steels are two categories of strong steels. However, in the past decade their application has been hindered by limited plasticity, the addition of expensive alloying elements, and processing challenges of bulk materials. Here, we report that the surface of a carburized Fe-Mn-Si martensitic steel with extremely low alloying elements can be practically fabricated into an amorphous-nanocrystalline nanolaminate structure. Atom probe tomography (APT) and nano-beam diffraction (NBD) of a hard turned surface together with molecular dynamics (MD) simulations reveal that the original cementite structure experiences a size-dependent amorphization and phase transformation during heavy plastic deformation. Molecular dynamics simulations reveal that the martensite/cementite interface serves as a nucleation site for cementite amorphization at a threshold strain of 11% at room temperature, and the cementite can become disordered as the cementite is strained further when the cementite particle is relatively small. A three dimensional carbon rich network is established thanks to the high binding energy of carbon-dislocation interaction and dislocation aided transport of C atoms. The graded nanolaminates exhibit a surface hardness of ~11.3 GPa, which exceeds the value of previously reported UFG steels. The verified practical and economical approach of producing a hard alloy with considerable ductility and toughness could provide expanded opportunities for producing an amorphous-crystalline hybrid structure in both steels and other alloy systems.
CM6.3: Discrete Dislocation Dynamics
Session Chairs
Tuesday PM, April 18, 2017
PCC North, 100 Level, Room 126 C
4:30 PM - *CM6.3.01
Dislocation Junctions and Line Length Distribution during Strain Hardening of Face-Centered Cubic Metals
Wei Cai 1 , Rayn Sills 2 , Nicolas Bertin 1 , Amin Aghaei 1
1 , Stanford University, Stanford, California, United States, 2 , Sandia National Laboratories, Livermore, California, United States
Show AbstractUnderstanding plasticity and strength of crystalline materials in terms of the physics of microscopic defects has been a long-standing goal of materials research. Over the last two decades, much effort has been placed on the prediction of stress-strain curve of single crystals through large-scale dislocation dynamics (DD) simulations. If successful, DD can thus provide a quantitative link, which has been lacking to date, between dislocation physics at the atomistic scale and crystal plasticity at the continuum scale.
Using a newly developed time integrator, DD simulations are able to predict the strain hardening rates in FCC Cu under [001] loading. The predicted hardening rates are consistent with stage II of the quasi-static stress-strain response observed in experiments, i.e. on the order of μ/200. By changing rules on unit mechanisms in DD simulations, we determine the relative importance of different dislocation reactions on the hardening rate. We find that glissile junctions are the most important junction type for hardening, with collinear and Lomer junctions second most important. Interestingly, the lengths of the dislocation lines between junctions are found to be exponentially distributed during a wide range of simulation conditions. A Boltzmann-type theory is developed to explain the dislocation line length distribution, and the role of different junctions on the hardening rate revealed by DD simulations.
This work was supported by Sandia National Laboratories (R.B.S.) and by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-SC0010412 (W.C.). 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.
5:00 PM - *CM6.3.02
3D DD Investigations of the Role of Interfaces on Dislocation Plasticity
Marc Fivel 1 , Hareesh Tummala 1 2 , Mohamed Hammad 2 , Thomas Pardoen 2 , Hosni Idrissi 2 , Laurent Delannay 2
1 , SIMaP, St Martin d'Heres France, 2 Université Catholique de Louvain, iMMC, Louvain-La-Neuve Belgium
Show AbstractIn this paper, we illustrate the role of interfaces in the dislocation organization and the consequence on the mechanical response through several investigations based on 3D discrete dislocation dynamics (DD) simulations. In a first example, we perform DD simulations on individual grains of same volume but different aspect ratios to understand the influence of the grain shape on the slip system activity and on the back stresses produced. Then, a polycrystalline version of the DD code coupled with a finite element method is used to perform simulations of a nanocrystalline free standing thin film of Palladium with varying both the texture and grain size distributions. In a last example, spherical nanoindentation of Cu is simulated and compared to simulations where a graphene layer has been deposited on the indented surface.
For each cases, DD simulation results are compared to experiments in order to reveal the physics at the origin of the macroscopic response.
5:30 PM - CM6.3.03
Discrete Dislocation Dynamics Simulations of Pop-In Events in Nanoindentation
Hengxu Song 1 , Hakan Yavas 1 , Erik van der Giessen 3 , Stefanos Papanikolaou 1 2 3
1 Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 3 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, 2 Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia, United States
Show Abstract
A two-dimensional discrete dislocation model is developed for the statistics of pop-in events during nanoindentation of a single crystalline sample. The sample is assumed to contain bulk dislocation sources as well as surface dislocation sources. Indentation is performed by using a cylindrical indenter that is either displacement or load controlled. By varying the indenter radius, we investigate the role and correlation of the pop-in events (stress/force drops or displacement jump) with bulk dislocation sources. For large indenter radii, pop-in events are controlled by surface dislocation sources while for small indenter radii (in comparison to average dislocation source spacing), the magnitude of the pop-in strongly depends on the size of the indenter. We also apply in-plane stresses up to the yield point on the sample surface prior to indentation. We investigate the role of in-plane stresses on the character of the pop-in events.
5:45 PM - CM6.3.04
On a Causal Stroh Formalism and Mach Cones Radiated by Fast Dislocations in a Linear-Elastic Anisotropic Medium
Yves-Patrick Pellegrini 1
1 , Commissariat a l'Energie Atomique et aux Energies Alternatives, Arpajon France
Show AbstractFew analytical works have previously dealt with fields radiated by fast-moving dislocations in a linear-elastic anisotropic medium, in the supersonic regime where the dislocation creates Mach cones [1,2].
In the supersonic regime, Mach cones are built over time as caustics of expanding wavefront sets, which is a causal process. This talk reviews the matter, and explains how a simple modification of Stroh's formalism [3] for uniformly-moving dislocations endows it with causal properties. This modification is derived from of an analysis of the radiation condition in the Green tensor of the elastodynamic wave equation. It allows for a straightforward computation of velocity-dependent field expressions that are valid whatever the dislocation velocity. Fields radiated by Somigliana dislocations can thus be computed without needing to consider subsonic and supersonic cases separately.
Form those results, a general explanation [3] is provided for Payton's atypical `inversed' Mach cones [1] (in which the cone aperture is directed towards the direction of motion), and a simple criterion for their existence is derived in terms of slowness surfaces. These considerations are illustrated by full-field calculations from analytical formulas for a dislocation of finite width in Fe, and confirmed by Huygens-type geometric constructions of Mach cones from ray surfaces [3].
References:
[1] R.G. Payton, Steady state stresses induced in a transversely isotropic elastic solid by a moving dislocation, Z. Angew. Math. Phys. 46 (1995) 282-288.
[2] D.M. Barnett, J.A. Zimmerman, Nonradiating dislocations in uniform supersonic motion in anisotropic linear solids, in: P. Schiavone, C. Constanda, A. Mioduchowski (Eds.), Integral Methods in Science and Engineering, Birkhauser, Boston, 2002, pp. 45–49.
[3] Y.-P. Pellegrini, Causal Stroh formalism for uniformly-moving dislocations in anisotropic media: Somigliana dislocations and Mach cones, Wave Motion 68 (2017), 128-148.
CM6.4: Poster Session: Dislocation Microstructures and Plasticity
Session Chairs
Wednesday AM, April 19, 2017
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - CM6.4.01
Electrical Properties of Dislocations within the Nitride Based Semiconductors Gallium Nitride and Indium Nitride
Erfan Baghani 1 , Stephen O'Leary 1
1 , University of British Columbia, Kelowna, British Columbia, Canada
Show AbstractDislocation lines affect the electrical and optical properties of semiconductors. In this research, the effect that the threading dislocation lines have on the free electron concentration and the electron mobility within gallium nitride and indium nitride is investigated. A formulation is developed for obtaining the screening space charge concentration and the corresponding electrostatic potential profile surrounding the dislocation lines. The resultant electrostatic potential profile is then used to compute the associated electron mobility, limited by scattering from the charged dislocation lines. As part of this research, a Gibbs factor formalism is also developed that can readily obtain the occupation statistics of the defect sites associated with the threading dislocation lines.
9:00 PM - CM6.4.03
Non-Isothermal Precipitation Hardening of AZ91 Magnesium Alloy
Panthea Sepehrband 1 , Anneliese Bals 1
1 , Santa Clara University, Santa Clara, California, United States
Show AbstractApplication of Magnesium alloys in automotive industry is still limited, despite the high strength-to-weight ratio of the alloys, which make them an attractive candidate for increasing fuel efficiency. The relatively poor mechanical properties of magnesium have impeded its use. Precipitation hardening has been known as a viable approach for improving the properties of the properly designed magnesium alloys. Controlling precipitation behavior and promoting homogeneous nucleation of precipitates, which allows for fine, evenly distributed precipitates to form within the grain structure, is expected to enhance mechanical properties of the alloy, and enabling it to achieve higher strength and better ductility. Thus far, main efforts regarding the improvement of magnesium alloys mechanical properties have been geared toward isothermal heat treatment. In this research, non-isothermal aging treatments are designed with the idea of increasing rate of homogenous precipitation during the early stages of aging at low temperatures, and rising diffusion rate, and therefore, rate of precipitate growth at elevated temperatures during the later stages of aging. The precipitation hardening behavior during non-isothermal aging of AZ91C, a magnesium alloy with 9 weight percent aluminum and 1 weight percent zinc, is explored. The experiment includes microhardness measurements of samples that have undergone dual step, isothermal, and non-isothermal aging at heating rates ranging between 0.1oC/min and 3oCmin. Data shows that slower heating rates lead to achieving a higher peak of hardness at lower temperatures. Results have been analyzed to investigate the kinetics of precipitation hardening in the magnesium alloy AZ91.
Symposium Organizers
Yang Xiang, Hong Kong University of Science and Technology
Stefan Sandfeld, TU Bergakademie Freiberg
Yao Shen, Shanghai Jiao Tong University
Jian Wang, University of Nebraska–Lincoln
CM6.5: Grain Boundaries and Dislocations
Session Chairs
Stefan Sandfeld
Peter Voorhees
Wednesday AM, April 19, 2017
PCC North, 100 Level, Room 126 C
9:00 AM - *CM6.5.01
A Disconnection Model for Grain Boundary Structural, Mechanical and Dynamical Properties
David Srolovitz 1 , Jian Han 1 , Spencer Thomas 1 , Jerry Quek Siu Sin 2 , Yong-Wei Zhang 2
1 , University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 , IHPC, Singapore Singapore
Show AbstractGrain boundaries play important roles in the mechanical behavior of polycrystalline materials beyond classical Hall-Petch behavior. Grain boundaries can (1) be driven to migrate by applied stresses or capillarity, (2) slide (or not), (3) absorb, transmit or emit lattice dislocations, (4) thermally roughen, (5) participate in grain rotation, .... In this presentation, we will discuss how all of these phenomena can be represented in a unified framework and can be applied to understand the role grain boundaries play in both microstructure evolution and in plasticity. Our approach rests on the bicrystallogphy of DSC dislocations and their associated disconnections. We demonstrate how grain boundary properties are not simply associated with individual types of disconnections but rather a spectrum of disconnections that can be predicted based upon bicrystallorgaphys, disconnection energetics and statistical mechanics ideas. We show that while theories exist for many of the individual phenomena enumerated above (usually containing empirical parameters), our disconnection-based approach brings many of these together in a fully predictive and self-consistent manner. We will develop the basic ideas of this theory, outline a set of predictions, and tests based on atomistic simulation results.
9:30 AM - *CM6.5.02
Three-Dimensional Grain Growth—The Role of Dislocations
Akinori Yamanaka 2 , Kevin McReynolds 1 , Peter Voorhees 1
2 , Tokyo University of Agriculture and Technology, Tokyo Japan, 1 , Northwestern University, Evanston, Illinois, United States
Show AbstractWe investigate grain growth and grain rotation in a body-centered cubic bicrystal composed of a spherical grain embedded in a single crystal matrix by three-dimensional phase-field crystal simulations. The structure and time evolution of the dislocation networks formed on the grain boundary during the capillary-driven grain shrinkage are examined. The results for initially spherical grains reveal the formation of hexagonal dislocation networks (HDNs) in the grain boundary. We demonstrate that the anisotropic distribution of the HDNs is responsible for the asymmetric shrinkage of the embedded grain and clarify the mechanisms of dislocation reactions during the grain shrinkage, which include dissociation, recombination, and annihilation of dislocations. For large misorientations, the high density of the HDNs leads to very small grain rotations. However, if the misorientation is small, the lack of dislocation reactions during grain shrinkage results in grain rotation of the shrinking three-dimensional grain.
10:00 AM - CM6.5.03
Continuum Framework for Dislocation Structure, Energy and Dynamics of Dislocation Arrays and Low Angle Grain Boundaries
Luchan Zhang 1 , Yejun Gu 2 , Yang Xiang 1
1 , Hong Kong University of Science and Technology, Hong Kong Hong Kong, 2 , Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractWe present a continuum framework for dislocation structure, energy and dynamics of dislocation arrays and low angle grain boundaries which may be nonplanar and nonequilibrium. We define a dislocation density potential function on the dislocation array surface or grain boundary to describe the orientation dependent continuous distribution of dislocations. The continuum formulation incorporates both the long-range dislocation interaction and the local dislocation line energy, and is derived from the discrete dislocation model. We also develop numerical methods to solve this continuum model. The continuum model is applied to dislocation structure and energy of grain boundaries in fcc crystals. This work was partially supported by Hong Kong Research Grants Council General Research Fund 605410 and 606313.
10:15 AM - CM6.5.04
Dislocation Assisted Rafting in Nickel-Based Superalloys—A Coupled Phase-Field/Continuum Dislocation Dynamics Model
Stefan Sandfeld 1 , Ronghai Wu 1
1 , University Erlangen-Nuremberg, Fuerth Germany
Show AbstractNickel-based superalloys play a major role in many technologically relevant high temperature applications. Understanding and predicting the evolution of the phase microstructure during high temperature creep together with the evolution of the dislocation microstructure is a challenge that up to date has not yet been fully accomplished.
Our two-dimensional coupled phase-field/continuum dislocation dynamics model explains microstructural mechanisms which are important during the early stage of rafting in a single crystal system. These simulations show that the long range shear stresses due to γ/γ' misfit and external loading drive dislocations towards the horizontal γ/γ' interfaces where microstructures consisting predominantly of geometrically necessary dislocations are formed. Such dislocation accumulations, in turn, change the elastic energy near the γ/γ' interfaces and result in γ/γ' precipitate rafting into horizontal direction. In this work, we study the influence of different dislocation densities on the rafted γ/γ' morphologies. We show how experimental creep strains can then be used to paramterize our simulations. Finally, we show first results from an extended simulation which additionally to dislocation glide also incorporates dislocation climb.
10:30 AM - CM6.5.05
Multiscale Simulation of Dislocation-Interface Reactions in Heterogeneous Materials
Liming Xiong 1 , Rigelesaiyin Ji 1 , Valery Levitas 1 , Shuozhi Xu 2 , David McDowell 2 , Youping Chen 3
1 , Iowa State University, Ames, Iowa, United States, 2 Department of Mechanical and Aerospace Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 3 Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractThe objective of this work is to determine the fundamental mechanism of the reactions between dislocations and a variety of material interfaces, including tilt grain boundaries (GBs) in silicon, twin boundaries (TBs) in copper, the crystalline-crystalline interfaces (CCIs) in multilayered fcc/bcc/fcc metallic composites as well as the amorphous-crystalline interfaces (ACIs) in Cu/CuxZr1-x nanolaminates using a predictive concurrent atomistic-continuum (CAC) model. With a fully atomistic resolution at the interface and coarse-grained (CG) atomistic resolution in the regions away from interfaces, this multiscale computer model requires significantly less computational cost than that by a fully atomistic simulation. Most importantly, the atomistic natures of dislocation nucleation, migration, and interactions are still preserved in the CG region away from the atomic-scale interfaces. As such, the atomic-scale interfacial structure and the long-range stress gradient from dislocation piling up on the interfaces are simultaneously incorporated into one multiscale computational framework. This enables the main mechanisms underlying the dislocation-interface interactions to be unraveled from the atomic to the microscale. For instance, our multiscale simulations of bi-crystalline Cu show that the dislocation-TB reaction always follow the recombination-redissociation process, without forming any TB dislocations in process of recombination. In multilayered Cu/CuxZr1-x, the multiscale computational results show that dislocation cores will spread at the ACIs when they approach CuxZr1-x. To maintain the deformation compatibility between the crystalline Cu layers and amorphous CuxZr1-x layers, a large number of shear transformation zones (STZs, the plasticity carrier in amorphous materials) will nucleate in the amorphous CuxZr1-x phases. In contrast, in bi-crystalline silicon with clean tilt GBs or fcc/bcc/fcc nanolaminates with well-defined PBs, the microscale dislocation pile-up on the GBs or PBs is found to assist phase transformation (PTs) of Si I-Si II and bcc-fcc, respectively. Our results elucidate the discrepancies between atomistic simulations and experimental observations of dislocation-interface reactions, and will highlight the importance of directly concurrent multiscale modeling of dislocation-interfaces reactions in heterogeneous materials.
10:45 AM - CM6.5.06
Dislocations on ∑5{013} Grain Boundaries in Mono-Cast Si—Atomistic Structure and Effects on Mechanical Properties
Yutaka Ohno 1 , Kentaro Kutsukake 1 , Momoko Deura 1 , Ichiro Yonenaga 1 , Hideto Yoshida 2 , Seiji Takeda 2 , Yasuo Shimizu 3 , Naoki Ebisawa 3 , Koji Inoue 3 , Yasuyoshi Nagai 3
1 , IMR, Tohoku University, Sendai Japan, 2 , ISIR, Osaka University, Osaka Japan, 3 , The Oarai Center, IMR, Tohoku University, Oarai Japan
Show AbstractDislocations in polycrystalline materials can accumulate on grain boundaries (GBs) during crystal growth, deformation and annealing processes, and such GB dislocations would be responsible for the GB dynamics such as cracking and migration of GBs. Also, the dynamic property would be modified via the segregation of impurity atoms at the GB dislocations. Therefore, a comprehensive knowledge of the mechanical property of GB dislocations depending on their atomistic structure is indispensable for a practical use of polycrystalline materials by optimizing the structural condition of GB dislocations in the materials.
Here we discuss the atomistic structure of GB dislocations in quasi-single crystalline silicon (Si) for solar cells produced by multiple Si seeds (a so called mono-cast Si) [1], which can enhance the conversion efficiency of solar cells in comparison with the conventional polycrystalline Si at a low cost. It is proposed that, the efficiency can be enhanced via the intentional introduction of functional GBs to suppress the propagation of GBs with artificially arranged Si seeds [2]. In the present work, functional ∑5{013} GBs were artificially introduced, and dislocations were accidentally generated on the GBs during the crystal growth. Atomic arrangement and three dimensional distribution of impurity atoms around the dislocations were, respectively, determined by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and by atom probe tomography (APT) with a low impurity detection limit about 0.005 at.% on a GB plane simultaneously with a high spatial resolution of 0.4 nm [3].
HAADF-STEM revealed that, a ∑5(013) GB segment has a misorientation of about 0.5 degrees with respect to the ∑5{013} coincidence orientation, and it contains GB dislocations with the Burgers vector of a/10[013], extending along [100], arranged at an interval of about 20 nm. High-resolution APT revealed that, nickel (Ni), copper (Cu) and oxygen atoms segregated along the GB dislocations. The number of segregating impurity atoms per unit GB area for Ni and that for Cu were in a trade-off correlation with that for oxygen, with respect to the misorientation angle. The oxygen number was small in a GB with a small misorientation angle, and the GB exhibited GB cracking. The cracking behavior depending on the structural condition of GB dislocations will be discussed.
[1] Y. Ohno, et al., Appl. Phys. Lett. 109 (2016) 142105.
[2] K. Kutsukake, et al., IEEE J. Photovolt. 4 (2014) 84.
[3] Y. Ohno, et al., Appl. Phys. Lett. 103 (2013) 102102.
CM6.6: Multiscale Models of Dislocations
Session Chairs
Wednesday PM, April 19, 2017
PCC North, 100 Level, Room 126 C
11:30 AM - *CM6.6.01
Comparative Atomistic-Continuum Modeling of Nanoscopic Dislocation Processes in Single and Bicrystals
Bob Svendsen 1 2 , Jaber Rezaeimianroodi 1
1 , RWTH Aachen University, Aachen Germany, 2 Microstructure Physics and Alloy Design, Max-Planck Institute for Iron Research, Duesseldorf, North Rhein Westfalia, Germany
Show AbstractThe purpose of this work is the development of continuum models for dislocation processes in nanocrystals. This is based in particular on detailed theoretical and computational comparison of such models with atomistic modeling based on molecular statics (MS) and molecular dynamics (MD). Continuum models considered theoretically in the current work include field dislocation mechanics (FDM: e.g., Acharya, JMPS 58, 766-778, 2010; Zhang et al., JMPS 84, 145-195; Gbemou et al., Int. J. Plast. 82, 241-259, 2016), generalized Peierls-Nabarro (Xiang et al., Acta Mat. 56, 1447-1460, 2008), phase-field dislocation dynamics (PFDD: e.g., Hunter et al., Phys. Rev. B 84, 144108, 2011), atomistic phase-field microelasticity (APFM: Mianroodi, Svendsen, JMPS 77, 109-122, 2015), and phase field crystal (PFC: e.g., Berry et al., Phys. Rev. B 86, 224112, 2012; B 89, 214117, 2014). Computational comparisons of PFDD, APFM, MS and MD are carried out in the context of the simulation of elementary dislocation processes such as dissociation, core and stacking fault formation, glide, Lomer-Cottrell locking, as well as dislocation-interface interaction in bicrystals.
12:00 PM - CM6.6.02
Improvements of the Peierls-Nabarro Model and Its Applications
Guisen Liu 1 , Xi Cheng 1 2 , Jian Wang 3 , Kaiguo Chen 4 , Yao Shen 1
1 , Shanghai Jiao Tong University, Shanghai China, 2 , Stanford University, Stanford, California, United States, 3 , University of Nebraska–Lincoln, Lincoln, Nebraska, United States, 4 , China Academy of Engineering Physics, Mianyang, Sichuan, China
Show AbstractPrediction of core structure and Peierls stress of dislocations is of fundamental concern in understanding and designing the plasticity and mechanical properties of crystalline materials. Peierls-Nabarro (PN) model is an attractive approach to study core structure and Peierls stress of dislocations for its simplicity and efficiency in incorporating the nonlinear feature of the dislocation core into the long range elastic fields. Despite considerable progress has been made to the original PN model, it remains a challenge to apply it to quantitatively predict core structure and Peierls stress for complex structures, and dislocation properties under external loading. In this work, substantial improvements of the PN model have been made based on the semi-discrete variational Peierls-Nabarro (SVPN) model, and the improved SVPN model is applied to dislocations in FCC and BCC metals, and the model predictions are validated by MD simulations.
Firstly, the SVPN model is improved from two aspects by accounting for (1) the large displacement gradient effects and (2) the nonlocality in the dislocation core region. (1) An enhanced SVPN model is proposed by incorporating a gradient energy term into the energy functional, to consider the influence of both the discreteness of atoms and the quick variations of the displacement in the dislocation core. By appropriately calibrating the model against MD simulation on core structure, the enhanced SVPN model can accurately predict core structure, and consequent precise Peierls stress for dislocations in FCC metals, within 1~4 times the prediction from MD calculations. (2) A nonlocal SVPN model is developed by incorporating the nonlocal atomic interactions into the SVPN model, where the nonlocal coefficient is directly computed from core structure. Compared with the enhanced SVPN mode, the nonlocal model further improves accuracy of the Peierls stress of both planar-extended core dislocations in FCC and condensed core dislocations in BCC metals. Moreover, the nonlocal model is generalized to three dimension, and can be extended to quantitatively predict Peierls stress for dislocations in complex structures.
The variation of Peierls stress of dislocations in FCC metal with respect to applied stress, i.e., the Escaig stress is further investigated by the enhanced SVPN model, and calculations show that Peierls stress pseudo-periodically oscillates and the oscillation gradually damps as Escaig stress increases. This pseudo-periodic variation of Peierls stress can be mathematically described by the combination of a sinusoidal and an exponential function, and further accounted for by the variation of stacking fault width between two partials during their movement under applied stresses. MD simulations further examined pseudo-periodic variation of Peierls stress, validating the improved SVPN model’s capability of predicting sophisticated behavior of dislocation under external loading.
12:15 PM - CM6.6.03
Multiscale Model for Interlayer Defects in Bilayer Material
Shuyang Dai 1 , Yang Xiang 3 , David Srolovitz 2
1 , Wuhan University, Wuhan China, 3 , Hong Kong University of Science and Technology, Hong Kong Hong Kong, 2 , University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractWe present a multiscale model to describe the interlayer defects in bilayer materials. The model incorporates both the elasticity of each layer and the first-principle calculation informed interaction between two layers, i.e., the 3D generalized stacking-fault energy based upon the disregistry between two layers. The force balance between these two contributions determines the structure. We apply this approach to determine the structure and energetics of twisted bilayer material. In tBLG, two distinct, modified Moiré structures are observed. The breathing mode, stable at large twist angle, has small amplitude (opposite sign) buckling of the two layers. The bending mode is characterized by large amplitude (same sign) buckling of the layers. The latter gives rise to a distorted Moiré pattern consisting of a twisted dislocation structure. The relaxation of the Moiré structure reduces the symmetry and increases the period of the tBLG. Our model agrees well with the atomistic results. An analytical description is developed based on the obtained structural features. We also study the buckling of the twisted heterogeneous bilayer material.
12:30 PM - CM6.6.04
Viscous Evolution in a Phase Field Model for Dislocations with Forest Hardening
Patrick Dondl 1 , Matthias Kurzke 2 , Stephan Wojtowytsch 3
1 , Albert-Ludwigs-Universität Freiburg, 79104 Freiburg Germany, 2 Mathematical Sciences, University of Nottingham, Nottingham United Kingdom, 3 Department of Mathematical Sciences, Durham University, Durham United Kingdom
Show AbstractWe consider a phase field model for dislocations introduced by Koslowski, Cuitino, and Ortiz in 2002. The model describes a single slip plane and consists of a Peierls potential penalizing non-integer slip and a long range interaction modeling elasticity. Forest dislocations are introduced as a restriction to the allowable phase field functions: they have to vanish at the union of a number of small disks in the plane. Garroni and Müller proved large scale limits of these models in terms of Gamma-convergence, obtaining a line-tension energy for the dislocations and a bulk term penalizing slip. This bulk term is a capacity stemming from the forest dislocations.
In the present work, we show that the contribution of the forest dislocations to the the viscous gradient flow evolution is small. In particular it is much slower than the timescale for other effects like elastic attraction/repulsion of dislocations, which, by a recent result due to del Mar Gonzales and Monneau is already slower than the time scale from line tension energy. Overall, this leads to an effective behavior like a gradient flow in a wiggly potential. On the other hand, of course, when adding a driving force in the direction of increasing slip, one needs to spend the energy to overcome the obstacles. The forest dislocations thus act like a dissipation for increasing slip, but their effect on the propagation is absent for decreasing slip.
12:45 PM - CM6.6.05
Heterogeneous Residual Stress in Nanocrystalline Cu
Lei Cao 1 , Arkaprabha Sengupta 2 , Daniel Pantuso 2 , Marisol Koslowski 3
1 Mechanical Engineering, University of Nevada, Reno, Reno, Nevada, United States, 2 Logic Technology Development, Intel Corporation, Hillsboro, Oregon, United States, 3 School of Mechanical Engineering, Purdue University, West Lafayette, Indiana, United States
Show AbstractHeterogeneous residual stress field developed during the manufacturing process of thin films plays a key role in the damage formation in MEMS and NEMS devices. For example, the inhomogeneous stress field drives the vacancy diffusion, leading to the void growth in semiconductor interconnects; the stress concentration assists the microcracks evolution, leading to fracture in thin films. The stress heterogeneity is caused by texture, dislocations, twinnings, and grain boundaries. To quantify the distribution of residual stress in thin films, we carry out phase-field dislocation dynamics (PFDD) simulations in nanocrystalline Cu subjected to cyclic loading. PFDD tracks the activity of collective partial and full dislocations and their interactions with grain boundaries. Inter- and intra-granular stress heterogeneity is found to arise from misorientations, grain size distribution, and grain boundary characteristics. These results help to evaluate the effect of microstructure in stress induced voiding by identifying potential voiding sites. In addition, PFDD simulations provide important metrics of residual stress and yield strength as a function of grain misorientation and grain size distribution, which are commonly missing components in continuum, system-level simulations. During cyclic loading, reverse plastic strain is also observed and is found to increase with grain size inhomogeneity. Finally, slip transmission is found to be influenced greatly by the orientation of slip systems in neighboring grains.
CM6.7: Dislocation Microstructures I
Session Chairs
Wednesday PM, April 19, 2017
PCC North, 100 Level, Room 126 C
2:30 PM - *CM6.7.01
The Coupling of Phase Transformations and Plasticity in NiTi Shape Memory Alloys
Peter Anderson 1 , Kathryn Esham 1 , Harshad Paranjape 1 , Sivom Manchiraju 1 , Michael Mills 1 , Lee Casalena 1
1 Materials Science and Engineering, The Ohio State University, Columbus, Ohio, United States
Show AbstractShape memory alloys (SMAs) offer the potential for a vast array of applications, including actuators for reconfigurable blades on rotorcrafts, variable geometry chevrons on aircraft engines, and flap actuators on aircraft wings. However, there are considerable materials challenges that include functional and structural fatigue under repeated actuation. This talk focuses on the underlying deformation phenomena in SMAs at the micro- and nano- structural scale. Recent transmission electron microscopy studies show the development of plasticity when SMAs are heated and cooled, even in the absence of an external load. This is captured in a new type of phase-field finite-element simulation that captures the details of rate-dependent crystal plasticity in the vicinity of moving austenite-martensite interfaces. The results show that plasticity has the capacity to refine the structure of martensite upon cooling and to alter the ease with which martensite forms during cooling. A coupled experimental-simulation approach reveals the underlying nature of this coupling between transformation and plasticity. This work is supported, in part, by The Department of Energy, Basic Energy Sciences.
3:00 PM - *CM6.7.02
Dislocation-Templated 1-nm Gd Nano-Fiber Patterns in Mg Alloys
Guozhen Zhu 1 , Yangxin Li 1 , Yao Shen 1 , Li Jin 1 , Jian Wang 2
1 , Shanghai Jiao Tong University, Shanghai China, 2 , University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show AbstractMicrostructure engineering, including manipulating the shape and distribution of strengthening units at multiple length scales, has been a widely applied strategy for tailoring the mechanical properties of structural materials. In addition to these well-studied strengthening units, e.g., precipitates, whiskers, grain boundaries etc, dislocation-based structural units1-3 have gained recent attention because of additional possibilities in varying their shape and arrangement. A prerequisite for optimizing the dislocation-based structural units is the clear understanding of their detailed atomic arrangements.
Focusing on a binary Mg-Gd alloy, we reported a new microstructure, which contains self-assembled hexagonal 1-nm Gd nano-fiber patterns, associated with the dislocation template.4 This Gd nano-fiber patterns form after the hot-extrusion and post-annealing processes, the generally economical approaches. The nano-fibers are perpendicular to the basal plane of the hexagonal close-packed lattice of Mg, resulting a strong inhibiting effect on the basal slips, the easiest slip system. On the other hand, these nano-fibers less impede non-basal slips since the glide of non-basal dislocations has much less chances of cutting these nano-fibers. Therefore, the self-assembled Gd nano-fibers can selectively increase the critical resolved shear stresses of basal slips and thus provide an opportunity of designing high-strength Mg alloys with promising ductility. Our results open up a new path of manipulating microstructure for the purpose of engineering advanced Mg alloys.
Acknowledgement: We acknowledge funding from National Natural Science Foundation of China and thank Ms. Meiyue Shao for preparing TEM samples.
References:
1. Kuzmina M., Herbig M., Ponge D., Sandlöbes S. and Raabe D., 2015, “Linear complexions: Confined chemical and structural states at dislocations”, Science. 349, 1080-1083.
2. Liu H., Gao Y., Xu Z., Zhu Y.M., Wang Y. and Nie J.F., 2015, “Guided self-assembly of nano-precipitates into mesecrystals”, Scientific Reports. 5, 16530.
3. Hull D. and Bacon D.J., 2001, “Introduction to dislocations”, 4th edition, Elsevier Science Publishers, North-Holland.
4. Li Y.X., Wang J., Chen K., Shao M., Shen Y., Jin L. and Zhu G.-z., “Self-patterning Gd nano-fiber in Mg-Gd alloys”, reversion submitted.
CM6.8: Dislocation Microstructures II
Session Chairs
Wednesday PM, April 19, 2017
PCC North, 100 Level, Room 126 C
4:30 PM - *CM6.8.01
Enhanced Fracture Toughness of Mg/Nb Laminated Composites
Nan Li 1 , Youxing Chen 1 , Jian Wang 2 , Amit Misra 3 , Nathan Mara 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 , University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 3 , University of Michigan, Ann Arbor, Michigan, United States
Show AbstractEnhancing fracture toughness of Mg alloys has been actively investigated due to its huge potential as structural materials in the aerospace and automotive industries. However, Mg and its alloys, with a hexagonal close packed structure, present lacked ductility and poor deformability. Therefore, it is still challenging to improve the deformability while maintaining the high flow strength of Mg and Mg alloys. Here, we propose to use Mg/Nb laminated composites to enhance the fracture toughness, in which a high density of interfaces are designed and manufactured to enhance both the yield strength and ductility. The mechanical strength of the Mg/Nb multilayers measured from pillar compression achieve a high value of ~1.1 GPa. Besides such high strength, we also demonstrated such layered structure can enhance fracture toughness associated with highly activated basal slip and interface shearing between Mg and Nb interfaces. In situ three-point bending experiments have been performed in a SEM. Mg/Nb interfaces play a dominant role to block crack propagation and blunt crack tip. The stress concentration at crack tip is significantly released by shearing interfaces parallel to Mg basal planes. Along with the experimental observation, we also investigate the crack propagation in Mg/Nb by molecular dynamics.
5:00 PM - *CM6.8.02
Monitoring the Elastic Response of Individual Subgrains of a Dislocation Structure during Small Changes in the Imposed Strain
Wolfgang Pantleon 1
1 , DTU Mechanical Engineering, Kongens Lyngby Denmark
Show AbstractWith high-resolution reciprocal space mapping it is possible to identify individual subgrains in the deformation structure of selected grains by their unique orientation. For significantly large plastic strains, new subgrains emerge and dislocation structure reorganizes. During small strain intervals, on the other hand, the behavior of each individual subgrain can be traced in-situ. Changes in the internal elastic strains of the subgrains (with respect to the average elastic strain of the grain) during the initial transient after a strain path change reveal an intriguing size effect for this microplastic regime. Additionally, the corresponding changes in the internal elastic strains during an elastic unloading-reloading sequence are reported. These experimental observations on the elastic response of individual subgrains form an ideal basis for comparison with the evolution of dislocation structures predicted by different simulation techniques.
5:30 PM - CM6.8.03
About the Role of the Microstructure in the Plasticity of Sub-Micron Al and Be Wires
Frederic Mompiou 1 , Marc Legros 1
1 , CEMES-CNRS, Toulouse France
Show AbstractThe origin of the improved mechanical properties of sub-micron single crystals and whiskers is still debated, but studies generally concentrate solely on size effects. In comparison, the role of the initial microstructure and defect content, linked to the crystal size, has been given less consideration.
In this presentation, i would like to highlight the importance of the dislocation content and the role played by the external surface on the triggering of plasticity in both Al and Be sub-micron wires investigated by in-situ transmission electron microscopy (TEM).
The wires, obtained by selective etching of Al/Al2Cu and Al/Be eutectic alloys, all exhibit a thin Al oxide outer layer. Al wires present a large variability in dislocation density while Be wires parallel to their c-axis are usually dislocation free.
In Al, we show that multiplication of dislocations through intermittent spiral sources directly causes a power-law increase of the yield stress with decreasing cross-sectional size. The size effect and resulting mechanical response are directly linked to the initial defect density and the distance between the source and the surface. In the absence of dislocations, fibers elastically reach high stresses with limited to no plasticity, reminiscent of whisker behavior.
A similar fragile-like behavior is also observed in dislocation free Be wires. In this case moreover, the plastic deformation is strongly dependent on the orientation of the crystal with respect to the straining axis. When strained along their c axis, wires tend to twin. In twinned area, ductile behavior was observed due to the favorable orientation for prismatic slip. Twin nucleation and propagation is thought to be triggered by surface nucleation. Because of the presence of a remaining Al oxide surrounding the wire, we show that the deformation may require dislocations moving along the fiber axis. Our observations indicate that these dislocations are thought to move in or close to a remaining Al/Al oxide layer at the wire surface.
Ref:
- F Mompiou, M Legros, A Sedlmayr, DS Gianola, D Caillard, and O Kraft. Source-based strengthening of sub-micrometer al fibers. Acta Materialia, 60:977–983, 2012
- F. Mompiou, M. Legros, C. Ensslen, and O. Kraft. In situ tem study of twin boundary migration in sub-micron be fibers. Acta Materialia, 96:57 – 65, 2015.
5:45 PM - CM6.8.04
Experimental Studies of Dislocation Density and Stress Distributions near Grain Boundaries in Deformed Materials
Jun Jiang 2 1 , Yi Guo 3 1 , Thomas Britton 4 , Angus Wilkinson 1
2 Mechanical Engineering, Imperial College London, London United Kingdom, 1 , University of Oxford, Oxford United Kingdom, 3 , Empa-Swiss Federal Laboratories for Materials Science and Technology, Thun Switzerland, 4 Materials, Imperial College London, London United Kingdom
Show AbstractSEM and synchrotron X-ray have seen significant advances over the last decade and these developments are now delivering new insights into the deformed state that help understand deformation processes performance limits of metallic polycrystals.
Cross-correlation-based analysis of EBSD patterns obtained in the SEM is now rather routine and allows mapping of elastic strain variations from which and lattice rotation tensors to be mapped routinely in the SEM, from which type III intragranular stress distributions can be determined, along with lattice rotations from which geometrically necessary dislocation (GND) densities can be estimated. Statistical analysis of data from OFHC Cu polycrystals deformed to various extents in uniaxial tension will be presented. Correlations between stress metrics, GND densities and position in the microstructure (ie Euclidian distance from the nearest grain boundary or triple junction) will be explored. The results show a tendency for the larger stresses and GND densities to be found close to these microstructural features at which compatibility of slip in the neighbouring grains needs to be accommodated, however the correlation coefficients are not very high indicating that multiple conditions need to be fulfilled for ‘hot spots’ to develop.
In Ti samples deformed to lower strain levels the influence of individual slip bands are much more obvious and we have used cross-correlation EBSD to measure stress and GND densities close to intersections between slip bands and grain boundaries. This has been extended into 3 dimensions by use differential aperture micro-beam Laue X-ray diffraction microscopy (DAXM), performed at beam line 34-ID-E at the Advanced Photon Source (APS). When slip systems in the two grains are well aligned slip bands often meet at the grain boundary and EBSD reveals little significant concentration of either stress or GND density. At the other extreme when slip systems are very badly align a slip band in the softer grain typical stops at the grain boundary and EBSD reveals a clear stress concentration as a result of the dislocation pile-up in accord with the classic Eshelby-Frank-Nabarro model. A third intermediate case was unexpectedly revealed where slip bands were blocked by the grain boundary but no obvious stress concentrations where revealed by EBSD, however in these cases lattice curvature analysis indicated that GND densities were increased either in a localised feature associated with the head of the pile-up or in a more diffuse distribution along the grain boundary suggesting that localised plastic flow relaxes the stress concentration.
Symposium Organizers
Yang Xiang, Hong Kong University of Science and Technology
Stefan Sandfeld, TU Bergakademie Freiberg
Yao Shen, Shanghai Jiao Tong University
Jian Wang, University of Nebraska–Lincoln
CM6.9: Atomistic Simulations of Dislocations
Session Chairs
Chun-Wei Pao
Arun Prakash
Thursday AM, April 20, 2017
PCC North, 100 Level, Room 126 C
9:00 AM - *CM6.9.01
Plasticity of Nanocrystalline Thin Films—New Insights from Atomistic Simulations
Arun Prakash 1 , Eva Preiss 1 , Aviral Vaid 1 , Benoit Merle 1 , Erik Bitzek 1
1 Department Materials Science and Engineering, Institute I, Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Erlangen Germany
Show AbstractNanocrystalline thin metallic films are a complex and interesting material system that find potential applications in many technologically relevant fields. They often display enhanced mechanical properties in comparison to their bulk counterparts. Recent experiments on nanocrystalline Au thin films have, nevertheless, shown that such structures evidence macroscopic brittle failure in the presence of a notch, with strong plastic localization observed directly in front of the notch. The detailed mechanisms leading to such localization, however, remain unclear.
In this work, we investigate the deformation behavior of Au and Ag thin films – both free standing films as well as films on an amorphous substrate – using large scale atomistic simulations. The initial samples, with (111) textured columnar grains are generated by means of a constrained Voronoi tessellation technique so as to obtain an optimal and realistic in boundary (GB) and triple junction (TJ) network, and subsequently, notches/slits of different radii are introduced. Both notched and notch-free specimens are then subjected to uniaxial tensile strain. We perform detailed analysis of the deformation mechanisms and the stress state in the individual grains. The results clearly show that both dislocation-mediated and GB mediated plasticity are localized in the region ahead of the notch/slit in the deformed free standing notched thin films. Furthermore, we observe copious amounts of deformation twins directly in front of the notch. These twins result in both surface undulations and a reduction in film thickness, particularly around GBs and TJs. In comparison, the films on a substrate show almost no twinning even in the presence of a notch. These results of thin films are compared to those of bulk nanocrystalline samples with both columnar and fully 3D microstructures.
The deformation behavior and the stress states in individual grains are, furthermore, discussed in the context of mesoscale simulation frameworks. The results from the simulations are compared with experiments to obtain insights into the deformation mechanisms and localization observed in bulge tests of nanocrystalline thin films.
9:30 AM - CM6.9.02
Plastic Deformation of Nanostructures Induced by Focused Ion Beam Irradiation—Insights from Atomistic Simulations
Cheng-Lun Wu 1 , Chun-Wei Pao 1 , David Srolovitz 2
1 , Academia Sinica, Taipei Taiwan, 2 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Show AbstractFocused Ion Beams (FIB) are versatile tools with cross-disciplinary applications from the physical and life sciences to archaeology. Nevertheless, the nanoscale patterning precision of FIB is often accompanied by defect formation and sample deformation. Since the introduction of FIB in the late 1970s, the mechanisms leading to FIB-induced deformation remain elusive. In this study, we revealed the fundamental mechanisms governing the large-scale plastic deformation of nanostructures undergoing FIB processes by a series of GPU-accelerated, large-scale molecular dynamic simulations. We revealed that the primary mechanism leading to nanostructure deformation during FIB processes is the mass transport to the surface of material caused by energetic ion bombardment. By quantitatively analyzing the amount of volume removed from film interior, a surprisingly simple linear correlation between atomic volume removed from the film bulk and film deflection angle, regardless of incident ion energies and currents, is revealed. Hence, the present study demonstrates that by controlling the direction of mass transport by properly controlling incident ion energy, it is possible to manipulate the plastic deformation of nanostructures, thereby fabricating nanostructures with complex three-dimensional geometries.
9:45 AM - CM6.9.03
Exploring the Energy Landscape for Screw Dislocation Motion in Tantalum
Amit Samanta 1 , Vasily Bulatov 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractMotion of screw dislocations control many important features of plastic deformation in BCC materials. Consequently, numerous studies have focused on understanding the stability of the various core structures, namely the easy-core (E), the hard-core (H) and the soft-core (S). Using results from ab initio DFT calculations for a prototypical BCC metal, tantalum, I will show that the small periodic supercell sizes used in DFT calculations necessitate careful accounting for non-linear elasticity and that, once the latter is indeed accounted for, dislocation core energies can be extracted with very high precision. We predict that the ground state of the screw dislocation in tantalum is the non-polarized E-core at pressures from 0 to 7 Mbar, the H-core is metastable and S-core is unstable. Our DFT calculations points to the existence of an additional metastable state that we call split-core, and the energy difference between the H-core and split-core determines how a screw dislocation moves. Together with an in depth analysis of the relative stability of the H, E and S-core structures and the transition pathways joining these structures, we are able construct a potential energy landscape from ab initio calculations that can describe the motion of screw dislocations in tantalum.
10:15 AM - CM6.9.05
Atomistic Simulations of Dislocation/Precipitate Interactions in Mg-Al Alloys
Amitava Moitra 2 , Javier Segurado 1 , Javier Llorca 1
2 , IMDEA Materials Institute, Getafe, Madrid, Spain, 1 , IMDEA Materials Institute & Technical University of Madrid, Getafe, Madrid Spain
Show AbstractHardening of Mg-Al alloys by precipitation is due to the interaction of basal dislocations (which present the lowest critical resolved shear stress of the slips systems and twinning) with Mg17Al12 precipitates. Strengthening of Mg-Al by precipitation is much less efficient than in other metallic alloys (e.g. Al) and this behavior has been attributed to geometrical effects, as the Mg17Al12 precipitates grow as thin plates parallel to the basal plane. However, to the authors’ knowledge, no detailed atomistic simulations of the dislocation-precipitate interaction have been carried out to ascertain the strengthening potential of these precipitates and this is the objective of this talk.
Analyses of the dislocation/precipitate interaction in the athermal limit were carried out by means of molecular statics. In particular, the critical resolved shear stress necessary to overcome the precipitates was determined as a function of the precipitate size (including the precipitate thickness) and offset from the dislocation glide plane and compared with predictions of classical continuum models. Moreover, the prevalence of the different mechanisms of dislocation/precipitate interaction (precipitate shearing, dislocation looping or cross-slip, leading partial cutting with trailing partial looping, diffusionless climb, defect nucleation at the dislocation–precipitate contact point, etc.) was ascertained in each condition. These results provided valuable information about the precipitate hardening mechanisms in Mg-Al and suggested new avenues to improve the mechanical properties of Mg-Al alloys.
10:30 AM - CM6.9.06
Frictional Properties of Multi-Asperity Surfaces at the Nanoscale
Arun Nair 1 , Raghu Santhapuram 1
1 , University of Arkansas, Fayetteville, Arkansas, United States
Show AbstractAsperities are considered an unevenness of surfaces, or surface roughness. Surfaces that are finely polished are still considered uneven at the atomic scale. This unevenness of surface reduces the actual contact area when two surfaces come into contact. Understanding surface asperities are very important because the friction and wear properties of two materials depend on the nano scale contact between the material’s surfaces. Many experimental studies have concluded that surface texture can help improve contact characteristics and reduce the frictional forces between surfaces. We use molecular dynamics simulations to study the frictional and mechanical response of an aluminum surface with cylindrical and spherical asperities that resemble true surfaces. Nanoindentation and scratch tests were carried out using different indenter radiuses on spherical and cylindrical asperities, and the results are compared to surfaces without asperities. We observe that the coefficient of friction (COF) is lower for spherical asperity surfaces, if the indenter radius is less than or equal to 4 nm, and the COF is lower for cylindrical asperity surfaces, if the indenter radius is greater than or equal to 5 nm. Finally, the COF decreases with increasing indenter radius for multi-asperity geometries studied here. The atomic mechanisms corresponding to the observed frictional response of the surfaces are explained by dislocation nucleation and propagation in the system and adhesion forces. These studies are expected to guide experiments to design multi-asperity surfaces for tribological applications.
10:45 AM - CM6.9.07
Symmetry Coupling between Phase Transitions and Crystalline Defects
Yipeng Gao 1
1 , The Ohio State University, Columbus, Ohio, United States
Show AbstractPhase transition is one of the most efficient and effective means of producing desired internal defect structures, which essentially dictate the physical properties of crystalline materials. It has been observed that a variety of defects at different dimensions (e.g., 1D defects of dislocations, 2D defects of grain boundaries, 3D defects of domain structures, etc.) are generated accompanying with structural phase transitions, which suggests an intrinsic connection between defects and phase transitions. Since crystalline defects are associated with crystal symmetry while phase transition is a symmetry-breaking process, there should be a symmetry coupling between them. However, such a coupling relation has not been recognized and established, not only because of diversified nature of defects but also due to the lack of theoretical description. We show that the type of generated defects is dictated by the broken symmetry during a phase transition, which covers a large number of defects of distinctively different dimensions. By utilizing a new theoretical framework based graph theory, we show that a hidden pathway can be discovered through crystal symmetry analysis in typical popular material systems, e.g., iron-based alloys, titanium-based alloys and NiTi alloys, which results in the generation of characteristic types of defect structures during phase transitions. The defect generation through the hidden pathway is distinctively different from plasticity because it is dictated by the modulus softening during phase transitions. Such an intrinsic connection between phase transitions and defect generation provides a theoretical foundation for crystallographic design of domain and defect structures in defect engineering.
CM6.10: Dislocation Core Related Mechanicsm
Session Chairs
Satish Rao
Christopher Weinberger
Thursday PM, April 20, 2017
PCC North, 100 Level, Room 126 C
11:30 AM - CM6.10.01
Screw Dipole Annhilation in FCC Crystals
Satish Rao 1 , L.P. Kubin 2 , Christopher Woodward 3 , T.A. Parthasarathy 3 4
1 , Ecole Polytechnique Federale Lausanne, Switzerland, Lausanne Switzerland, 2 , French Aerospace Lab ONERA, Palaiseau France, 3 Air Force Research Laboratory, Wright-Patterson Air Force Base, Wright-Patterson AFB, Ohio, United States, 4 , UES Inc., Dayton, Ohio, United States
Show AbstractAtomistic simulations are used to study the structure of a screw dislocation intersecting an edge dipole in FCC Cu. It is shown, that for edge dipole heights < 3nm, atomic jogs (3 111 layer thick) are formed on the screw dislocation in the minimum energy configuration, if the distance between one of the edge dislocations in the edge dipole and the screw dislocation is not greater than 3 111 atomic planes. It is proposed that cross-slip, initiated at such 3 111 plane thick jogs, lead to the annhilation of screw dipoles, in PSB channels at saturation stress and at tiii stress in monontonic deformation . Screw dipole heights, for which there is complete annhilation at 0K due to cross-slip initiated at these atomic jogs, is investigated using atomistic simulations. These simulation results are expected to be useful in understanding dynamic recovery initiation in FCC crystals.
11:45 AM - CM6.10.02
Microplasticity in Ag3Sn—Experiments and Modeling
Christopher Weinberger 1 , Ian Bakst 1 , Keith Dusoe 2 , Mohammadreza Bahadori 3 , Hang Yu 3 , Paul Canfield 4 , Seok-Woo Lee 2
1 , Colorado State University, Fort Collins, Colorado, United States, 2 , University of Connecticut, Storrs, Connecticut, United States, 3 , Drexel University, Philadelphia, Pennsylvania, United States, 4 , Iowa State University, Ames, Iowa, United States
Show AbstractThe intermetallic Ag3Sn, and similar structured intermetallics like Cu3Sn, are common in new lead free solders as well as dental amalgams where they can act as both strengthening phases as well as ductile phases in these alloys. While the constitutive behavior of composites containing Ag3Sn has been studied, there is little information regarding the mechanisms of plastic deformation in the intermetallic itself. To understand the dual role these intermetallics play in alloys, we use experiments and modeling to explore the dislocation and twinning mechanisms that give rise to plastic deformation in single crystals of Ag3Sn. TEM, bulk deformation, and micropillar compression tests give insight into the fundamental carriers of plastic deformation including identification non-basal slip in these hcp-derived materials. Density functional theory calculations in combination with analytical theory demonstrates that the basal plane is the easy slip system whose energetics of slip are similar to many ductile metals. However, twinning and non-basal slip are both required (and experimentally observed) to allow for the activation of a sufficient number of degrees of freedom to support general deformation. The ordering of the alloy inhibits many of the twinning and slip mechanisms that are operative in hcp metals, which limits ductility.
12:00 PM - CM6.10.03
Dislocation Trajectory and Schmid Law Deviation in BCC Metals
Lucile Dezerald 1 , David Rodney 2 , Emmanuel Clouet 3 , Lisa Ventelon 3 , Francois Willaime 3
1 , Institut Jean Lamour, Nancy France, 2 , Université Lyon 1, Lyon France, 3 , CEA Saclay, Saclay France
Show AbstractPlasticity in body-centered cubic (bcc) metals is known to be atypical at low temperatures. One of their most surprising features is a marked dependence of the yield stress on crystal orientation, which is in clear violation with the Schmid law applicable in most other metals. These properties are known to arise from the glide of 1/2<111> screw dislocations that undergo strong core effects at the atomic scale. Here, we used ab initio calculations based on the Density Functional Theory (DFT) to investigate 1/2<111> screw dislocation core properties and their influence on macroscopic plasticity in the following bcc transition metals: V, Nb, Ta, Mo, W and Fe. We show that, at atomic scale, 1/2<111> screw dislocations glide in {110} planes only on average. The path they follow is systematically shifted towards the twinning region. The amplitude of this deviation is metal dependent and can directly be linked to the dislocation Peierls potential, the two-dimensional energy landscape of the dislocation in the {111} plane. We propose to modify the Schmid law to account for this deviation by projecting the applied stress on the deviated trajectory rather than on the average {110} glide plane. This new law agrees with both experimental and DFT measurements of Peierls stress variations with crystal orientation and enables understanding why Schmid law deviation is metal dependent. We can now predict Schmid law deviation in all BCC crystals from simple atomistic calculations and explain this well-known property characteristic of BCC plasticity.
12:15 PM - CM6.10.04
Grain and Indentation Size Effect of Nanocrystalline Ceramic Nanoindentation
Heonjune Ryou 2 1 , Kathy Wahl 1 , Edward Gorzkowski 1 , Boris Feigelson 1 , James Wollmershauser 1
2 , ASEE, Washington, District of Columbia, United States, 1 , US Naval Research Laboratory, Washington, District of Columbia, United States
Show AbstractNanomaterials have unique and interesting properties that are often related to high ratio of atoms on interfaces or grain boundaries to the grain interior atoms. Typically, smaller microstructures exhibit improved strength and hardness. However, measured material properties are often superimposed with phenomena that are unique to a specific measurement method and the size of the crystallites. Recent developments in processing and sintering of bulk nanostructured ceramics have allowed creation of fully dense volumetric ceramics with grain sizes much smaller than 100 nm. These materials and advances in indentation instrumentation open a new pathway to study the fundamental physical behavior of ceramics across various grain sizes (~10 nm to single crystal) in multiple indentation length scales (nano to micro). In this work, nano-grain size MgAl2O4 spinel samples were fabricated by sintering nanocrystalline spinel powders. The samples were evaluated by nanoindentation to observe the indentation size effect (method specific) and the grain size effect (material specific). The results were compared to larger grain size spinel samples. We will discuss the distinct roles of indentation size effect and grain size influencing the measured and intrinsic mechanical properties of nanostructured oxides.
12:30 PM - CM6.10.05
Role of Dislocation Core Structures in Tension/Compression Asymmetry in HCP Titanium Using DFT and MEAM
Max Poschmann 1 , Mark Asta 1 , Daryl Chrzan 1
1 Materials Science and Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractTitanium alloys are known to exhibit a tension-compression asymmetry in the critical resolved shear stress for dislocation slip (R. Chait, 1973. Scripta Metallurgica, 7(4), 351–354). In hcp α-titanium alloys the glide of a-type screw dislocations with burgers vector <11-20> on the {1-100} (prismatic) planes plays an important role in plastic deformation. Neeraj et al. observed tension-compression asymmetry for these dislocations and suggested that the asymmetry may arise from a change in the core structure of the dislocations (T. Neeraj, M. F. Savage, J. Tatalovich, L. Kovarik, R. W. Hayes, & M. J. Mills, 2005. Philosophical Magazine, 85(2–3), 279–295). The important role of dislocation core structure in controlling tension/compression asymmetry is well known in bcc metals (V. Vítek, R. C. Perrin, & D. K. Bowen, 1970. Philosophical Magazine, 21(173), 1049–1073), but the configuration of a-type screw dislocation cores in titanium is not fully understood in part because their compactness has prevented complete experimental analysis. Because their core structure is likely related to their behavior, gaining a better understanding of these cores through atomistic modeling would be of significant value to predicting the properties of alloys.
Under no applied stress, Clouet et al. have demonstrated that the difference in energies of screw dislocation cores spread on different planes can be very small and yet still have significant implications for plasticity in α-titanium (E. Clouet, D. Caillard, N. Chaari, F. Onimus, & D. Rodney, 2015. Nature Materials, 14(9):931–936). Their calculations indicate that core spreading on the first-order pyramidal plane is the stable configuration, but that the barrier to cross slip into the prismatic plane followed by slip on the prismatic plane is lower than that for slip on the pyramidal plane. We find that for sufficiently converged calculations our results agree that the pyramidal core is the stable configuration under no applied stress. Further, we find that non-shear stresses can have significant effects on the core structure of a-type screw dislocations in titanium and thereby influence the energy barrier to slip. In particular, we find for certain directions of applied stress that compression (tension) enhances the pyramidal (prismatic) character of the dislocation core. Core structures of these dislocations are computed using both density functional theory and modified embedded atom method to achieve high accuracy and understanding of cell size effects. The nudged elastic band method is used in combination with both techniques to calculate barrier energies and transition paths for dislocation slip. This allows us to determine how non-Schmid effects can influence dislocation mobility by changing the dislocation core structure. Our results suggest that the tension-compression asymmetry may be attributable to these effects.
This research is supported by Office of Naval Research grant N00014-12-1-0413.
12:45 PM - CM6.10.06
Study of Kink Propagation along Screw Dislocation in Body Centered Cubic Iron
Anshuman Choudhury 1 , Laurent Proville 1
1 DEN, SRMP, CEA-Saclay, Gif sur Yvette, ile de France, France
Show AbstractIn body centered cubic metals and alloys the screw dislocation glide is obstructed by the Peierls barrier, which can be overcome either by thermal activation or by the application of stress. TEM images from in-situ straining experiments (at 100K and applied stress of 200 MPa) performed by Daniel Caillard (CEMES Toulouse) show what he calls 'jerky motions' i.e. the dislocation glide proceeds through long jump of many Peierls barriers whereas the standard Peierls process predicts jumps of one Peierls hill at a time. Since there is no existing theory to accurately predict such motion at atomic-scale, we have chosen to work with atomic simulations at very low temperature and see how it behaves at different levels of increasing stresses.
Molecular Dynamics (MD) allows us to work with several millions of atoms to simulate glide of screw dislocations through the computation of the individual atom dynamics. In bcc metals, the screw dislocations glide by the formation of a kink pair (KP) in the next Peierls valley and its propagation along the dislocation line. It is also of keen interest to observe the subsequent propagation of kinks along the dislocation line under the effect of the applied stress. The NEB method [3] employed to find the saddle state for the KP formation works by producing a number of images of the system. This requirement is computationally unfavorable to work with very large number of atoms. For this purpose we run the NEB in a reduced simulation cell to compute the KP saddle state and extend it equally in both directions along the dislocation line by placing straight dislocations in the extensions. Finally we obtain a dislocation length of 0.52 μm, of size comparable with D. Caillard’s experiments.
Using different inter-atomic potentials [1, 2], we have performed MD simulations below the Peierls stress to examine the energy dissipation through the crystal at different stress levels and different temperatures to tentatively explain the jerky motion of screw dislocations.
1. Proville L., Rodney R., and Marinica M.C., "Quantum effect on thermally activated glide of dislocations." Nature materials 11.10 (2012): 845-849.
2. Gordon P. A., Neeraj T., and Mendelev M. I., "Screw dislocation mobility in bcc metals: a refined potential description for α-Fe." Philosophical Magazine 91.30 (2011): 3931-3945.
3. Sheppard D., Terrell R., and Henkelman G., "Optimization methods for finding minimum energy paths." The Journal of chemical physics 128.13 (2008): 134106.
CM6.11: Dislocations and Nanoparticles
Session Chairs
Dan Mordehai
Stefan Sandfeld
Thursday PM, April 20, 2017
PCC North, 100 Level, Room 126 C
2:30 PM - *CM6.11.01
The Temperature-Dependent Strength of Metallic Nanoparticles
Dan Mordehai 1 , Roman Kositski 1 , Koren Shreiber 1 , Doron Chachamovitz 1
1 , Technion, Haifa Israel
Show AbstractThe strength of nanoscale metallic specimens which are initially pristine from dislocations is much higher than their bulk counterparts due to the need to nucleate dislocations on their surfaces to deform them plastically. The process of nucleating dislocations, which is a thermally activated process, depends on local stresses and temperature. In this talk we explore computationally the strength of faceted defect-free nanoparticles and how it depends on the temperature. In particular, we propose a computational scheme to calculate the activation barriers for dislocation nucleation at the onset of plasticity. With atomistic simulations we show that the vertices of faceted nanoparticles are preferable sites for dislocation nucleation under compression. The stress gradients near the vertices yield both a shape and a size effect in strength. The larger stress gradients in the smaller nanoparticles leads to larger compressive stresses to nucleate a dislocation at the vertices. In addition, different nanoparticle shapes lead to different gradients, which in turn may also eliminate the effect of size. This result is demonstrated with nanoparticles of various material types. The compressive stress to nucleate dislocations was found to decrease with temperature. With the help of classical nucleation theory, we propose a model to quantify the size and temperature dependency. The model is employed to extract from the simulation the activation energy, volume and entropy to nucleate dislocations at the vertices of the nanoparticles.
3:00 PM - CM6.11.02
Coupling Dislocations and Cavities with Phase Field Methods
Antoine Ruffini 1 , Alphonse Finel 1 , Yann Le Bouar 1
1 , LEM CNRS/ONERA, Chatillon France
Show AbstractIn material science, one of the ubiquitous difficulties is to describe the development and evolution of mesoscale microstructures whose properties are closely controlled by phenomena requiring a microscale description. For example, single-crystal nickel-based superalloys used in aerospace industry can exhibit micropores acting as potential sources of fatigue failure. The pores can be removed by hot isostatic pressure (HIP). The difficulty lies in modeling the physics of pore annihilation since it involves a wide variety of mechanisms which themselves stand at a much lower scale (dislocations, microcracks…) [1].
Numerically, in strategies consisting in describing some of these processes, atomistic simulations usually fail to account for realistic systems with high enough space and time scales. Continuum methods, such as the phase-field, thus appear as potentially well adapted alternatives. To this end, we have recently proposed a new phase-field model to explicitly couple individual cracks and dislocations [2-3]. Notably, this model allows us to consider the physical entities at their characteristic time scale, in a numerical formulation that enables the management of complex free surfaces.
In this presentation, the actual model extended to face-centered cubic geometry will be exposed and illustrated in the context of pore annihilation by plastic flow in single-crystal nickel-based superalloys. A way of introducing the competition between plasticity and surface energy on the morphology of nanopores will also be discussed.
[1] A. Epishin, B. Fedelich, T. Link, T. Feldmann, I. L. Svetlov, Mater. Sci. and Eng. A, Vol. 586 (2013) pp. 342-349.
[2] A. Ruffini, A. Finel, Acta Mater., Vol. 92 (2015) pp. 197-208.
[3] A. Ruffini, A. Finel, J. Colin, J. Durinck, Scripta Mater., Vol. 113 (2016) pp. 222-225.
3:15 PM - CM6.11.03
Solute Effects on Dislocation Core Structure in BCC Metals and Consequences on Dislocation Mobility
Berengere Luthi 1 , Lisa Ventelon 1 , David Rodney 2 , Francois Willaime 3
1 DEN-SRMP, CEA, Université Paris Saclay, Gif-Sur-Yvette France, 2 ILM, CNRS, Université Claude Bernard Lyon 1, Lyon France, 3 DEN-DMN, CEA, Université Paris Saclay, Gif-Sur-Yvette France
Show AbstractThere is ample experimental evidence of the effects of interactions of solute atoms with dislocations on metal properties, such as pipe diffusion, Cottrell atmospheres, and dynamical strain ageing. In this work we investigated, using ab initio electronic structure calculations, the interaction between interstitial solute atoms and screw dislocations in several body-centered cubic (bcc) metals.
First considering carbon in bcc Fe, our calculations suggest a strong interaction of carbon solutes with screw dislocation cores inducing a spontaneous reconstruction of the core structure towards a low-energy configuration, where unexpectedly, the dislocation core adopts a hard-core configuration, which is unstable in pure metals [1]. The solute atoms are at the center of regular trigonal prisms formed by the Fe atoms inside the dislocation core. This local configuration is similar to the building unit of Fe3C cementite, which is composed of both corner- and edge-sharing trigonal iron prisms centered on carbon atoms [2]. The interaction energy between the dislocation and the solute atoms is even more attractive with carbon in other bcc metals of group VI (W and Mo), where the carbides WC and MoC exclusively consist of face-sharing prisms, exactly like in dislocation cores. Based on a thermodynamic model at thermal equilibrium, we show that this strongly attractive interaction leads to a core saturation by solute atoms, even at very low carbon bulk concentration. The same core reconstruction is obtained with other octahedral interstitial solutes (B, N, O) in Fe. Consequences on the dislocation mobility in Fe(C) and relations to dynamical strain ageing will be discussed based on our NEB calculations, showing that the mobility of the transformed dislocations is very low. This agrees with recent in-situ TEM observations, which show that in the regime of dynamical strain ageing of steels, the mobility of screw dislocations is strongly reduced [3]. The double-kink formation and migration on decorated dislocations will also be addressed.
[1] L. Dézerald et al., Phys. Rev. B 89, 024104 (2014).
[2] L. Ventelon et al., Phys. Rev. B 91, 220102(R) (2015).
[3] D. Caillard, J. Bonneville, Scripta Mater. 95, 15 (2015).
3:30 PM - CM6.11.04
Dynamics of Interactions between Point Defects and Dislocations in BCC Iron
Luis Casillas 1 , Haixuan Xu 1
1 , University of Tennessee, Knoxville, Tennessee, United States
Show AbstractThe interaction between point defects and dislocations plays a central role in the microstructural evolution, plastic deformation behavior and radiation resistance of structural materials. Previous investigations of this interaction are mostly based on the elastic theory. In this work we investigate the dynamics of the interaction of vacancies and interstitials with perfect edge and screw dislocations in bcc iron. The saddle point distribution of a point defect near a dislocation is sampled using self-evolving atomistic kinetic Monte Carlo (SEAKMC). The anisotropic effects caused by the dislocation on migration energy barriers are examined. Then the probability and life time of a defect to be absorbed by a dislocation is determined based on KMC simulations. The results show that the dynamics of interaction between point defects and dislocations controls the interaction probability and the interaction radius is different from the estimation from the elastic theory. We examined the migration paths of defects and found the presence of repulsion zones for dumbbell in the presence of screw dislocations.
3:45 PM - CM6.11.05
Dislocation Dynamics Simulations of Spatial Distribution Effect in Particle Strengthening
Jian Sheng Wang 1 , Herng-Jeng Jou 2 4 , Gregory Olson 3 2
1 , Retired, Pasadena, California, United States, 2 , QuesTek Innovations, LLC, Evanston, Illinois, United States, 4 , On Leave, San Jose, California, United States, 3 , Northwestern University, Evanston, Illinois, United States
Show AbstractThe 3D interaction between dislocations and strengthening particles and the effects of their spatial distribution were investigated with a dislocation dynamic (DD) simulation code. Statistical measures were developed to quantify the local spatial inhomogeneity of particles and implemented in the code to generate various degrees of clustering/anti clustering spatial distributions. Three types of particles are studied: Orowan, shearing and mixed, i.e., within the Orowan/shearing transition regime. In general, clustering decreases the critical resolved shear stress (CRSS) comparing with square-lattice distribution in Orowan regime, but increase CRSS in shear-cutting. The effect in the transition regime is in between, i.e., the peak stress is insensitive to the distribution. Quantitative correlations between the change in CRSS and the degree of the inhomogeneity of particle spatial distributions were obtained.
CM6.12: Irradiation and Dislocations
Session Chairs
Thursday PM, April 20, 2017
PCC North, 100 Level, Room 126 C
4:30 PM - *CM6.12.01
Radiation-Induced Helium Nanobubbles Enhance Ductility in Submicron-Sized Single-Crystalline Copper
Weizhong Han 1
1 , Xi'an Jiaotong University, Xi'an China
Show AbstractThe workability and ductility of metals usually degrade with exposure to irradiation, hence the phrase “radiation damage”. Here, we found that Helium (He) radiation can actually enhance the room-temperature deformability of submicron-sized copper. In particular, Cu single crystals with diameter of 100 nm to 300 nm and containing numerous pressurized sub-10 nm He bubbles, become stronger, more stable in plastic flow and ductile in tension, compared to fully dense samples of the same dimensions that tend to display plastic instability (strain bursts). The sub-10 nm He bubbles are seen to be dislocation sources as well as shearable obstacles, which promote dislocation storage and reduce dislocation mean free path, thus contributing to more homogeneous and stable plasticity. Failure happens abruptly only after significant bubble coalescence. The current findings can be explained in light of Weibull statistics of failure, and the beneficial effects of bubbles on plasticity. These results shed light on plasticity and damage developments in metals and could open new avenues for making mechanically robust nano- and micro-structures by ion beam processing and He bubble engineering. Nano Letters 16 (2016) 4117-4124
5:00 PM - *CM6.12.02
Irradiation Effects on Hardening and Strain Bursts at the Microscale
Yinan Cui 1 , Giacomo Po 1 , Nasr Ghoniem 1
1 Mechanical and Aerospace Engineering Department, University of California, Los Angeles, Los Angeles, California, United States
Show AbstractNumerous defect clusters are produced after materials are subjected to irradiation. These defects will definitely impede dislocation motion, similar to the solute atoms. The unique aspect of irradiation defects, compared with strong solute atoms, is that irradiation defects can strongly interact with gliding dislocations and may even be damaged, such as forming the defect clear channel. In order to accurately and effectively describe irradiation defects in discrete simulation or continuum theoretical model, important problems arise: whether simply introducing the defect-induced stress field is enough to capture the irradiation hardening? Whether the reactions between irradiation defects and gliding dislocations also contribute a lot to the hardening, or the reactions mainly lead to the subsequent deformation localization or strain softening? On the other hand, whether irradiation effects inhibit or promote strain burst remained a mystery. The constraint to dislocation motion seems to inhibit strain burst, while the gradual formation of clear channel seems to promote larger burst. In order to unravel these mysteries, we present here a systematic computational study of plastic deformation of irradiated and unirradiated submicron specimens subjected to an externally applied constant stress or displacement rates. 3-D Discrete Dislocation Dynamics (DDD) simulations of FCC (Cu) and BCC (Fe) crystals are carried out. The computer simulations are based on our code for the Mechanics Of Defect Evolution Library (MODELib). Irradiation defects are modelled in two ways. One is through the direct modelling of discrete irradiation defects, which can capture plentiful physical insights. The other is through field description of irradiation defect density, which is more efficient for dealing with ultra-high defect density and large size. Based on the simulation results, the coupling effect of size and irradiation on hardening is disclosed. Previous irradiation hardening models are revisited. The common and different features in irradiated FCC and BCC crystals are analysed. In addition, the conditions when strain burst is promoted or inhibited under irradiation condition are revealed.
5:30 PM - CM6.12.03
Physically-Based Crystalline Law for Irradiated Steels
Ghiath Monnet 1 , Ludovic Vincent 2
1 , EDF - R&D, Moret sur Loing France, 2 SRMA, CEA, Gif sur Yvettes France
Show AbstractRecently, a set of Dislocation Dynamics (DD)-based constitutive equations of plastic deformation in BCC iron single crystals have been reported [1]. These equations account for lattice friction, controlling plastic flow at low temperature, and for jog drag, control the behavior at high temperature. The Critical Resolved Shear Stress (CRSS) appears as a superposition of a lattice friction and a line tension terms. The latter is shown to depend strongly on the former term. At low temperature, the line tension contribution may even vanish if the obstacle density is not large enough. In the present work, these equations have been generalized to irradiated BCC low-alloying steels.
In a first step, the Hall-Petch effect, inherent to polycrystalline materials, is phenomenologically implemented as athermal component of the CRSS, proportional to the inverse of the square root of the grain size.
The second step is dedicated to the implementation of radiation effects on the mechanical behavior. Thanks to recent DD simulations of dislocation interactions with dislocation loops and solute clusters, radiation hardening can be expressed in simple equations. Radiation defects are treated as local obstacles increasing the line tension contribution to the CRSS.
As an application of technological interest, the crystalline law was adapted for Reactor Pressure Vessel (RPV) steel and integrated over 500 grain orientations in order to compute the homogenized mechanical behavior. Results show that radiation hardening decreases at low temperature, in agreement with experimental investigations.
[1] G. Monnet, L. Vincent, B. Devincre, Acta Mater, 61, 6178–6190 (2013).
5:45 PM - CM6.12.04
Atomistic Based Study of Creep in Model Materials
Marie Landeiro Dos Reis 1 , Laurent Proville 1 , Maxime Sauzay 2
1 DMN/SRMP, CEA, Saclay France, 2 DMN/SRMA, CEA, Saclay France
Show AbstractThe study of creep is essential in the field of materials for nuclear reactors which components close to fissile matter are submitted to hight temperature and low stress. The theory used to predict the long time behavior of such components has been often criticized and tentatively improved. It yields a creep velocity varying as a power law of the applied stress. However, our comparison with experimental data show that this theory does not allow us to predict correctly the creep behavior of pure materials and different steels over a wide range of stress. In order to obtain a satisfactory agreement with the different sets of experimental data, we propose an atomistic based model where the deformation is controlled by the dislocation glide through a random distribution of obstacles. At very low stress the dislocation glide is assisted by the climb as in the standard theory whereas for higher stresses the thermal activation of dislocation glide proves sufficient to pass the obstacles. In the latter regime the standard theory of stress power law fails. The adjustment of the model parameters allows us to identify the typical characteristics of the random obstacles and potentially to identify them by comparison with atomistic calculations.
Symposium Organizers
Yang Xiang, Hong Kong University of Science and Technology
Stefan Sandfeld, TU Bergakademie Freiberg
Yao Shen, Shanghai Jiao Tong University
Jian Wang, University of Nebraska–Lincoln
CM6.13: Interfaces and Dislocations
Session Chairs
Friday AM, April 21, 2017
PCC North, 100 Level, Room 126 C
9:30 AM - CM6.13.01
A Concurrent Atomistic-Continuum Study of Sequential Slip Transfer of Curved Dislocations across Tilt Grain Boundaries
Shuozhi Xu 1 , Liming Xiong 2 , Youping Chen 3 , David McDowell 1
1 , Georgia Institute of Technology, Atlanta, Georgia, United States, 2 , Iowa State University, Ames, Iowa, United States, 3 , University of Florida, Gainesville, Florida, United States
Show AbstractSequential slip transfer across grain boundaries (GB) plays an important role in grain size-dependent plastic deformation in polycrystalline metals. In spite of extensive studies in modeling individual phases and grains, well accepted criteria of slip transfer across GBs and models of predicting irreversible GB structure evolution are still lacking. 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. Although a few concurrent multiscale methods have been developed and employed to investigate GB slip transfer, these methods are limited by the need to pass defects from a continuum dislocation dynamics domain to the atomistic region near the interface or by the need for significant computational effort devoted to adaptive remeshing to admit dislocations. In this work, large scale concurrent atomistic-continuum (CAC) simulations [1,2] are performed to address the slip transfer of dislocation pile-ups across tilt GBs in Cu and Ni. Results suggest the viability of the CAC method to describe the interface reactions with fully-resolved atomistics while preserving the net Burgers vector and associated long range stress fields of curved dislocations. In addition, we explore the role of specific tilt GB structures and dislocation character in interface absorption-desorption reactions, including evolution of the structure of the interface. The history effect of a sequence of dislocation reactions with the interface is also identified in light of irreversible evolution of the GB with each encounter.
[1] L. Xiong, G.J. Tucker, D.L. McDowell, Y. Chen, Coarse-grained atomistic simulation of dislocations, J. Mech. Phys. Solids, 59, 160 (2011)
[2] S. Xu, R. Che, L. Xiong, Y. Chen, D.L. McDowell, A quasistatic implementation of the concurrent atomistic-continuum method for FCC crystals, Int. J. Plast., 72, 91 (2015)
9:45 AM - CM6.13.02
Novel Dislocation Dynamics Model to Study Contact Deformation of Self-Affine Metal Surfaces
Syam Parayil Venugopalan 1 , Lucia Nicola 1
1 , Delft University of Technology, Delft Netherlands
Show AbstractThe contact mechanics of solids with self-affine rough surfaces is a topic of great practical importance since most of the man-made or natural surfaces exhibit self-affinity to some extent. Contact between metal surfaces results already at small loads in plastic deformation of the surface asperities. In self-affine surfaces the asperities span various length scales, and at the micron-scale their plastic response is size dependent. Size dependent plasticity can be captured by discrete dislocation plasticity simulations [1,2]. However, so far, only simulations for very simple surface geometries have been carried out using dislocation dynamics, e.g. two-dimensional surfaces with sinusoidal profile.
To extend the study to plastic deformation of self-affine surfaces under contact loading, a new modeling technique is developed that combines DDP [3] with Green's function molecular dynamics (GFMD) [4]. GFMD is a boundary value method that enables one to study the elastic response of a body subjected to an external load by modeling only the surface. The elastic stress field inside the solid can be deduced from the surface through analytical solutions. The dislocation dynamics are governed by constitutive rules, following [3].
The advantage of the new method, Green’s Function Dislocation Dynamics, is that it is computationally much less costly than conventional DDP.
Green’s Function Dislocation Dynamics keep track of the real contact area and contact pressure during (size-dependent) plastic deformation of the self-affine surface.
References
[1] Deshpande V, Needleman A, and Van der Giessen E 2004, Discrete dislocation plasticity analysis of friction, Acta Mater 52(10), 3135-3149
[2] Sun F, Van der Giessen E and Nicola L 2012, Plastic flattening of a sinusoidal metal surface: A discrete dislocation plasticity study, Wear 296, 672-680
[3] Van der Giessen E and Needleman A 1995, Discrete dislocation plasticity: a simple planar model, Modell. Simul. Mater. Sci. Eng. 3, 689-735
[4] Campaña C and Müser MH 2006, Practical Green's function approach to the simulation of elastic semi- infinite solids, Phys Rev B 74(7), 075420
10:00 AM - CM6.13.03
Cohesive Zone Failure of Al-Cu Misfit Dislocation Networks Using Hybrid Adhesion Method
Nicholas Brown 1 2 , Enrique Martinez 2 , Jianmin qu 3
1 Theoretical and Applied Mechanics, Northwestern University, Evanston, Illinois, United States, 2 Materials Science & Technology, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 Mechanical Engineering, Tufts University, Medford, Massachusetts, United States
Show AbstractA molecular dynamics study on misfit dislocation networks between the FCC aluminum-copper interfaces is presented. These interface dislocations are initially identified under relaxed conditions and then subjected to a uniaxial loading condition perpendicular to the interface plane. The purpose of the analysis is to study the evolution of the misfit dislocation networks under a load, until a failure mode is observed in the cohesive zone. The resulting fracture surfaces exhibit trace patterns caused by the initial misfit dislocations. This work models the structures using a hybrid adhesion method to simulate the attractive and repulsive forces between dissimilar material surfaces, typically defined by interatomic potentials. The purpose is twofold: (1) to track dislocation developments under uniaxial loading between an Al-Cu interface, (2) to identify the advantages of using an adhesive-based bonding relationship over a traditional force-field in thin-film delamination applications, and to highlight several limitations in utilizing this approximation simulation technique within molecular dynamics.
10:15 AM - CM6.13.04
3D Microstructural Characterization of Aluminum Alloys using Transmission X-Ray Microscopy (TXM)
C. Shashank Kaira 1 , Sudhanshu Singh 1 3 , Christopher Kantzos 1 , Antony Kirubanandham 1 , Vincent De Andrade 2 , Francesco De Carlo 2 , Nik Chawla 1
1 Materials Science and Engineering, Arizona State University, Tempe, Arizona, United States, 3 Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur India, 2 Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois, United States
Show AbstractPrecipitation-strengthened alloys are ubiquitously used in almost all structural applications and their superior mechanical performance can be attributed to the complex distribution of precipitates in the matrix. These different precipitate morphologies play a significant role in deciding the alloy’s mechanical response. Several models exist which describe interaction of dislocations with precipitates but these require experimental ratification. Conventional characterization techniques like transmission electron microscopy and atom probe tomography cannot be used prior to mechanical characterization and also do not sample statistically relevant regions. 3D X-ray Nanotomography using Transmission X-ray Microscopy (TXM) has been employed to characterize the 3D microstructure present in Aluminum-Copper alloys. High temperature in situ studies were performed owing to its high spatial resolution, non-destructive nature and quick acquisition time. Coupling this with Micromechanical testing and EBSD, has allowed us to establish accurate structure-property relationships to better predict the alloy’s deformation behavior.
10:30 AM - CM6.13.05
Work Hardening Stages, Structural Transitions and Scaling Laws
Darcy Hughes 1
1 , Consultant, Fremont, California, United States
Show AbstractAn experimentally determined and fully described deformation microstructure is employed to unravel the origin of the intermediate and latter stages of work hardening with increasing stress and strain in fcc metals. The potent characteristic of this microstructural evolution via dislocation slip is the development of a hierarchical structure ranked in size scale that exhibits transitional stages in parallel to the evolving hardening stages. Grain subdivision by two types of boundaries describes this hierarchy which includes: 1) incidental dislocation boundaries, which form by statistical trapping of dislocations (IDBs), and 2) geometrically necessary boundaries (GNBs), which delineate regions that deform either with different slip systems or with a different strain partitioning on the same systems. Structural parameters for both of these boundary types have been measured in a transmission electron microscope as a function of strain from small to very large strains. Parameters include boundary topology, spacing, misorientation angle and crystal orientation. Boundary dislocation densities are calculated from these measurements. The coexistence and hierarchy of GNBs and IDBs introduces an essential topological twist when compared to many other studies of hardening and recovery. At large strain the interplay between structure, crystallographic texture and slip systems cause a high fraction of gliding dislocations to reach a GNB before encountering an IDB. These GNBs with medium to high angle misorientations are thereby strong sinks for dislocations thereby altering the hardening rate. A new dynamic recovery mechanism is also active at large strain whereby sharp triple junctions joining long GNBs, migrate thereby reducing boundary area. Scaling laws link the joint evolution of structural parameters within an additive hardening law, thereby predicting strength. This mechanism-based model may inspire enlightened engineering to produce extremely fine scale, stable and strong metals.
10:45 AM - CM6.13.06
Using In Situ SEM Fatigue Test and Advanced Ex-Situ TEM Techniques to Investigate Fatigue Mechanisms in Single and Bi-Crystal Nickel Micropillars
Vahid Samaeeaghmiyoni 1 , Jonas Groten 4 , Hosni Idrissi 1 2 , Ruth Schwaiger 3 , Dominique Schryvers 1
1 EMAT, Physics, University of Antwerp, Antwerpen Belgium, 4 , Joanneum Research, Graz Austria, 2 Institute of Mechanics, Materials and Civil Engineering, Université catholique de Louvain, Louvain-la-Neuve Belgium, 3 Institute for Applied Materials, Karlsruhe Institute of Technology, Karlsruhe Germany
Show AbstractFatigue is one of the main failure modes in engineering materials. Since the role of surfaces and interfaces in the fatigue failure is critical, understanding the fundamental mechanisms governing the dislocation structures at interfaces and surfaces at the early stages of fatigue can be helpful to design and improve the fatigue resistance.
In the present study, in-situ SEM loading-unloading cycles have been performed on single and bi-crystal micropillars with well-known orientations as revealed by EBSD. Careful characterizations of the nature and the distribution of dislocations, the character and the local structure of the interface as well as the mechanisms controlling the interaction between these defects under cyclic loads were performed using conventional and advanced post mortem TEM including diffraction contrast imaging, nano-strain mapping in TEM as well as automated crystallographic orientation mapping in TEM (ACOM TEM).
Pronounced ratcheting and higher accumulated strains in the single crystal micropillars compared to the bi-crystals was observed. It has been attributed to the formation of dislocation walls in the single crystal micropillars. The bi-crystal micropillars exhibit a uniform tangled dislocation structure indicating that the grain boundaries hinder/postpone the formation of dislocation walls in these systems. Nanostrain mapping revealed by advanced nanobeam precession electron diffraction showed that most of the dislocations confined in the walls are of edge type. Local internal stresses at the walls have also been measured. Orientation mapping by ACOM-TEM revealed local changes of the crystal orientation at the walls. Furthermore, systematic contrast analyses on dislocations in the channels (i.e., between the walls) proved that mostly slip systems with high Schmid factor have been activated and that most of the dislocations in the channels were of screw type. Based on these results, the role of the free surface in fatigue is discussed and compared with bulk fatigued crystals.