Dierk Raabe Max- Planck Institut für Eisenforschung Düsseldorf
Raul Radovitzky Massachusetts Institute of Technology
Surya R. Kalidindi Drexel University
Marc Geers Technische Universiteit Eindhoven
Monday AM, November 30, 2009
Room 107 (Hynes)
9:30 AM - **HH1.1
Defect Dynamics of Irradiated Microstructures.
Tom Arsenlis 1 , Moono Rhee 1 , Gregg Hommes 1 Show Abstract
1 Condensed Matter and Materials Division, LLNL, Livermore, California, United States
The desire to increase the utilization of fuel in nuclear reactor cores is driving a renewed interest in modeling the degradation of materials in such environments in hope of producing predictive models of materials performance and life limits. Under these conditions, material microstructures may develop second phase precipitates, noble gas bubbles, prismatic dislocation loops, vacancy clusters, and other nano-scale defect structures. Dislocation dynamics simulation tools, designed to simulate the interaction of dislocations leading to strength and strain hardening, can be simply augmented to create defect dynamics simulation tools by including the enumerated irradiation defects through the use of Eshelby inclusions. In these defect dynamics simulations, the defects interact through their elastic strain fields and though a series of rules detailing their reaction at defect-defect intersections. Such tools can be used to simulate the effect of dislocation networks on the microstructural evolution during irradiation and the effect of irradiated microstructures on the evolution of dislocation networks during mechanical loading. Details of the implementation of Eshelby inclusions into the Parallel Dislocation Simulator (ParaDiS) code project will be discussed, and initial results of prototypical strain hardening simulations in the presence of prismatic loops and volumetric inclusions will be presented. Initial functional forms for defect-based coarse grained models of irradiation hardening/embrittlement will be offered.
10:00 AM - HH1.2
A Discrete Dislocation Plasticity Analysis of the Bauschinger Effect in Thin Films.
Siamak Shishvan 1 3 , Lucia Nicola 2 , Erik Van der Giessen 3 Show Abstract
1 Dept. of Structural Engineering, University of Tehran, Tehran Iran (the Islamic Republic of), 3 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, 2 Materials Science and Engineering, Delft University of Technology, Delft Netherlands
The Bauschinger effect in plastic flow of polycrystalline materials is generally understood to be caused by inhomogeneous deformation during loading, leading to residual stress upon unloading. This inhomogeneity can be intragranular, due for instance to dislocation pile-ups, and/or intergranular, caused by variations in texture or grain sizes. Some recent experiments on thin metallic films have revealed a remarkable Bauschinger effect , even in films that are so thin that they have only a single grain across the thickness, provided that they harden.We here study the origin of the Bauschinger effect in thin films by means of a two-dimensional discrete dislocation plasticity model. The model considers an array of grains having different orientations, with plastic deformation being represented by the motion of discrete dislocations that get blocked by the grain boundaries. Thus, the model explicitly incorporates the effects of grain size and orientation, as well as the piling-up of dislocations, which gives rise to size dependent plastic flow as shown previously for passivated films with columnar grains in . A recent innovation of the model in  is the introduction of a two-dimensional constitutive rule to represent Frank--Read sources taking into account that the grain size limits the available and effective dislocation source length and thereby its strength .While the latter model has demonstrated to be able to quantitatively capture grain size and film thickness effects in unpassivated as well as passivated thin films, the issue addressed here is what the origin of the Bauschinger effect is. The predicted Bauschinger effect clearly has an intra-granular contribution due to inhomogeneous slip, but the interaction between grains in a two-dimensional model needs further consideration. In particular, columnar-grained films are essentially a series connection of grains, which does not in any way reflect the interaction between grains normal to the loading direction. To remedy this within a two-dimensional context we explore the use of an affine deformation assumption on the grain level. While the resulting constraint has limited influence on the response of multi-layer films because of the grain interactions in the through-thickness direction, in columnar films it leads to predictions that are in good agreement with the experimental data in . Rajagopalan J, Han JH, Saif MTA. Scr Mater 2008;59:734 Nicola L, Xiang Y, Vlassak JJ, Van der Giessen E, Needleman A. J Mech Phys Solids 2006;54:2089. Shishvan SS, Van der Giessen E. "Distribution of dislocation source length and size dependent yield strength in freestanding thin films" (submitted for publication)
10:15 AM - HH1.3
Local Plasticity During Nanoindentation of Nanocrystalline FCC Metals using Quasicontinuum Simulation and Nanomechanical Experiment.
Frederic Sansoz 1 , Virginie Dupont 1 , Travis Gang 1 , Kevin Stevenson 1 Show Abstract
1 School of Engineering, University of Vermont, Burlington, Vermont, United States
Predicting the integrity of metallic thin films deposited on semiconductors for microelectromechanical systems (MEMS) applications requires a precise understanding of surface effects on plasticity in materials with nano-sized grains. Several models of plastic yielding for metal indentation have been proposed based on the nucleation and propagation of lattice dislocations, and their interaction with grain boundaries beneath penetrating tips. However, model refinement is needed to include the characteristics of materials whose grain size is much smaller than the typical plastic zones found in contact experiments. Particularly, cooperative deformation processes mediated by grain boundaries, such as grain rotation, deformation twinning, and stress-driven grain coarsening, can simultaneously emerge for very small grain sizes (< 20 nm), thus making a predictive understanding of plastic yielding elusive. This presentation summarizes our recent progress in using multiscale modeling and atomic force microscopy-based nanoindentation experiments in order to gain fundamental insight into the underlying mechanisms of surface plasticity in nanocrystalline face-centered cubic metals deformed by nanoscale contact probes. We first present two numerical approaches to model contact-induced plasticity in nanocrystalline aluminum, the quasicontinuum method and parallel molecular dynamics simulation, and discuss the role of a grain boundary network on the incipient plasticity of nanocrystalline Al films deformed by wedge-like cylindrical tips, as well as the processes of stress-driven grain growth in nanocrystalline films subjected to nanoindentation. Second, we present new low-force experiments using diamond tips mounted on AFM cantilevers, which reveal unusually-strong inhomogeneities in plastic flow and hardness during nanoindentation of Ni electrodeposits with a mean grain size of 18 nm.
10:30 AM - HH1.4
Reversible Plasticity under Nanoindentation of fcc Metals.
Gerolf Ziegenhain 1 , Herbert Urbassek 1 Show Abstract
1 Physics, University , Kaiserslautern Germany
Recently, it has been demonstrated that nanoindentation fo fcc metals may give rise to a regime of `reversible plasticity'. This phenomenon is of particular interest, as it underlines the difficulties of generalizing the macroscopic concept of plasticity on the atomistic level. Using atomistic simulations we investigate the indentation of single-crystalline Cu for both an ideal and a stepped (111) surface. Both systems exhibit an intermediate regime of `reversible plasticity', characterized by the formation of extended stacking faults, which heals entirely upon withdrawal of the indenter. The existence of the reversible regime reveals that, on the atomistic scale, ture (irreversible) plastic deformation is characterized by material transport rather than by the nucleation of stacking faults. We establish a criterion - based on the total displacement of particles - to determine after which indentation depth plasticity is generated irreversibly in the material.Recently the existence of reversible plasticity in FCC-metalsunder nanoindentation has been unveiled.Using atomistic simulations we investigate the indentation of single-crystalline Cufor both an ideal and a stepped (111) surface. Both systemsexhibit an intermediate regime of such reversible plasticity,characterized by the formation of extended stacking faults, which healentirely upon withdrawal of the indenter. Its existence reveals that, on the atomistic scale, plasticdeformation is characterized by material transport rather than by thenucleation of stacking faults. We establish a criterion --based on the total displacement of particles -- to determine after whichindentation plasticity is generated irreversibly in the material.
11:15 AM - **HH1.5
Effect of Microstructural Length Scales on the Plastic Deformation.
Reinhard Pippan 1 , Stephan Scheriau 1 , Christian Rehrl 1 , Christian Motz 1 , Martin Rester 1 Show Abstract
1 , Erich Schmid Institute of Materials Science OEAW, Leoben Austria
It is well known that constrain of slip is one of the most important parameter controlling the plastic deformation behavior of metals and alloys. Despite the vast number of research activities in the last century, we are far away from a complete understanding of these phenomena. Like other groups, we have applied new types of local analyses techniques (EBSD, Local deformation analysis, micromechanical tests, etc.) on different types of materials from single crystalline to nanocrystalline materials. The aim of this paper is to give an overview about the results of those experiments especially related to the constrain in dislocation movement. The different phenomena occurring at the different length scales will be discussed. Special attention will be devoted to the consequences of these results for the modelling of plasticity.
11:45 AM - HH1.6
Predicting Dislocation Mobility From Explicit Atomistic Details: A Kinetic Monte Carlo Study.
Mukul Kabir 1 , Timothy T. Lau 1 , David Rodney 1 2 , Sidney Yip 1 3 , Krystyn J. Van Vliet 1 Show Abstract
1 Department of Materials Science and Technology, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Science et Ingenierie des Materiaux et Procedes, Grenoble Institute of Technology, Grenoble France, 3 Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
In metals with appreciable concentrations of both point defects and line defects, interactions among defect species such as vacancies and dislocations contribute significantly to types and rates of mechanical deformation. Typically, computational simulation of such complex materials considers either the point defects (via Newtonian particle simulation approaches such as molecular dynamics) or the line defects (through elastic line simulation approaches such as dislocation dynamics). Here, we consider the case of heavily deformed body-centered cubic (alpha) iron, and develop multiscale simulations that aim to predict the creep response of macroscopic crystalline samples with atomistic fidelity to these point-line defect interactions. We report kinetic Monte Carlo simulations of dislocation climb at elevated temperatures. Our approach explicitly incorporates energy barriers associated with vacancy-dislocation interactions as determined from atomistic calculations, and enables observations of self diffusivity and climb over experimentally relevant timescales. We find that, as vacancies approach edge dislocation cores, energetic barriers to vacancy migration rapidly decrease. Consequently, the diffusivity of iron lattice vacancies is accelerated for those vacancies in the vicinity of dislocation cores. We also find the climb velocity to be a complex function of applied stress through the corresponding dislocation density and vacancy supersaturation density. The calculated macroscopic creep rates at elevated temperatures is in quantitative agreement with available experiments in terms of the so-called creep stress exponent of power law creep, and in qualitative agreement with our calculated dependence of the vacancy migration activation barrier on temperature and applied stress.
12:00 PM - HH1.7
Failure Analysis of Multilayer Permeation Barrier Structures in Flexible Electronics.
Teng Li 1 2 , Matthew Tucker 1 Show Abstract
1 Department of Mechanical Engineering, University of Maryland, College Park, Maryland, United States, 2 Maryland NanoCenter, University of Maryland, College Park, Maryland, United States
The functional organic materials used in flexible electronics are extremely vulnerable to the attack of environmental water vapor. Developing high performance permeation barrier for flexible electronics has been a significant challenge. A new design of permeation barrier that consists of multilayers of alternating nanoscale inorganic (i.e., Al2O3) and organic (i.e., polymers) thin films, is emerging as a possible solution. Flexible electronics are subject to cyclic, large deformation during their service life. While the organic layers in a multilayer permeation barrier can sustain large deformation, the brittle Al2O3 thin layers fractures at small strains. The fracture of the brittle inorganic layers substantially increases the water vapor permeation through the barrier, leading to the degraded device function. Here we report a systematic study of the failure analysis of the multilayer permeation barrier structures using finite element method. Our focus is placed on the debonding along the inorganic/organic interfaces as well as the fracture of the inorganic layers. The parametric study leads to quantitative understanding of the structure/materials design of the multilayer barriers to achieve better performance. We also show that adding a thin protective layer on the top of the multilayer permeation barrier can significantly enhance the deformability of the barrier.
12:15 PM - HH1.8
Study of Crack Propagation in FCC Materials using Strain Gradient Viscoplasticity.
Prateek Nath 1 , W. Curtin 1 , A. Needleman 1 Show Abstract
1 , Brown University, Providence, Rhode Island, United States
Classical plasticity ignores the role of stress enhancement due to accumulation of net Burgers vector of dislocations associated with plastic strain gradient. However such stress enhancements give rise to size effects in plasticity at the micron scale, and can necessary to predict fracture toughness of, for example, coatings and bi-materials. Discrete dislocation simulations capture these effects of dislocation accumulation but are computationally intensive. Here a viscoplastic version of the Gurtin's strain gradient crystal plasticity is developed as an alternative approach to include the accumulation of net burgers vector that is consistent with standard rate-dependent crystal plasticity models. The viscoplastic strain gradient model is then applied to predict crack propagation and toughness in FCC materials as a function of the additional length scale parameters in the model.
Monday PM, November 30, 2009
Room 107 (Hynes)
2:30 PM - **HH2.1
Size Dependent Strength and Ductility Optimization of Multiphase Steels.
Thomas Pardoen 1 , Pascal Jacques 1 , Thierry Massart 2 Show Abstract
1 , Université catholique de Louvain, Louvain-la-Neuve Belgium, 2 , Université Libre de Bruxelles, Brussels Belgium
Fine and ultra fine grained single phase steels suffer from a lack of ductility associated to a limited strain hardening capacity, similar to other fine grained metals. The lack of strain hardening originates from complex mechanisms of interactions between dislocations and grain boundaries. The multiphase strategy is commonly used in steels to restore ductility while preserving the strength enhancement associated to a small grain size. Dual phase and TRIP assisted multiphase steels constitute two important examples. The number of material parameters related to these complex and fine microstructures is much larger than in single phase ferritic steels and physics based multiscale modelling is essential for guiding the optimization of these materials. New generations of dislocation informed continuum models with size effects are available nowadays to make this step. A key ingredient is to incorporate information regarding grain boundaries. Grain boundaries are not perfect impenetrable barriers to dislocations but oppose an evolving constraint on the plastic flow, involving complex strain gradient effects and back stress variations. A 1-D phenomenological model with varying confinement at the grain boundaries has been developed within the Kocks-Mecking framework. A second, more advanced strain gradient plasticity model with evolving higher order boundary conditions at the grain boundaries has been implemented in a 2-D finite element code within a finite strain setting. The two models predict the increase of strength and the loss of ductility associated to a grain size reduction in single phase steels. These two models have been used to better understand the competition between the different microstructural and flow parameters affecting the strength/ductility balance of dual phase steels and TRIP assisted multiphase steels. For instance, the optimum dual phase microstructures are predicted as a function of grain size, volume fraction of second phase and C content. Also, the strain gradient based model captures the strong strain hardening enhancement associated to the continuous appearance of new impenetrable interfaces when submicron size austenite region transform into martensite in TRIP steels.
3:00 PM - HH2.2
A Non-local Dislocation Based Constitutive Hardening Model in Crystal Plasticity Finite FEM.
Philip Eisenlohr 1 , Eralp Demir 1 , Franz Roters 1 , Dierk Raabe 1 Show Abstract
1 , Max-Planck-Institut fuer Eisenforschung, Duesseldorf Germany
Continuum-based variational formulations for describing the elastic-plastic deformation of anisotropic heterogeneous crystalline matter are commonly referred to as crystal plasticity finite element models. They are used for microstructure-based mechanical predictions of complex microstructures exposed to complicated boundary conditions as well as for engineering design and performance simulations involving polycrystalline and polyphase anisotropic media. In this presentation we present a novel dislocation-based constitutive model that is used in conjunction with such a crystal plasticity finite element solver. A spatial heterogeneity in slip activity entails an imbalance in the sign of the local dislocation density, i.e., it results in so-called geometrically necessary dislocations with a net Burgers vector differing from zero. Provided that the gradient of such heterogeneity is large, the signed excess dislocation density biases the mechanical response and causes size-dependent effects. A continuum dislocation density-based model for crystal plasticity is proposed, which naturally accounts for above mentioned behavior. In this model, the microstructure of crystals is simplified as signed dislocation densities of edge and screw character on each slip system. The local density evolution results from reactions among and flux of dislocations. The internal stress due to the spatial distribution of excess dislocation content is taken into account in its full tensorial nature and superimposes the applied stress tensor which then drives dislocation motion. The model is solved at the integration point level of a finite element discretization and directly uses neighboring integration point data for the derivation of spatial gradients. We present applications of this new formulation in the field of bi- and oligocrystalline deformation.
3:15 PM - HH2.3
Surface Form Memory in NiTi by Indentation-Planarization: Modeling of Subsurface Plastic Zones.
Xueling Fei 1 , Corey O'Connell 1 , David Grummon 1 , Yijun Zhang 2 , Yang-tse Cheng 3 Show Abstract
1 Dept. of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan, United States, 2 , Conveyor Dynamics Inc, Bellingham, Washington, United States, 3 Chemical and Materials Engineering Department, University of Kentucky, Lexington, Kentucky, United States
Deep penetration of a spherical indenter into a martensitic NiTi shape memory alloy creates complex subsurface deformation zones that can drive subsequent thermally-induced two-way cyclic displacement of the indent profile. An indent made in this way is deep at low temperature and becomes shallower when warmed - a displacement that may be cycled indefinitely. If the indented surface is subsequently ground flat, warming causes a protrusion to appear at the site. We have termed this ability to thermally drive reversible flat-to-bumpy transitions “Surface Form Memory” (SFM). This effect may have applications to special optical devices, assembly of micromachines, bond-release, micro forging, micro joining, electrical switching, microconnectors, and variable heat transfer devices, among many others.The ability to achieve reversible SFM is related to the existence of a subsurface deformation zone in which plastic strains exceed those that can be accommodated by martensite detwinning reactions. These strains produce high dislocation densities that are thought to underlie the observed two-way effects. A finite element model has been used to analyze the strain distribution in the subsurface region, allowing an approximate determination of the extent of zones experiencing slip plasticity and martensite detwinning. Finally, using an indent replication technique, we show that SFM can perform very substantial mechanical work when displacing against a base-metal substrate. Comparison with the FE model indicates that the full energy density associated with NiTi actuator alloys (~1 MJm-3) is expressed in the SFM event.Keywords:NiTi Indentation Finite Element Model Two-Way Shape Memory
3:30 PM - HH2.4
Modeling of Subgrain Growth Kinetics: 3D Monte-Carlo Simulation.
Tomoaki Suzudo 1 , Hideo Kaburaki 1 , Mitsuhiro Itakura 2 Show Abstract
1 , Japan Atomic Energy Agency, Tokai-mura Japan, 2 , Japan Atomic Energy Agency, Tokyo Japan
Development of the subgrain size with time at annealing stages reveal a kinetics having the form, Dtn-D0n=Kt, where Dt is the subgrain size at time t; n a constant called a growth exponent; K a temperature dependent rate constant. Some experimental results indicates n=2.5-7 that is significantly larger than the value measured in normal grain growth (n=~2). The goal of this study is to computationally clarify the origin of this difference by using a 3D simulation model.A major microstructural difference between subgrains and normal grains is the difference of misorientation at their boundaries; the misorientation at the subgrain boundaries is smaller and usually less than 10 degrees. Because the mobility of the boundary depends on the misorientation, we modeled the subgrain growth kinetics using a three-dimensional Monte-Carlo method that adopts a well-known relation between the misorientation and the boundary mobility.The initial state of the simulation is created by assigning a crystallographic orientation to each sub-divided polyhedron produced in advance by the Voronoi tessellation. The crystallographic orientations are given by a method utilizing a random-walk process in the orientation space that creates a realistic misorientation distribution. This method also provides a desired mean misorientation by selecting an appropriate length of the random-walk process.The computational results indicated that cases with the low mean misorientation resulted in high n up to ~9 while those with the high mean misorientation led to n=~2: This coarsely agrees with experiments. Further detailed analysis showed that early annihilation of the high-angle boundaries causes the high grain-growth exponent.
4:15 PM - **HH2.5
Deformation of Ultra-fine-grained and Nanocrystalline Metals: Role of Dislocation-grain Boundary Interaction.
Alexander Hartmaier 1 , Rebecca Janisch 1 , Naveed Ahmed 1 , Xiaohui Zeng 1 Show Abstract
1 ICAMS, Ruhr-University Bochum, Bochum, NRW, Germany
The mechanical strength of grain boundaries determines the deformation behavior of ultra-fine-grained and nanocrystalline metals. Sliding of grain boundaries has been suggested contribute significantly to the plastic deformation, on top of conventional dislocation plasticity. Furthermore, grain boundaries and triple lines are potential sites of crack initiation and crack advance. A multiscale model is presented in which the mechanical properties of grain boundaries under shear and tension are calculated by electronic structure methods within the density functional theory. From the resulting tensile force-displacement curves physical parameters like the work of separation, maximum stress and displacement across the interface are derived. For the case of shearing and grain boundary sliding generalized gamma-surfaces are calculated that can be used to classify the different types of grain boundaries into three categories. The ab initio results can be used to parameterize cohesive zone models that describe the mechanical behavior of interfaces directly on the continuum scale. Preliminary results obtained with a simplified model indicate that grain boundary sliding itself is not a significant deformation mechanism during plastic deformation of nanocrystalline and ulta-fine grained metals, but rather facilitates recovery processes close to grain boundaries and thus weakens the material.
4:45 PM - HH2.6
Stress Evolution and Cracking Behavior During the Formation of Hierarchically-structured Thermal Barrier Coatings Deposited by Plasma Spray.
Kentaro Shinoda 1 , Alfredo Valarezo 1 , Brian Choi 1 , Yang Tan 1 , Toshio Nakamura 1 , Sanjay Sampath 1 Show Abstract
1 Center for Thermal Spray Research (CTSR), Stony Brook University, Stony Brook, New York, United States
Plasma-sprayed thermal barrier coatings (TBCs) exhibit a hierarchical microstructure in multiscales, ranging from nanometer grains, micro-sized splats, tens of micrometers pass layers, to mesoscale coatings. This layered structure resembles those of natural systems such as nacre, which potentially increases the toughness of such a thermomechanical coating. Although significant researches have been investigated this complex microstructure, much is left to incorporate in coating design due to complex deposition phenomena; non-particle state parameters such as the deposition rate and the substrate temperature can affect the microstructure as well as in-flight particle state parameters. In this study, we have used a novel in situ curvature system to examine deposit formation dynamics and stress evolution during deposition.Yttria-partially-stabilized zirconia (YSZ) powder was deposited with a conventional plasma spray torch onto aluminum and stainless steel beams. The stress evolution during the deposition was monitored via curvature change of the substrate during coating deposition, with the back side substrate temperature measured as well. The deposition rate and the substrate temperature were varied by controlling the powder feed rate, robot raster speed, and cooling jet. Particle temperature and velocity were also monitored.Main contribution of the stress evolution during deposition (named the evolving stress) was associated with constrained rapid solidification and cooling down of the particles (quenching stress). Increasing the feed rate and decreasing the raster speed concurrently increased the evolving stress, but the feed rate variation exhibited stronger effect on the stress state than that of raster speed. On the contrary, the evolving stress did not monotonically increase as increasing the substrate temperature. Since the higher feed rate results in shorter splat impact interval, the feed rate increase can cause the increase in the local deposition temperature where particle plume impingement and coating deposition were occurred. Thus, the evolving stress is expected to have a positive correlation with the local deposition temperature, which is difficult to measure directly. This finding can help in designing coatings such as deliberately introducing vertical cracks for enhancing in-plane coating compliance.
5:00 PM - HH2.7
Phase Field Simulations of Elastic Deformation Driven Grain Growth.
Michael Tonks 1 , Paul Millett 1 , Tapan Desai 1 , Dieter Wolf 1 Show Abstract
1 , Idaho National Laboratory, Idaho Falls, Idaho, United States
Deformation can have a large influence on grain boundary migration, altering both the grain growth kinetics and the evolving grain structure. In this work, a phase field grain growth simulation is coupled with a linear elastic stress calculation to model deformation driven grain growth. The model is verified by comparing the predicted behavior to atomic scale simulation results in bicrystals and analytical grain boundary migration expressions. Our simulations indicate that grains oriented such that they have a higher elastic stiffness in the load direction tend to have less stored elastic energy, and therefore tend to grow. The applied load does not change the exponent at which the average grain area grows, but it does alter the steady state grain size distribution and the final orientation distribution.
5:15 PM - HH2.8
Ab initio Determined Fundamental Materials-design Limits in Mg-Li-X (X = Al, Si, Zn, Ca, Cu) Ternaries.
Martin Friak 1 , William Counts 1 , Dierk Raabe 1 , Joerg Neugebauer 1 Show Abstract
1 Computational Materials Design, Max Planck Institute for Iron Research, Duesseldorf Germany
Ab initio calculations are becoming increasingly useful in designing new engineering alloys as these simulations accurately predict basic material properties only knowing the atomic composition of the material. In this paper, fundamental thermodynamic and elastic properties of a few selected bcc Mg-Li-X ternaries are calculated using density-functional theory (DFT) and compared with available experimental data. These DFT-determined properties are used to calculate engineering parameters such as (i) specific Young's modulus (Y/ρ) or (ii) bulk over shear modulus ratio (B/G) differentiating between brittle and ductile behavior. The engineering parameters are then used to identify alloys that have application-taylored mechanical properties needed for a light weight structural material. Previously, our multi-physics and multi-disciplinary approach allowed to reveal an inherent and fundamental materials-design limit in Mg-Li system where it is not possible to maximize both Y/ρ and B/G by changing only the composition or local atomic order of a binary alloy (Acta Mater. 2009, 57, 69-76). In order to by-pass these newly identified limitations in the binary system, the study has been extended by screening for potentially beneficial alloying elements in B2-MgLi-X (X = Al, Si, Zn, Ca, Cu).
5:30 PM - HH2.9
Applications of Homogenization Methods for Crystal Plasticity to Strongly Heterogeneous Polycrystalline Materials.
Ricardo Lebensohn 1 , Pedro Ponte Castaneda 2 , Martin Idiart 3 Show Abstract
1 MST-8, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 Facultad de Ingenieria, Universidad Nacional de La Plata, La Plata Argentina
In this talk we present recent advances in second-order homogenization of the plastic response of polycrystalline materials with strong contrast in local properties, due to, e.g., the presence of intergranular cavities, and/or strong directionality due to, e.g. low-symmetry crystal structure. The proposed method is validated by comparison with Fast Fourier Transform (FFT)-based unit cell calculations. Applications to the prediction of microstructure evolution in voided polycrystals, including the effects of triaxiality, texture-induced anisotropy, pore morphology and rate-sensitivity are presented and discussed.
5:45 PM - HH2.10
Mechanical Characterization of Microconstituents in Materials by Micromechanical Testing Technique.
Kazuki Takashima 1 , Masaaki Otsu 1 , Mitsuhiro Matsuda 1 Show Abstract
1 Materials Science & Engineering, Kumamoto University, Kumamoto Japan
The mechanical properties of materials are dominated by their microstructure, including grain-boundary precipitates, etc. It is, therefore, important to evaluate the mechanical properties of each microstructural constituent to develop materials with superior mechanical properties. However, it is rather difficult to measure the mechanical properties of each microstructural constituent, because the size of the microconstituents is of the order of microns. We have developed a machine that enables the mechanical testing of micro-sized materials. This testing machine has been developed for measuring the mechanical properties of MEMS materials (mainly, thin film materials). To date, tensile, bending, fracture toughness, and fatigue tests have been carried out for specimens with dimensions of approximately 10 μm using this testing machine. The size of these specimens is smaller than the grain diameter of ordinary bulk materials, thus enabling the direct measurement of the mechanical properties of individual microconstituents, including the strength of grain boundaries, fracture toughness of precipitates, interfacial strength between the matrix and secondary phase, etc. We have developed a micromechanical characterization method and a preparation technique for micro-sized test pieces from the microconstituents of the bulk material. In this study, some mechanical characterization results (interfacial fracture toughness measurement of lamellar structured PST TiAl alloys, tensile and fracture behavior of LPSO and matrix phases in Mg-Zn-Y alloys, etc.) are presented. In addition, the concept of multi-scale materials design based on the mechanical properties of microconstituents is discussed.
HH3: Poster Session
Monday PM, November 30, 2009
Exhibit Hall D (Hynes)
9:00 PM - HH3.1
Hydration of Portland Cement: Microstructure & Morphology.
Nicanor Prendes 1 , Ignacio Echegoyen 2 , Esperanza Menendez 3 Show Abstract
1 MEB-EDX, Ministry of Public Works, Madrid, Madrid, Spain, 2 Materiales y Estructuras, ISDEFE SA, Madrid, Madrid, Spain, 3 Ciencia de Materiales, ICCET-CSIC, Madrid, Madrid, Spain
If we accept the hydration concept as a the result of a series of physicochemical reactions in the mixing of a slightly soluble, extremely basic pH, cement, with a limited volume of water, giving birth to a precipitate whose microstructure is mechanically resistant, always related to a set of factors, both extrinsic (time, curing conditions, etc.) and intrinsic (relative humidity, pH, etc.), we can determine the chemical characteristics of the phases developed inside concrete. This process discriminates against the type of reaction that generates inside the concrete, as well as his sequence of formation, which helps to explain, from morphologic criteria and texturales, the different present phases in the same mortar, since it is the case of the Alkali Silica Reaction and the sulphates attack. The determination of this sequence is important to estimate the scope and expansion of the concrete, as well as the origin of his deterioration.Alkali-Silica Gels is often observed in close vicinity to, or intimately mixed, with ettringite, the product of the hydration reaction of Portland cement clinker aluminates with sulphates. The ettringite is usually observed in open spaces such as air void, the cement paste and aggregate interface, or through-paste cracks by S.E.M. technique. This has led some authors to conclude that the alkali-silica reaction (ASR) may be influenced by the presence of sulphates or other sulphur-bearing phases (pyrite and/o pirrotite). On the other hand, there are numerous evidences which show that ettringite will recrystallize into open spaces whether or not the alkali-silica reaction occurred. The issues complicating the picture are those of proper distinction between so-called primary and secondary ettringite, and of the effect of the curing temperature on the ettringite decomposition and the conditions for its subsequent recrystallization and redepositions for subsequent recrystallization and redeposition in the same system.The our purpose is to review the available data of S.E.M. and compare them with observations from examination of concrete, and clarify the relationships between expansion of cracking and the observed of ASR product and ettringite.
9:00 PM - HH3.2
Micro-tensile Testing of Mg-Zn-Y Alloys Including LPSO Phase.
Yuichi Nagatomi 1 , Yushi Kawakami 2 , Masaaki Otu 1 , Kazuki Takashima 1 , Yoshihito Kawamura 1 Show Abstract
1 , Graduate School of Science and Technology, Kumamoto University, Kumamoto Japan, 2 , Industrial Technology Center of SAGA, Saga Japan
Mg-Zn-Y alloys consisting of α-Mg and LPSO (Long Period Stacking Ordered) phases have higher strength, ductility and heat resistance compared with conventional magnesium alloys. The LPSO phase is considered to affect strengthening mechanism of these alloys, but it is not clearly identified why LPSO phase improves their mechanical properties because the size of LPSO phase is in the micro-meter range. We have then applied micro-tensile testing method to investigate the mechanical properties and the deformation behavior of these alloys in the micro-meter scale. Two types of extruded Mg97Zn1Y2 and Mg88Zn5Y7 alloys were used. The both alloys include the LPSO precipitates in α-Mg matrix as a secondary phase, and the volume fraction of LPSO phase in Mg97Zn1Y2 alloy was 26% and that in Mg88Zn5Y7 alloy was 80%. Thin foils with a thickness of 20 μm were prepared from these alloys by mechanical polishing, and micro-sized tensile specimens with dimensions of parallel part 50×20×20 μm3 were fabricated by FIB (Focused Ion Beam) machining. Micro-tensile tests were performed by the micro-tensile testing equipment which we had developed. The tensile strength for micro-sized Mg88Zn5Y7 alloy was higher than that for micro-sized Mg97Zn1Y2 alloy, but the fracture occurred by brittle manner for Mg88Zn5Y7 alloy. This result indicates that LPSO phase increases the tensile strength, but decreases ductility. In addition, the tensile strength parallel to the extrusion direction is higher than that perpendicular to the extrusion direction for both types of the alloys. This is due to the basal textures of LPSO phase formed during extrusion.
9:00 PM - HH3.3
Molecular Dynamics Based Observation of Interaction Between Grain Boundaries and Lattice Defects.
Toshihiro Kameda 1 Show Abstract
1 Engineering Mechanics and Energy, University of Tsukuba, Tsukuba, Ibaraki, Japan
To investigate the unique behavior of the severe plastic deformed metallic material, the interaction between grain boundaries (GB) and possible lattice defects induced during their making process is observed with molecular dynamics simulations. The simulation model contains several types of GBs and different sizes of defects, and their locations from the material surface are also varied. The number of atoms in each simulation is about 0.1 million. Through these numerical experiments of copper crystals with several types of GBs and sizes of defects, dislocation behaviors near GB and defects are studied. The results shows that (1) the lattice defects near grain boundaries can act as dislocation sources and could stimulate the dislocation activities, (2) the grain boundaries behave as both dislocation sink and source, (3) the activity as dislocation sink and source is higher when defects are located near surface. Acquiring more information about the structures and densities of defects, and types of grain boundaries, we might be able to establish the maximum enhancement method of the advantage of severe plastic deformed materials.
9:00 PM - HH3.4
Micro-Scale Fracture Testing of Mg-Zn-Y.
Shun Matsuyama 1 , Tetsuya Sakamoto 1 , Masaaki Otsu 1 , Kazuki Takashima 1 , Yoshito Kawamura 1 Show Abstract
1 , Kumamoto University, Kumamoto Japan
When considering the improvement in fracture property of multi-phase alloy, it is important to examine the fracture toughness of each phase and interface between phases. However, the size of each phase is often in order of micro-meters, and the present macro-scale fracture tests can not be applied to obtain fracture toughness of each phase and their interfaces. We have developed a testing machine, which enables the measurement of fracture property of micro-scale specimens. In this study, we have applied this technique to investigate the fracture behavior of two-phase Mg-Zn-Y alloys on the micro-meter scale. Two types of Mg-Zn-Y alloys (Mg97Zn1Y2 and Mg88Zn5Y7) were used in this investigation. These alloys consist of α-Mg and long period stacking ordered (LPSO) phases. The volume fraction of LPSO phase in Mg97Zn1Y2 was 26% and that in Mg88Zn5Y7 was 86%. Micro-scale cantilever specimens with dimensions of 10×20×50 μm3 were prepared from extruded Mg97Zn1Y2 and Mg88 Zn5Y7 alloys by focused ion beam (FIB) machining. Notches with a width of 0.5 μm and a depth of 4.5 - 5.0 μm were also introduced into the micro-sized specimens by FIB. Fracture tests were carried out using a mechanical testing machine for micro-sized specimens. Since fracture toughness value (KIC) were not able to obtain as the specimens size was too small to satisfy the plane strain condition, fracture toughness was measured as provisional KQ values. The KQ values of Mg97Zn1Y2 alloy were 0.8 - 1.2 MPam½ and those of Mg88Zn5Y7 alloy were 1.2 - 3.0 MPam½. As the volume fraction of LPSO phase in Mg88Zn5Y7 alloy was higher than that in Mg97Zn1Y2 alloy, this indicates that LPSO phase increases fracture toughness of Mg-Zn-Y alloys. The crack profile in Mg97Zn1Y2 alloy was observed and the crack was found to be deflected by LPSO phase. This suggests that LPSO phase also increases the crack growth resistance of this alloy.
9:00 PM - HH3.5
Electron Backscatter Diffraction and Nanoindentation of Nickel Powder Particles Impacted at High Velocity.
Yu Zou 1 , Dina Goldbaum 1 , Richard Chromik 1 , Eric Irissou 2 , Jean-Gabriel Legoux 2 , Stephen Yue 1 , Jerzy Szpunar 1 Show Abstract
1 Mining and Materials Engineering, McGill University, Montreal, Quebec, Canada, 2 Industrial Materials Institute (IMI), National Research Council Canada (NRC), Boucherville, Quebec, Canada
Micron-sized nickel powder particles were deposited at supersonic velocity on a steel substrate to form a coating using the method of cold gas spraying. The microstructural evolution and hardness distribution of the impacted powder particles were investigated using electron backscatter diffraction and nanoindentation, respectively. We found ultrafine grains, in the size of 200 nm, with relatively high defect density and hardness in the particle/particle interfacial regions. Formation of these ultrafine grains is interpreted in terms of rotational dynamic recrystallization caused by adiabatic shear instability.
9:00 PM - HH3.6
Formation of Ultrafine Microstructure during Uniaxial Warm-Compressive Deformation in an Fe-0.67%C Steel for Railway Wheels.
Yuji Yasumoto 1 , Kazuyuki Handa 1 2 , Yoshisato Kimura 1 , Yoshinao Mishima 1 Show Abstract
1 Materials science and Engineering, Tokyo Institute of Technology, Yokohama Japan, 2 Materials Technology Division, Railway Technical Research Institute, Kokubunji Japan
A high carbon steel with 0.67%C having a ferrite-pearlite microstructure is used for railway wheels because it provides enough strength and toughness, which ensure safe transportation. However, a few problems concerning safety remain unsolved, including so-called tread thermal cracks generated on the tread surface under the actual service conditions. The cause of these cracks is both heavy rolling contact with a rail and cyclic frictional heat from braking. We have to remove them by cutting so that fine cracks never grow into harmful ladder cracks.In the previous work, we focused on the relationship between crack generation and microstructural change near the tread surface. We elucidated that ultrafine microstructure is formed as the surface affected layer in the region about 100 μm depth below the contact surface. This ultrafine microstructure consists of ferrite grains with the average size less than 1 μm and spheroidized cementite particles having uniform distribution. Since the growth of fine cracks seems to be interrupted in the affected layer, it is interesting to understand how microstructure change affects crack generation. Thus, we have proposed to experimentally reproduce ultrafine microstructure by uniaxial warm compressive deformation using a universal type mechanical testing machine. The objective of the present work is to understand the mechanism of ultrafine microstructure formation in the surface affected layer of railway wheels and the effect of microstructural changes on mechanical properties.The uniaxial compressive deformation was conducted at strain rates of 10-1 and 1.0 s-1 up to total true strain of 0.5 and 0.7. The deformation temperatures are from 673 K to 973 K, which covers surface temperatures of railway wheels. Specimens were cooled to room temperature in the furnace, and annealed for 1 h at each deformation temperature. Microstructures were observed by optical and scanning electron microscopy. We have succeeded to reproduce ultrafine microstructure, which is quite similar to that of the surface affected layer, under the deformation condition at the strain rate of 1.0 s-1 up to 0.7 true strain at 873 K. The actual railway service condition can be estimated to be equivalent to the above condition. Electron backscattered diffraction analyses clarify that the ultrafine ferrite grains are surrounded by high-angle grain boundaries and low-angle subboundaries, and that the formation of ultrafine microstructure at temperatures lower than A1 is a recovery process associated with the rearrangements of accumulated dislocations. Spheroidized cementite is considered to play an important role to prevent the coarsening in ferrite grain refinement. In the case of deformation at 673 K, pearlite lamellar is not fragmented by neither deformation nor subsequent annealing for 1 h, and thereby, microstructural refinement is not occurred at 673 K.
9:00 PM - HH3.7
Constrained Deformation of TiAl PST Crystals.
Kengo Goto 1 , Kyosuke Kishida 1 , Haruyuki Inui 1 Show Abstract
1 Department of Materials Science and Engineering, Kyoto University, Kyoto Japan
The lamellar microstructure in TiAl/Ti3Al two-phase alloy is of special interest since the fracture toughness and creep resistance of TiAl/Ti3Al two-phase alloys with the fully lamellar microstructure are superior to those of any other microstructure types. We have been conducting a systematic study of the deformation mechanisms of the fully lamellar microstructure using polysynthetically twinned (PST) crystals through conventional uniaxial loading tests as well as the constraint tension or compression tests. In conventional uniaxial tension or compression, PST crystals except those with the lamellar boundaries perpendicular to the loading axis were found to deform in a highly anisotropic manner. When the compression axis is parallel to the lamellar boundaries, the macroscopic deformation occurs so that the specimen dimension measured in the direction parallel to the lamellar boundaries increase, leaving the dimension measured in the direction perpendicular to the lamellar boundaries almost unchanged. Such anisotropic macroscopic deformation was in good agreement with the anisotropy predicted on the basis of the operative deformation modes determined by TEM analysis of the deformation structures in six orientation variants in the TiAl lamellae. When deformation in the direction parallel to the lamellar boundaries is restricted in plane strain compression of PST crystals with the lamellar boundaries parallel to the loading axis, marked increases in yield stress and work-hardening rate are observed as compared to the case of the conventional compression test. These variations in mechanical properties are discussed by comparing the operative deformation modes determined by TEM analysis and those deduced by the Taylor model.
9:00 PM - HH3.8
Effect of Hard Phase on the Texture Evolution of Subgrain in IF and DP Steels During Biaxial Deformation.
Do Hyun Kim 1 , Suk Hoon Kang 1 , Seoung-Bum Son 1 , Heung Nam Han 1 2 , Kyu Hwan Oh 1 Show Abstract
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Materials Science and Engineering, Carnegie-Mellon University, Pittsburgh, Pennsylvania, United States
Subgrain textures of interstitial free (IF) and dual phase (DP) steels were compared during biaxial tensile deformation. Deformation stage for biaxial tension was specially designed for this study and electron backscattering diffraction (EBSD) was used to perform texture measurement with increasing biaxial strain. The martensite which exists on the ferrite boundaries was uniformly distributed in DP steel. Several individual ferrite grains were selected to measure the average orientations, local misorientations, orientation spread and orientation rotation rate of grains with increasing strain, and these experimental data were compared between IF and DP steels to analyze the hard phase effect.
9:00 PM - HH3.9
Nanoindentation into Structured Surfaces - Dependency on the Structure Size.
Gerolf Ziegenhain 1 , Herbert Urbassek 1 Show Abstract
1 , TU Kaiserslautern, Kaiserslautern Germany
Using atomistic simulations we investigate the indentation ofsingle-crystalline Cu surfaces. Besides an ideal defect-free surface, westudy different types of steps and adatom-islands of different shapes andsizes. Three different regimes of plastic deformation are identified, whichdepend on the size of the islands. While the deformation for medium andlarge islands is understandable in terms of surface steps, for smallislands a new mechanism has been identified: Under the load of the indentersmall islands of adatoms are pushed into the substrate and transported awayin the form of prismatic loops.
Dierk Raabe Max- Planck Institut für Eisenforschung Düsseldorf
Raul Radovitzky Massachusetts Institute of Technology
Surya R. Kalidindi Drexel University
Marc Geers Technische Universiteit Eindhoven
Tuesday AM, December 01, 2009
Room 107 (Hynes)
9:00 AM - **HH4.1
Global and Local Issues in Modeling Metallic Polycrystals.
David McDowell 1 2 Show Abstract
1 Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 School of Materials Science & Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Modeling plasticity associated with dislocation processes over a broad range of length and time scales is among the most challenging scientific problems due to the long range fields in dislocated crystalline metals, complexity of many body interactions, and associated self-organizational processes at multiple length and time scales. Advances in experimental and computational capabilities in the last decade have fused to foster increased understanding of grain level phenomena such as cyclic slip, dislocation substructure, and crack formation in fatigue, as well as mesoscopic polycrystalline phenomena such as shear localization, effects of grain boundary networks and crystallographic texture on yield, plastic flow and fracture, and large scale behavior such as diffuse necking that plays a role in establishing biaxial forming limits. However, several basic challenges remain to be fully addressed. This talk will provide perspective regarding nuances of treatment of heterogeneity in modeling microstructure evolution, which is relevant to the issue of whether localized (i.e, grain or sub-grain scale) or homogenized polycrystalline responses or properties are of interest. Moreover, consideration of uncertainty of mechanisms is relevant to the development of hierarchical and concurrent multiscale models, treatment of size effects, and modeling concepts to address the role of grain boundaries in polycrystals. Combined bottom-up and top-down modeling strategies are essential for calibrating approaches that consider the complexity of slip mechanisms and slip system interactions, as well as the role of grain boundaries. Depending on the purpose or goal of the simulations, consideration of high fidelity details in rendering microstructure morphology in simulations may or may not be crucial, with the latter being the case for comparative parametric studies to support materials design or to predict macroscopic stress-strain response.
9:30 AM - HH4.2
Modeling the Effect of Asymmetric Rolling on Microstructure and Ductility in AA5182.
Catalin Picu 1 , Antoinette Maniatty 1 , Renge Li 1 , Fujun Xu 1 , Devin Pyle 1 Show Abstract
1 MANE, RPI, Troy, New York, United States
A multiscale modeling effort is used to model the asymmetric rolling process, to predict the resulting microstructure (grain structure and orientations), and to investigate the effect of the microstructure on ductility in AA5182. Dynamic strain aging in Al-Mg alloys, such as AA5182, is associated with dislocations interacting with Mg solute atoms and leads to a negative strain rate sensitivity, which, in turn, has an adverse effect on formability. On the other hand, asymmetric rolling, where the speed of the upper and lower rolls differ to impart shear in addition to compression, may reduce the grain size and impart a shear texture that can improve formability. In this work, we develop models at the dislocation, grain, and macroscale to study these effects. At the dislocation scale, we study the interaction of dislocations with obstacles of different types and the resulting impact on strain rate sensitivity. At the grain scale, using models informed from dislocation level models and deformation histories based on macroscale models, we study the evolution of microstructure during asymmetric rolling and the effect of microstructure on formability. At the macroscale, we model the asymmetric rolling process and we also use microstructurally informed models to investigate the effect of microstructure and microstructural mechanisms on formability.
9:45 AM - HH4.3
Plastic Yield Criteria in Nanocrystalline Metals at High Strain Rates: An Atomistic Study.
Avinash Dongare 1 2 , Arunachalam Rajendran 3 , Bruce LaMattina 4 , Mohammed Zikry 2 , Donald Brenner 1 Show Abstract
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 Mechanical Engineering, University of Mississippi, University, Mississippi, United States, 4 , U.S. Army Research Office, Research Triangle Park, Mississippi, United States
The plastic deformation mechanisms of nanocrystalline materials depend on the interplay between dislocation and grain boundary processes. A reduction in grain size results in an increase in yield strength of materials, a relation known as the Hall-Petch effect. Recent studies indicate that the increase in strength with decreasing grain size reaches a maximum after which further a decrease in the grain size (less than ~ 15 nm) results in the weakening of the metal due to the shift in the dominating mechanism of plastic deformation from dislocation induced plasticity in the case of coarse grained materials to grain boundary sliding in the case of ultra-small grain sizes. The commonly used phenomenological yield criteria for polycrystalline metals and alloys are the Tresca and the von Mises criteria where deformation of polycrystalline metals is primarily due the motion of dislocations. As a result, it can be expected that the yield criterion needs to be modified to account for the change in deformation mechanisms at the ultrafine grain size (<= 10 nm) of nanocrystalline metals. Large-scale molecular dynamics (MD) simulations are used to understand the macroscopic yield behavior of nanocrystalline Cu with an average grain size of 6 nm at high strain rates. Three aspects of deformation behavior are studied: The tension-compression strength asymmetry, biaxial yield surface, and the three-dimensional yield surface. The calculated biaxial yield surface and the three-dimensional yield surface indicate that the von Mises type yield criterion may be used to study deformation at high strain rates for nanocrystalline Cu at grain sizes in the inverse Hall-Petch regime.
10:00 AM - HH4.4
Thermoplastic Elastomers: Multiscale Modeling, Microstructure Evolution and Macroscopic Instabilities.
Pedro Ponte Castaneda 1 , Oscar Lopez-Pamies 2 , Vikranth Racherla 3 Show Abstract
1 , University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 , S.U.N.Y., Stony Brook, New York, United States, 3 , I.I.T., Kharagpur India
Thermoplastic elastomers (TPEs) are block copolymers made up of “hard” (glassy) and “soft” (rubbery) domains that self-segregate on a length scale of a few tens of nanometers. Under typical processing conditions, TPEs also develop a “granular” structure at the micron level, which is similar to that of metal polycrystals. Therefore, TPEs can be described as materials with (continuum) heterogeneities at two different length scales. In this talk, we will develop constitutive models for TPEs with lamellar morphology, where the grains are made up of the same, perfect, lamellar structure (single crystal) with randomly varying lamination directions (crystal orientations). In particular, based on experimental evidence, we consider two types of such materials: “oriented” and “unoriented” samples. The oriented TPEs are highly ordered, near-single-crystal systems with slightly varying grain orientations, while the unoriented samples are initially isotropic due to an initially random distribution of orientations in the sample. It is found that for certain loading conditions--namely, for those with sufficiently large compressive deformations applied in the direction of the layers within the individual grains--the overall behavior of near-single-crystal TPEs becomes macroscopically unstable (i.e., it develops shear localization instabilities). The unoriented samples are also susceptible to instabilities, depending on the loading conditions. Finally, these instabilities can be related to the evolution of the underlying microstructure, which can be tracked experimentally in these systems.
10:15 AM - HH4.5
Microstructures and Mechanical Properties of FeNiMnAl Spinodal Alloys.
Xiaolan Wu 1 , Ian Baker 1 , Hongbin Bei 2 , Paul Munroe 3 Show Abstract
1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 School of Materials Science and Engineering, University of New South Wales, Sydney, New South Wales, Australia
The microstructure and mechanical behavior of the recently-discovered, very high strength FeNiMnAl spinodal alloys have been investigated. FeNiMnAl alloys with six different compositions were cast and their microstructure were studies by using X-ray diffraction (XRD) and transmission electron microscope (TEM). Two different types of microstructures were observed in those alloys, namely, f.c.c. and B2 two-phase alloys, or b.c.c. and L21 two-phase alloys. Based on the high temperature DSC measurement, all samples experienced two solid phase transformations (around 1093 K and 1393 K) before melting at ~1593 K during heating. Hardness measurements were performed as a function of annealing time at annealing temperature of 823 K. It is found that hardness increase in all samples at initial annealing stage (annealing 30 mins). At long time annealing, the change of hardness shows two trends. Some samples shows stable hardness values after long time annealing, while the hardness of other samples increases steadily probably due to the Mn-rich precipitates. We will discuss the relationships between the mechanical behavior and microstructures for these alloys. Research was supported by DOE Award #DE-FG02-07ER46392. Work in ORNL was sponsored by the U.S. Department of Energy: Division of Materials Sciences and Engineering.
11:00 AM - **HH4.6
Comparison of Slip, Slip Transfer, and Damage Nucleation in Experimental Observations and Crystal Plasticity Finite Element Simulations of CP Ti.
Thomas Bieler 1 , Yiyi Yang 1 , Leyun Wang 1 , YunJo Ro 2 , Philip Eisenlohr 2 , Martin Crimp 1 , Darren Mason 5 , Gene Ice 3 , Wenjun Liu 4 Show Abstract
1 Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan, United States, 2 Microstructure Physics and Metal Forming, Max-Planck-Institut für Eisenforschung, Düsseldorf Germany, 5 Mathematics and Computer Science, Albion College, Albion, Michigan, United States, 3 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 4 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois, United States
Heterogeneous deformation near grain boundaries in polycrystalline α-titanium was investigated using a combination of experimental characterization and Crystal Plasticity FEM (CPFEM) simulation. Samples were deformed incrementally by four-point bending until microcracks were observed in grains or at grain boundaries. Active deformation systems were then identified using orientation-imaging microscopy (OIM), differential image correlation (DIC), electron channeling contrast imaging (ECCI), 3-D x-ray diffraction, and atomic force microscopy (AFM) to obtain accurate z-displacements across deformation twins and dislocation slip bands. The deformation system activity in a patch of grains was quantitatively assessed based on these data, and compared with calculations based on a global stress Schmid factor hypothesis. Local strains measured by this analysis were compared with CPFEM simulation results. CPFEM simulation showed that dislocation slip in soft oriented grains at the early stage would alter the local stress state near grain boundaries significantly, affecting the deformation processes in the volumes close to the grain boundaries. The computational model’s sensitivity to grain boundary inclination beneath the sample surface was examined using information gained from 3D x-ray diffraction. This research was supported by NSF grant DMR-0710570 and DFG grant EL 681/2-1.
11:30 AM - HH4.7
High-Energy X-ray Diffraction for In-Situ Investigations in Thermo-Mechanical Processes.
Klaus-Dieter Liss 1 Show Abstract
1 The Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, New South Wales, Australia
The production and shaping of almost all metals and many other industrial materials depend on thermo-mechanical processes, which are the application of temperature and / or mechanical stress, not solely to obtain macroscopic forms but moreover to obtain a well defined microstructure, which finally defines the mechanical and other physical properties of the material. Conventionally, information of the microstructure after thermo-mechanic simulation is obtained ex-situ after quench from the process, destructive sample preparation and mostly surface sensitive investigation. Synchrotron high energy X-rays around 100 keV bear the advantage of deep penetration into the material and are brilliant enough to obtain direct time-resolved in-situ studies from the bulk while undergoing thermo-mechanic simulation. Large area high-resolution two-dimensional detectors are employed for a multi-dimensional exploitation of the diffraction patterns and are now fast enough to follow essential steps in real time. Examples shall be presented on selected metallic systems undergoing thermo-mechanical load revealing features like grain correlations upon phase transformations, grain refinement, subgrain formation, grain rotation, dynamic recovery, dynamic recrystallization, grain growth and the evolution of texture. In order to cope with ever increasing demands in industry-relevant processes, it is proposed, that future thermo-mechanic simulation takes place routinely in a high-energy X-ray beam.
11:45 AM - HH4.8
Integrated Experimental and Computational Modeling of the High Strain-Rate Behavior of Aluminum Alloys.
K. El-Khodary 1 , William Lee 1 , L. Sun 2 , Bryan Cheeeseman 3 , Donald Brenner 2 , Mohammed Zikry 1 Show Abstract
1 Mechanical and Aerospace Engineering, North Carolina State university, Raleigh, North Carolina, United States, 2 Materials Science, North Carolina State university, Raleigh, Maryland, United States, 3 WMRD, Army Research Laboratory, Aberdeen, Maryland, United States
The objective of this study was to identify the dominant microstructural mechanisms related to the high strength and ductile behavior of aluminum alloys and how high strain-rate loading conditions would affect the overall behavior of this alloy. Material characterization, molecular dynamic simulations, and microstructurally based finite element techniques calculations were undertaken to predict how Ω and θ’ precipitates and dispersed particles affect behavior at high strain-rates and physical scales that range from the nano to the micro. The predictions from the microstructural finite element model indicated that the precipitates continue to harden, and also act as physical barriers that impede the matrix from forming large connected zones of intense plastic strain. As the modeling indicates have indicated, and consistent with the experimental observations, the combined effects of θ’ and Ω preciptates, acting on different crystallographic orientations, enhance the strength, the ductility, and reduce the susceptibility of specific aluminum alloys to shear strain localization due to dynamic compressive loads.
12:00 PM - HH4.9
Direct Observation of Zr-4 Texture Evolution and Internal Strain Distribution during Multi-pass Welding Simulation.
Kun Yan 1 , Huijun Li 2 , Klaus-Dieter Liss 1 Show Abstract
1 , Australian Nuclear Science and Technology Organisation, Menai, New South Wales, Australia, 2 The Institute of Materials and Engineering, Australian Nuclear Science and Technology Orgonization, Menai, New South Wales, Australia
Texture evolution of Zircaloy-4 after different welding temperatures has been studied by high energy X-ray diffraction. Cyclic tensile experiments were conducted to simulate the internal strain of nuclear reactor tube which is bombed by neutrons along radial direction. Highly correlated grain orientation distribution was enforced by increasing welding temperature. Static and dynamic recrystallization were observed during cyclic tension while Burger’s relationship for hcp-bcc structure transformation were approved quantitatively. And the size effect during phase transformation with load was analysed as well.
12:15 PM - HH4.10
In-situ Neutron Diffraction Experiments as a Guide for Understanding the Microstructure Evolution during Deformation of Complex Materials.
Steven Van Petegem 1 , Alexander Evans 1 , Helena Van Swygenhoven 1 Show Abstract
1 NUM/ASQ, Paul Scherrer Institut, Villigen Switzerland
Predicting the development of intra- and intergranular stresses during deformation is a challenging task, especially for materials with a complex microstructure such as advanced steels and multiphase engineering components. A detailed knowledge of these so-called ‘microstresses’ is of utmost importance for understanding the influence of microstructure exerted on the mechanical properties, a rapidly growing field in which advanced crystal plasticity models play a crucial role. In particular combined finite element - elastoplastic self-consistent modeling has great potential but still needs further development based upon new experimental input. Time-of-flight (TOF) neutron diffractometers are ideally suited for the determination of microstresses in engineering components. Neutrons have a large penetration depth in typical engineering materials. Furthermore TOF instruments have the advantage over constant wave length diffractometers in that they provide a complete diffraction pattern containing the necessary structural information to determine microstresses.Here we present some recent results obtained at POLDI, the TOF diffractometer at SINQ (Paul Scherrer Institut). In particular we focus on the development of microstresses during uni-axial tensile deformation of some multiphase advanced steels. We report on the complex interplay between elastic and plastic anisotropy, which is responsible for the built-up large residual stresses. Furthermore we demonstrate how in-situ x-ray diffraction can be used as a complementary tool to reveal the role of those phases which are invisible for neutrons because of their low volume fraction and/or chemical nature.
12:30 PM - HH4.11
Interplay between Plastic Deformation and Martensitic Transformation in Polycrystalline NiTi Shape Memory Alloys.
Peter Anderson 1 , Sivom Manchiraju 1 , Ronald Noebe 2 , Santo Padula 2 Show Abstract
1 , The Ohio State University, Columbus, Ohio, United States, 2 , NASA Glenn Research Center, Cleveland, Ohio, United States
Plastic deformation and the martensitic phase transformation are two of the main deformation mechanisms in polycrystalline NiTi shape memory alloys (SMAs). The two mechanisms are strongly coupled. This coupling can be effectively used to optimize and train SMAs but it also can be detrimental, leading to reduced work output, early fatigue failure, and loss of shape memory properties.A microstructural finite element (MFE) model is developed to predict the macroscopic response of polycrystalline NiTi subjected to various stress and temperature states. The model captures the grain-to-grain redistribution of stress caused by both plasticity and phase transformation, thereby allowing each mechanism to affect the driving force for the other. Both deformation modes tend to operate simultaneously in polycrystals, because compatibility between grains introduces complex internal stress states—even for simple uniaxial loading. The stress redistribution from the phase transformation can induce plasticity and this in-turn can reduce the macroscopic transformation strain and stabilize residual martensite upon unloading.The coupling between plasticity and transformation is explored further by pre-straining the material in tension at elevated temperature (above Md temperature), so that austenite deforms plastically. Subsequent pseudoelastic tension tests (at 200 above Af temperature) show the early onset of transformation and pronounced hardening. Also, this pre-straining induces a two-way shape memory effect during subsequent no-load thermal cycling. Overall, the model has the potential to help identify optimal microstructures for stable, high-work-output SMAs and to design an efficient training process for SMA devices.
12:45 PM - HH4.12
Evaluation of the Generalized von Neumann Rule (MacPherson-Srolovitz Relation) Using the Phase-field Simulations.
Kunok Chang 1 , Carl Krill 2 , Long-Qing Chen 1 Show Abstract
1 Materials Science & Engineering, Penn State, University Park, Pennsylvania, United States, 2 Institut für Mikro- und Nanomaterialien, Universität Ulm, Ulm Germany
Fundamental understanding of grain growth has been a significant challenge in materials science for many decades. More than 50 years ago, von Neumann and Mullins derived an exact formula for the growth of grains in two dimensions, which yielded a rule both purely topological in content and rather intuitive in nature: a grain with more than six sides will grow, whereas one with fewer than six sides will shrink. In the three-dimensional case, there have been many attempts to develop an analogous expression for the growth rate of an individual grain based entirely on its topology. Recently, R.D. MacPherson and D.J. Srolovitz proposed an extension of the von Neumann rule to three dimensions: in their model, the growth rate of a grain is a function not only of its topology but also its size. Evaluating this model using real or simulated polycrystalline microstructures would be an important step to gaining greater understanding of grain growth. For this reason, we investigated the MacPherson-Srolovitz relation using a phase-field simulation of grain growth. With the Active Parameter Tracking (APT) algorithm, each grain is assigned a unique order parameter, which allows us to perform large-scale, coalescence-free simulations of grain growth. At each time step in the simulation, we can follow the migration of the boundaries of each individual grain. Thus, a rigorous validation of the MacPherson-Srolovitz relation becomes possible.